Domestic host availability modifies human-triatomine contact and host shifts of the Chagas disease vector Triatoma infestans in the humid Argentine Chaco

Domestic animals may affect human-vector contact and parasite transmission rates. We investigated the relationships between host-feeding choices, site-specific host availability, bug nutritional status, stage and abundance of Triatoma infestans Klug (Heteroptera: Reduviidae) in rural houses of Pampa del Indio during spring. We identified the bloodmeal sources of 865 triatomines collected in 70 sites from four main ecotopes. The main sources in domiciles were human (65.9%), chicken (23.4%) and dog (22.4%); dog (64.4%, 35.3%) and chicken (33.1%, 75.4%) in kitchens and storerooms, respectively; and chicken (94.7%) in chicken coops. Using random-intercept logistic regression clustered by domicile, the fraction of human-fed triatomines strongly decreased with increasing proportions of chicken- and dog-fed bugs, dropping from 96.4% when no chicken or dog slept indoors at night to 59.4% when both did. The fraction of dog-fed bugs significantly decreased with increasing human and chicken blood indices, and marginally increased with an indoor-resting dog. Mixed blood meals occurred 3.62 times more often when a chicken or a dog slept indoors. Host blood source did not affect mean body weight adjusted for body length and bug stage. Indoor-resting chickens and dogs greatly modified human-bug contact rates, and may be targeted with long-lasting systemic insecticides to suppress infestation.

Polyparasitism and zoonotic parasites in dogs from a rural area of the Argentine Chaco

Dogs play an important role as reservoirs and hosts of multiple pathogens shared with humans and wildlife, which contribute significantly to the global burden of disease. Here, we assessed the occurrence of a broad range of zoonotic and non-zoonotic parasites in dogs from a rural area in the humid Chaco; determined the occurrence of polyparasitism; and explored its association with selected risk factors. In total, 212 dogs were examined serologically to determine Trypanosoma cruzi infection and 152 of them also were examined for Ehrlichia canis, Borrelia bugderfori, Anaplasma phagocitophylum, Dirofilaria immitis and Toxoplasma gondii. Fecal samples from 85 dogs were examined for intestinal parasites. Seventeen parasite species were seen, 77% of which are zoonotic. The most prevalent parasites were Ancylostoma caninum (68.2%), T. gondii (55.3%, first report for dogs in Argentina), Giardia sp. (25.9%), Cryptosporidium sp. (20.0%), T. cruzi (16.5%), trematodes (15.3%) and Toxocara canis (14.1%). Polyparasitism was found in 96% of the dogs, with up to six parasite species in a single dog, and was significantly associated with age of dog but not with host body condition or sex. The most frequent pair of parasites found together were T. gondii-A. caninum (46%), A. caninum-T. cruzi (34%) and T. gondii-T. cruzi (27%). The prevalence of anemia and leukocytosis was significantly higher in dogs showing the worst body condition. Our findings likely reflect structural poverty, poor sanitation and lack of a safe water supply. Importantly, many of the prevalent parasites seen are threats to human health. 243 words.

An oral dose of Fluralaner administered to dogs kills pyrethroid-resistant and susceptible Chagas disease vectors for at least four months

New vector control tools that can fit into a broader integrated vector management strategy are notably lacking. We conducted a seven-month randomized trial to assess the efficacy of a single oral dose of Fluralaner (Bravecto^®) administered to dogs on the blood-feeding success, engorgement levels and mortality of pyrethroid-resistant and -susceptible Triatoma infestans third- and fifth-instar nymphs. The trial included 10 Fluralaner-treated and 10 placebo-treated (control) outbred healthy dogs residing in rural houses of the Argentine Chaco. Most (92.7%) of the 3017 triatomines exposed were able to blood-feed. Generalized linear models showed that blood-feeding success was not significantly modified by Fluralaner treatment, time posttreatment and their interaction. However, pyrethroid-susceptible fifth instars blood-fed significantly more frequently than susceptible third instars, and no significant differences were observed between the latter and resistant fifth instars. Engorgement levels were not significantly modified by Fluralaner treatment, time posttreatment and their interaction. Nearly all the triatomines that blood-fed on treated dogs up to 60 days posttreatment (DPT) died within 24 h regardless of pyrethroid susceptibility status combined with bug stage. Cumulative bug mortality over 4 days postexposure remained high over 90-120 DPT (70-81% in susceptible third and fifth instars, and 47-49% in resistant fifth instars), and was virtually nil at 210 DPT. Triatomines that fed on control dogs suffered marginal mortality (0-4%) except at 4 and 30 DPT. Fluralaner and xenointoxication are eligible for Phase III efficacy trials alone or combined with other methods in the frame of an integrated vector management strategy in areas with or without pyrethroid resistance.

Host-Feeding Sources and Infection With Trypanosoma cruzi of Triatoma infestans and Triatoma eratyrusiformis (Hemiptera: Reduviidae) From the Calchaqui Valleys in Northwestern Argentina

We assessed the prevalence of infection with Trypanosoma cruzi, parasite genotypes (discrete typing units, DTUs), and the host-feeding sources of domestic and peridomestic Triatoma infestans Klug and Triatoma eratyrusiformis Del Ponte in eight rural communities of the subandean Calchaqui valleys in northwestern Argentina. We sought to analyze their epidemiological role in the context of routine vector surveillance and control actions. Infection with T. cruzi was determined by optic microscopy or polymerase chain reaction (PCR) amplification of the hypervariable region of kinetoplast DNA minicircles. Parasite genotypes were identified through a multi PCR-based strategy. Bloodmeal contents were tested with a direct ELISA assay against nine antisera. Human sleeping quarters (domiciles) and peridomestic dry-shrub fences concentrated most of the T. infestans and T. eratyrusiformis infected with T. cruzi, respectively. The most frequent host-feeding sources of T. infestans were chickens (73.1%) in peridomiciles and humans (73.3%) in domiciles, whereas T. eratyrusiformis fed more often on cavid rodents (92.6%), which thrived in the dry-shrub fences. The main T. cruzi DTU identified in both vectors was T. cruzi I (TcI). Triatoma eratyrusiformis was implicated in the local circulation of TcI among cavies and perhaps mice, but infection with other typically domestic DTUs (TcVI and TcII/TcV/TcVI) indicated overlap between (peri)domestic transmission cycles in both vector species. Because dry-shrub fences were not targeted for routine insecticide spraying, they may act as sources of (peri)domestic reinfestation. Triatoma eratyrusiformis is an emergent secondary vector of T. cruzi and plays a significant role in the local transmission of T. cruzi.

Is the infectiousness of dogs naturally infected with Trypanosoma cruzi associated with poly-parasitism?

Interactions among different species of parasites co-infecting the same host could be synergistic or antagonistic. These interactions may modify both the frequency of infected hosts and their infectiousness, and therefore impact on transmission dynamics. This study determined the infectiousness of Trypanosoma cruzi-seropositive dogs (using xenodiagnosis) and their parasite load (quantified by qPCR), and tested the association between both variables and the presence of concomitant endoparasites. A cross-sectional serosurvey conducted in eight rural villages from Pampa del Indio and neighboring municipalities (northeastern Argentina) detected 32 T. cruzi-seropositive dogs out of 217 individuals examined for infection. Both the infectiousness to the vector Triatoma infestans and parasite load of T. cruzi-seropositive dogs examined were heterogeneous. A statistically significant, nine-fold higher mean infectiousness was registered in T. cruzi-seropositive dogs co-infected with Ancylostoma caninum and a trematode than in T. cruzi-seropositive dogs without these infections. The median parasite load of T. cruzi was also significantly higher in dogs co-infected with these helminths. An opposite trend was observed in T. cruzi-seropositive dogs that were serologically positive to Toxoplasma gondii or Neospora caninum relative to dogs seronegative for these parasites. Using multiple logistic regression analysis with random effects, we found a positive and significant association between the infectiousness of T. cruzi-seropositive dogs and co-infections with A. caninum and a trematode. Our results suggest that co-infections may be a modifier of host infectiousness in dogs naturally infected with T. cruzi.

Selective Insecticide Applications Directed Against Triatoma infestans (Hemiptera: Reduviidae) Affected a Nontarget Secondary Vector of Chagas Disease, Triatoma garciabesi

The control of nondomiciliated triatomine species adapted to peridomestic habitats represents a challenge because they are connected to sylvatic colonies, and pyrethroid insecticides have limited effects outdoors. The effects of residual insecticide spraying have rarely been assessed on secondary triatomines. Triatoma garciabesi (Carcavallo, Martinez, Cichero, Prosen & Ronderos, 1967) is a nontarget vector that inhabits the dry western Chaco region, and a member of the Triatoma sordida Stål 1859 complex. Little is known on the capacity of T. garciabesi to invade and establish viable domestic or peridomestic colonies, and on its response to residual insecticide sprays directed against Triatoma infestans Klug 1834. The presence and abundance of triatomines were assessed by timed manual collections annually or biannually (spring and fall) during 10 yr after a community-wide insecticide spraying campaign and selective insecticide sprays directed against T. infestans in a rural village of northwestern Argentina. T. garciabesi mainly occupied peridomestic habitats associated with chickens, and was unable to colonize human sleeping quarters. Trees with chickens occurred in nearly all houses and were infested in >25% of the occasions. The abundance of bugs at house-compound level was best explained by a generalized estimating equation model that included selective insecticide sprays during the previous semester (negative effects), chicken abundance (positive effects), seasonality, and their interactions. Our results suggest that insecticide applications targeting T. infestans affected the abundance of T. garciabesi, and reduced the likelihood of future infestation.

High levels of Trypanosoma cruzi DNA determined by qPCR and infectiousness to Triatoma infestans support dogs and cats are major sources of parasites for domestic transmission

The competence of reservoir hosts of vector-borne pathogens is directly linked to its capacity to infect the vector. Domestic dogs and cats are major domestic reservoir hosts of Trypanosoma cruzi, and exhibit a much higher infectiousness to triatomines than seropositive humans. We quantified the concentration of T. cruzi DNA in the peripheral blood of naturally-infected dogs and cats (a surrogate of intensity of parasitemia), and evaluated its association with infectiousness to the vector in a high-risk area of the Argentinean Chaco. To measure infectiousness, 44 infected dogs and 15 infected cats were each exposed to xenodiagnosis with 10-20 uninfected, laboratory-reared Triatoma infestans that blood-fed to repletion and were later individually examined for infection by optical microscopy. Parasite DNA concentration (expressed as equivalent amounts of parasite DNA per mL, Pe/mL) was estimated by real-time PCR amplification of the nuclear satellite DNA. Infectiousness increased steeply with parasite DNA concentration both in dogs and cats. Neither the median parasite load nor the mean infectiousness differed significantly between dogs (8.1Pe/mL and 48%) and cats (9.7Pe/mL and 44%), respectively. The infectiousness of dogs was positively and significantly associated with parasite load and an index of the host's body condition, but not with dog's age, parasite discrete typing unit and exposure to infected bugs in a random-effects multiple logistic regression model. Real-time PCR was more sensitive and less time-consuming than xenodiagnosis, and in conjunction with the body condition index, may be used to identify highly infectious hosts and implement novel control strategies.

Discrete typing units of Trypanosoma cruzi identified in rural dogs and cats in the humid Argentinean Chaco

The discrete typing units (DTUs) of Trypanosoma cruzi that infect domestic dogs and cats have rarely been studied. With this purpose we conducted a cross-sectional xenodiagnostic survey of dog and cat populations residing in 2 infested rural villages in Pampa del Indio, in the humid Argentine Chaco. Parasites were isolated by culture from 44 dogs and 12 cats with a positive xenodiagnosis. DTUs were identified from parasite culture samples using a strategy based on multiple polymerase-chain reactions. TcVI was identified in 37 of 44 dogs and in 10 of 12 cats, whereas TcV was identified in 5 dogs and in 2 cats -a new finding for cats. No mixed infections were detected. The occurrence of 2 dogs infected with TcIII -classically found in armadillos- suggests a probable link with the local sylvatic transmission cycle involving Dasypus novemcinctus armadillos and a potential risk of human infection with TcIII. Our study reinforces the importance of dogs and cats as domestic reservoir hosts and sources of various DTUs infecting humans, and suggests a link between dogs and the sylvatic transmission cycle of TcIII.

Processes affecting Aedes aegypti (Diptera: Culicidae) infestation and abundance: inference through statistical modeling and risk maps in northern Argentina

Understanding the processes that affect Aedes aegypti (L.) (Diptera: Culicidae) may serve as a starting point to create and/or improve vector control strategies. For this purpose, we performed statistical modeling of three entomological surveys conducted in Clorinda City, northern Argentina. Previous 'basic' models of presence or absence of larvae and/or pupae (infestation) and the number of pupae in infested containers (productivity), mainly based on physical characteristics of containers, were expanded to include variables selected a priori reflecting water use practices, vector-related context factors, the history of chemical control, and climate. Model selection was performed using Akaike's Information Criterion. In total, 5,431 water-holding containers were inspected and 12,369 Ae. aegypti pupae collected from 963 positive containers. Large tanks were the most productive container type. Variables reflecting every putative process considered, except for history of chemical control, were selected in the best models obtained for infestation and productivity. The associations found were very strong, particularly in the case of infestation. Water use practices and vector-related context factors were the most important ones, as evidenced by their impact on Akaike's Information Criterion scores of the infestation model. Risk maps based on empirical data and model predictions showed a heterogeneous distribution of entomological risk. An integrated vector control strategy is recommended, aiming at community participation for healthier water use practices and targeting large tanks for key elements such as lid status, water addition frequency and water use.

Genetic variability, phylogenetic relationships and gene flow in Triatoma infestans dark morphs from the Argentinean Chaco

The recent discovery of sylvatic populations of Triatoma infestans outside the Andean Valleys of Bolivia prompted an evolutionary question about the putative ancestral area of origin and dispersal of the species, and an epidemiological question regarding the possible role of these sylvatic populations in the recolonization process of insecticide-treated houses. The finding of a population of sylvatic melanic T. infestans (dark morphs) in the Argentinean dry Chaco at 7 km from a peridomestic bug population of typical coloration gave us the opportunity to test both questions simultaneously by employing phylogenetic and population genetic approaches. For this purpose we analyzed sylvatic and peridomestic bugs using sequence-based mitochondrial and nuclear markers (mtCOI and ITS-1) and microsatellites. Sylvatic bugs were confirmed to be T. infestans and not hybrids, and showed high levels of genetic variability and departures from neutral expectations for mtCOI variation. New ITS-1 and mtCOI haplotypes were recorded, as well as haplotypes shared with peridomestic and/or domestic bugs from previous records. The peridomestic population was invariant for ITS-1 and mtCOI, but showed variability for microsatellites and signatures of a population bottleneck, probably due to a limited number of founders. Phylogenetic analyses were consistent with the presence of ancestral haplotypes in sylvatic bugs. According to F-statistics and assignment methods there was a significant differentiation between sylvatic and peridomestic bugs and gene flow was low and asymmetric, with more bugs moving from the peridomicile to the sylvatic environment. These results support the hypothesis of the Chaco region as the area of origin of T. infestans, and a limited role of sylvatic melanic T. infestans in peridomestic infestation in the Argentinean Chaco.

Detection of "Candidatus Rickettsia sp. strain Argentina"and Rickettsia bellii in Amblyomma ticks (Acari: Ixodidae) from Northern Argentina

Ixodid ticks were collected from vegetation and from humans, wild and domestic mammals in a rural area in the semi-arid Argentine Chaco in late spring 2006 to evaluate their potential role as vectors of Spotted Fever Group (SFG) rickettsiae. A total of 233 adult ticks, identified as Amblyomma parvum, Amblyomma tigrinum and Amblyomma pseudoconcolor, was examined for Rickettsia spp. We identified an SFG rickettsia of unknown pathogenicity, "Candidatus Rickettsia sp. strain Argentina", in A. parvum and A. pseudoconcolor by PCR assays targeting gltA, ompA, ompB and 17-kDa outer membrane antigen rickettsial genes. Rickettsia bellii was detected in a host-seeking male of A. tigrinum. Amblyomma parvum is widespread in the study area and is a potential threat to human health.

Patterns of Aedes aegypti (Diptera: Culicidae) infestation and container productivity measured using pupal and Stegomyia indices in northern Argentina

A citywide control program of Aedes aegypti (L.) (Diptera: Culicidae) mainly based on the use of larvicides reduced infestations but failed to achieve the desired target levels in Clorinda, northeastern Argentina, over 5 yr of interventions. To understand the underlying causes of persistent infestations and to develop new control tactics adapted to the local context, we conducted two pupal surveys in a large neighborhood with approximately 2,500 houses and recorded several variables for every container inspected in fall and spring 2007. In total, 4,076 lots and 4,267 containers were inspected over both surveys, and 8,391 Ae. aegypti pupae were collected. Large tanks used for potable water storage were the most abundant and the most productive type of container, accounting for 65-84% of all the pupae collected. Therefore, large tanks were key containers and candidates for improved targeted interventions. Multivariate analysis showed that containers located in the yard, at low sun exposure, unlidded, filled with rain water, and holding polluted water were all more likely to be infested by larvae or pupae. When only infested containers were considered, productivity of pupae was most closely associated with large tanks and rain water. A stochastic simulation model was developed to calculate the expected correlations between pupal and Stegomyia indices according to the characteristics of the distribution of larvae and pupae per container and the spatial scale at which the indices were computed. The correlation between pupal and Stegomyia indices is expected to increase as infestation levels decline.

Molecular population genetics and phylogeography of the Chagas disease vector Triatoma infestans in South America

Knowledge of the genetic variability, population structure, and evolutionary history of Triatoma infestans may be useful for developing rational vector control strategies. A 661-bp fragment of the mitochondrial gene cytochrome oxidase I (COI) was sequenced and analyzed in bugs from Argentina, Uruguay, Peru, and Bolivia, including peridomestic, domestic, Andean, and Chaco sylvatic bugs. A total of 48 polymorphic sites among 37 haplotypes were described. Nucleotide variation fluctuated among samples, with the highest nucleotide diversity observed in seven Argentinean provinces. Within this group, some populations showed patterns of variability compatible with population expansions and/or fine-scale population structure, whereas others suggested population bottlenecks and/or population admixture processes. A maximum parsimony analysis of the haplotypes showed the presence of a Bolivian/Peruvian and an Argentinean/Uruguayan clade. Bolivian sequences were further divided in Chaco sylvatic and Andean domestic and sylvatic. Two different nested clades were found within the Argentinean/Uruguayan cluster. Analysis of molecular variance (AMOVA) and K(ST)* analysis supported a strong population structure in Argentina, where genetic differentiation was correlated with geographic distance. Departures from neutrality expectations and a nested cladistic analysis suggest a recent population expansion of T. infestans in Argentina, followed by restricted gene flow and patterns of isolation by distance. This expansion could have taken place as a two-wave process, as was shown by the phylogenetic analysis and signatures of population admixture in the southern most Argentinean populations.

Environmental and demographic factors determining the spatial distribution of Triatoma guasayana in peridomestic and semi-sylvatic habitats of rural northwestern Argentina

Triatoma guasayana (Wygodzinsky & Abalos), a sylvatic vector of Chagas' disease, occurs in natural and peridomestic habitats of the dry Chaco region of Argentina, Bolivia and Paraguay. Ten-year retrospective spatial analyses of peridomestic T. guasayana abundance in the rural community of Amamá were expanded to the neighbouring community of Trinidad in northwestern Argentina. The distribution of T. guasayana in domiciles, peridomiciles (storerooms, chicken coops and corrals) and natural habitats (bromeliads, dry cacti and logs) around houses (i.e. 'semi-sylvatic' habitats) was analysed. The distribution of the 316 T. guasayana specimens collected in domestic and peridomestic sites during 1993-2002 was significantly clustered in both communities. Searches confirmed that the spatial distribution of semi-sylvatic and peridomestic T. guasayana was determined by the joint effects of the local abundance of goats and the density of semi-sylvatic habitats. The integration of detailed entomological and demographic longitudinal data with geographic information system data, high-resolution satellite imagery, appropriate spatial and temporal analyses and field observations allowed us to infer the underlying processes determining the distribution of T. guasayana in rural communities. This approach may be applied to other sylvatic and peridomestic vectors of Chagas' disease in order to identify high-risk areas for targeted control or environmental management.

Domestic dogs and cats as sources of Trypanosoma cruzi infection in rural northwestern Argentina

The reservoir capacity of domestic cats and dogs for Trypanosoma cruzi infection and the host-feeding patterns of domestic Triatoma infestans were assessed longitudinally in 2 infested rural villages in north-western Argentina. A total of 86 dogs and 38 cats was repeatedly examined for T. cruzi infection by serology and/or xenodiagnosis. The composite prevalence of infection in dogs (60%), but not in cats, increased significantly with age and with the domiciliary density of infected T. infestans. Dogs and cats had similarly high forces of infection, prevalence of infectious hosts (41-42%), and infectiousness to bugs at a wide range of infected bug densities. The infectiousness to bugs of seropositive dogs declined significantly with increasing dog age and was highly aggregated. Individual dog infectiousness to bugs was significantly autocorrelated over time. Domestic T. infestans fed on dogs showed higher infection prevalence (49%) than those fed on cats (39%), humans (38%) or chickens (29%) among 1085 bugs examined. The basic reproduction number of T. cruzi in dogs was at least 8.2. Both cats and dogs are epidemiologically important sources of infection for bugs and householders, dogs nearly 3 times more than cats.

Seasonal variations in active dispersal of natural populations of Triatoma infestans in rural north-western Argentina

The flight dispersal of Triatoma infestans Klug (Hemiptera: Reduviidae) is one of the main mechanisms determining community re-infestation after control interventions. An empirical model of flight initiation coupled with data from a longitudinal study predicted that the flight dispersal of T. infestans would peak in summer. To test this prediction, longitudinal light trap collections were conducted during 3-8 nights in March (late summer), July (winter) and November (spring) 2003, and in March 2004 in a rural community in north-west Argentina. Following each light-trapping collection date, all peridomestic sites around light traps were inspected to assess the relative abundance and nutritional status of T. infestans at each site. A total of 21 adult and five nymph T. infestans, six Triatoma guasayana Wygodzinsky & Abalos, and nine Triatoma garciabesi Carcavallo et al. were collected in 96 light-trapping nights, whereas 696 T. infestans were collected from the peridomestic sites that surrounded the light traps. The arrival of T. infestans in the light traps occurred in 64% of catch stations and peaked in the summer surveys (10-14 bugs) compared with spring and winter surveys. When winds were < 5 km/h, the arrival of adult T. infestans at the light traps was significantly associated with maximum temperature and relative humidity. This is the first field report of seasonal variations in the flight dispersal activity of T. infestans.

Differential detection of Blastocrithidia triatomae and Trypanosoma cruzi by amplification of 24salpha ribosomal RNA genes in faeces of sylvatic triatomine species from rural northwestern Argentina

Flagellates indistinguishable from Trypanosoma cruzi were detected by microscopy in faecal samples of 2/110 Triatoma guasayana and 2/283 Triatoma garciabesi captured in a rural area of northwestern Argentina. Inoculation of faecal homogenates to mice followed by xenodiagnosis, haemoculture, histopathology and culture from cardiac homogenates, and PCR based on T. cruzi minicircle and nuclear sequences failed to detect T. cruzi infection, pointing to another trypanosomatidean. A PCR strategy targeted to the D7 domain of 24salpha ribosomal DNA genes amplified a 250 bp sequence from one T. guasayana and one T. garciabesi faecal lysate. Sequence analysis revealed 100% identity with 24salpha rDNA amplicons from Blastocrithidia triatomae obtained from faeces of reared Triatoma infestans bugs. Phylogenetic analysis clustered this sequence with C. fasciculata and L. major, separated from the Trypanosoma branch (bootstrap: 968/1000), in concordance with a Neighbour-joining dendrogram based on 18s rDNA sequences. This PCR procedure provides a rapid sensitive tool for differential diagnosis of morphologically similar trypanosomatids in field surveys of Chagas disease vectors and laboratory-reared triatomines used for xenodiagnosis.

Long-term reduction of Trypanosoma cruzi infection in sylvatic mammals following deforestation and sustained vector surveillance in northwestern Argentina

Long-term variations in the dynamics and intensity of sylvatic transmission of Trypanosoma cruzi were investigated around eight rural villages in the semiarid Argentine Chaco in 2002-2004 and compared to data collected locally in 1984-1991. Of 501 wild mammals from 13 identified species examined by xenodiagnosis, only 3 (7.9%) of 38 Didelphis albiventris opossums and 1 (1.1%) of 91 Conepatus chinga skunks were infected by T. cruzi. The period prevalence in opossums was four-fold lower in 2002-2004 than in 1984-1991 (32-36%). The infection prevalence of skunks also decreased five-fold from 4.1-5.6% in 1984-1991 to 1.1% in 2002-2004. Infection in opossums increased with age and from summer to spring in both study periods. The force of infection per 100 opossum-months after weaning declined more than six-fold from 8.2 in 1988-1991 to 1.2 in 2002-2004. Opossums were mainly infected by T. cruzi lineage I and secondarily by lineage IId in 1984-1991, and only by T. cruzi I in 2002-2004; skunks were infected by T. cruzi IId in 1984-1991 and by IIc in 2002-2004. The striking decline of T. cruzi infection in opossums and skunks occurred in parallel to community-wide insecticide spraying followed by selective sprays leading to very low densities of infected Triatoma infestans in domestic and peridomestic habitats since 1992; to massive deforestation around one of the villages or selective extraction of older trees, and apparent reductions in opossum abundance jointly with increases in foxes and skunks. These factors may underlie the dramatic decrease of T. cruzi infection in wild reservoir hosts.

PCR-based screening and lineage identification of Trypanosoma cruzi directly from faecal samples of triatomine bugs from northwestern Argentina

This study applied improved DNA extraction and polymerase chain reaction strategies for screening and identification of Trypanosoma cruzi lineages directly from faeces of triatomines collected in a well-defined rural area in northwestern Argentina. Amplification of the variable regions of the kinetoplastid minicircle genome (kDNA-PCR) was performed in faecal lysates from 33 microscope (MO)-positive and 93 MO-negative Triatoma infestans, 2 MO-positive and 38 MO-negative Triatoma guasayana and 2 MO-positive and 73 MO-negative Triatoma garciabesi. kDNA-PCR detected T. cruzi in 91% MO-positive and 7.5% MO-negative T. infestans, which were confirmed by amplification of the minicircle conserved region. In contrast, kDNA-PCR was negative in all faecal samples from the other triatomine species. A panel of PCR-based genomic markers (intergenic region of spliced-leader DNA, 24Salpha and 18S rRNA genes and A10 sequence) was implemented to identify the parasite lineages directly in DNA lysates from faeces and culture isolates from 28 infected specimens. Two were found to be infected with TCI, 24 with TCIIe, 1 with TCIId and 1 revealed a mixed TCI+TCII infection in the faecal sample whose corresponding culture only showed TCII, providing evidence of the advantages of direct typing of biological samples. This study provides an upgrade in the current diagnosis and lineage identification of T. cruzi in field-collected triatomines and shows T. cruziII strains as predominant in the region.

Identification and characterization of microsatellite markers in the Chagas disease vector Triatoma infestans (Heteroptera: Reduviidae)

Triatoma infestans, the main vector of Chagas disease in the southern cone countries, is the principal target of a regional elimination program. A better understanding of its dispersal, sources of reinfestation, and insecticide resistance is key to an effective control program. To address such problems, we identified and characterized 13 microsatellite loci of T. infestans. For each locus, primer sequences and PCR conditions are presented. Allele variability and frequency were analyzed in 59 T. infestans specimens from different rural communities in northwestern Argentina; nine loci were considered suitable for population genetic studies. Departure from Hardy-Weinberg equilibrium was detected in 10/13 loci with F(IS) values ranging from 0.04 to 0.91, indicating heterozygote deficit and a possible grade of sub-structure in the sample analyzed. Presence of null alleles in some loci cannot be discarded. The present work provides a promising tool to develop a population genetic study of natural populations of T. infestans in tandem with field studies and analyses of bug dispersal and the reinfestation process.

Feeding rates, nutritional status and flight dispersal potential of peridomestic populations of Triatomainfestans in rural northwestern Argentina

Triatoma infestans, the main vector of Chagas disease in Southern Cone countries, frequently infests peridomestic structures housing domestic animals. A total of 814 T. infestans collected from 35 different peridomestic sites in rural northwestern Argentina over 1-4 consecutive seasons was examined for recent blood meals and nutritional status. Bugs from goat or pig corrals had lower qualitative nutritional status and mean weight to length ratios (W/L) than those captured in chicken coops. Males systematically had lower qualitative nutritional status and W/L than females. Using logistic multiple regression, the daily feeding rates of T. infestans were significantly associated with season and stage but not ecotope, whereas the proportion of well-fed bugs varied significantly with all three factors. The seasonal trends in feeding rates and nutritional status were consistent with the local availability and breeding timing of domestic animals. The observed data fed into an empirical model predicted that the probability of flight initiation would peak in summer from pig or goat corrals, not chicken coops, and be insignificant in all ecotopes in spring and fall. Male T. infestans outnumbered females as potential fliers. This is the first study conducted in well-defined habitat units that shows significant heterogeneities in the feeding rates and nutritional status of triatomine populations linked to host demographics and management, and how these affect flight dispersal potential over seasons. Peridomestic bug populations are of great relevance as a source of domestic reinfestation and for the elimination of T. infestans.

Seasonal variations of microclimatic conditions in domestic and peridomestic habitats of Triatoma infestans in rural northwest Argentina

Our objectives were to describe and compare the seasonal variations in microclimatic conditions in domestic and peridomestic ecotopes infested by Triatoma infestans, the main vector of Chagas disease, in rural northwestern Argentina. Using data loggers for 4-7 consecutive days in October 2000, February, May and August 2001, microsite (interior of thatched roofs) and ambient temperatures or relative humidity (RH) were recorded simultaneously and compared with an external reference. The damping effects on microsite temperature and ambient RH increased successively from the goat corral to the chicken coop or pig corral, to the storeroom or human habitation. In the interior of the thatched roofs of the habitation and storeroom, average external minima were increased by +5.0 to +5.6 degrees C in all seasons, and average external maxima were decreased by -4.3 to -5.6 degrees C in October-May and -10.6 degrees C in August. On average, the human habitation damped extreme temperatures by 8 degrees C over a wider range than previously reported. The interior of the thatched roofs of the pig corral and chicken coop exerted intermediate damping effects that varied over seasons, whereas the goat corral's thatch showed little or no damping effects at all. The adobe-and-thatch habitation and storeroom presented nearly optimal and more suitable conditions for T. infestans populations than other open structures housing animals. Global climate changes are expected to affect peridomestic populations of T. infestans much more than domestic bug populations, provided other local conditions are held constant. Heterogeneous habitat conditions are expected to affect T. infestans population parameters, dispersal, control and infection with Trypanosoma cruzi.

Effects of partial housing improvement and insecticide spraying on the reinfestation dynamics of Triatoma infestans in rural northwestern Argentina

The long-term effects on domiciliary reinfestation by Triatoma infestans of smoothing the plaster of indoor walls prior to insecticide application (in Amamá village) relative to only insecticide application (in Trinidad-Mercedes villages) were evaluated in rural northwestern Argentina from 1992 to 1997. All domestic and peridomestic areas of each house were sprayed with 2.5% suspension concentrate deltamethrin at 25 mg/m(2) in October 1992, and infestations were assessed by various methods every 6 months. Domiciliary infestation decreased from 72-88% in 1992 to 6-17% in late 1995, to increase moderately thereafter without returning to baseline rates. Peridomestic sites were the first in becoming reinfested, and reached more abundant T. infestans populations than domiciliary areas. Domiciliary infestation rates and bug abundances were not significantly different between communities during surveillance. Domiciliary infestation rates in well-plastered houses were very low (5-9%) and approximately stable until 1996, but in houses with regular or bad plaster they consistently increased from 5 to 19-21% in both communities. Logistic multiple regression analysis showed that the likelihood of domestic infestation assessed through householders' collections was significantly and positively associated with the occurrence of an infested peridomestic site in the respective house, the occurrence of high-density domestic infestations before interventions, and well-plastered walls in 1996. Combining insecticide spraying and partial improvement of walls controlled domestic infestations and transmission of Trypanosoma cruzi effectively, but was not sufficient to eliminate T. infestans from the study area or increase the effectiveness of careful chemical control.

Modeling household transmission of American trypanosomiasis

American trypanosomiasis, or Chagas disease, caused by the protozoan parasite Trypanosoma cruzi and transmitted by blood-feeding triatomine bugs, is a chronic, frequently fatal infection that is common in Latin America. Neither adequate drugs nor a vaccine is available. A mathematical model calibrated to detailed household data from three villages in northwest Argentina shows that householders could greatly reduce the risk of human infection by excluding domestic animals, especially infected dogs, from bedrooms; removing potential refuges for bugs from walls and ceilings; and using domestically applied insecticides. Low-cost, locally practicable environmental management combined with intermittent use of insecticides can sustainably control transmission of T. cruzi to humans in rural Argentina and probably elsewhere.

Comparison between two artificial shelter units and timed manual collections for detecting peridomestic Triatoma infestans (Hemiptera: Reduviidae) in rural northwestern Argentina

A new artificial shelter unit was compared with segments of bamboo cane lined with pleated filter paper for detecting peridomestic Triatoma infestans Klug at Amamá and nearby rural villages in northwestern Argentina. The new shelter unit consisted of a black plastic, wide-mouthed jar with a screw cap on the top, and a removable central structure made of pleated corrugated paper. In devices exposed from February to December 1999 at 24 sites positive for T. infestans by timed manual collections with an irritant in April 1999, the cumulative percentage of sites with any sign of infestation detected by the shelter unit increased from 71% after 2 mo to 96% after 10 mo, whereas bamboo cane units concurrently detected only 12-42% of the sites. Sensitivity increased with time of exposure and the abundance of T. infestans per site. In 19 sites negative for T. infestans by inspection, shelters increasingly detected infestation at 16-63% of sites after 10 mo, whereas the bamboo canes only detected one infestation. Shelter units inspected three times over an 11-mo period were significantly more sensitive than a single manual search with an irritant performed in March 2000. Our study provided conclusive field evidence that the shelter unit was more sensitive for detecting peridomestic T. infestans than were timed manual searches, the standard reference method, or bamboo cane units. Rapid timed searches by skilled bug collectors during the early surveillance phase overlooked many peridomestic populations that, in the absence of control, inevitably would increase in abundance and repopulate treated areas.

Association between nutritional indicators and infectivity of dogs seroreactive for Trypanosoma cruzi in a rural area of northwestern Argentina

The association between the nutritional state of mongrel dogs naturally infected with Trypanosoma cruzi and their infectivity to Triatoma infestans bugs and immune response to Trypanosoma cruzi were studied in the rural village of Amamá, northwestern Argentina. All of the 97 evaluated dogs were classified into one of three categories of external clinical aspect (ECA) based on the degree of muscle development, external evidence of bone structures, state of the hair of the coat, existence of fatty deposits, and facial expression. ECA was significantly associated with two nutritional indicators, hematocrit and skin-fold thickness, but not with total serum proteins. For all dogs, hematocrit was significantly correlated with skin-fold thickness. The 2-year survival probability decreased significantly from 60.7% for dogs with good ECA to 45.9% and 31.2% for those with regular and bad ECA, respectively. The age-adjusted relative odds of infection for Triatoma infestans xeno-diagnosis nymphs that fed once on a dog seroreactive for Trypanosoma cruzi decreased significantly as ECA improved, when tested by multiple logistic regression analysis. A delayed hypersensitivity reaction was observed in all of the seroreactive dogs with good ECA but only in 45-50% of those with regular or bad ECA. Dogs with bad ECA had a 2.6 and 6.3 times greater probability of infecting triatomines after a single full blood meal than dogs with regular or good ECA, respectively. Our study shows that the reservoir competence of dogs for Trypanosoma cruzi was associated with ECA, which is a surrogate and valid index of nutritional state.

A simple method to identify triatomine (Hemiptera: Reduviidae) feces in sensing devices used in vector surveillance programs

We successfully applied the phenolphthalin (Kastle-Meyer) test used in forensic chemistry to distinguish between feces from triatomines and other domestic arthropods in sensing devices used for vector surveillance. All black or dark brown, but not white or yellow, fecal smears from laboratory-reared or field-collected Triatoma infestans Klug, Triatoma guasayana Wydgozinsky & Abalos, Triatoma sordida Ståhl (recently revalidated as Triatoma garciabesi Carcavallo, Cichero, Martínez, Prosen & Ronderos) tested positive, whereas dejecta from cockroaches and spiders, crickets, beetles, predatory bugs, and domestic flies tested negative. Black or dark brown dejecta from female Aedes aegypti L. and Cimex lectularius L. bedbugs also tested positive. In sellsing devices installed in bedrooms of 11 houses in Amamá, rural northwestern Argentina, where neither cimicid bedbugs nor argasid ticks had been found over the years, only 62% of the black or dark brown fecal smears attributed to triatomines by a skilled observer tested phenolphthalin-positive. After insecticidal spraying, when bedroom areas were not colonized by triatomines, only 33-40% of the black or dark brown fecal smears in sensor boxes attributed to triatomines by another skilled observer tested phenolphthalin-positive. Eleven (79%) ofthe 14 houses with dubious or nontypical triatomine feces tested phenolphthalin-positive at least once during 1993-1995. Our study introduces a low-cost, simple and effective procedure for the identification of triatomine feces. The test, as a helpful adjunct to sensing devices used in triatomine surveillance, will aid in the accurate detection of infestations and the determination of the need for insecticide application.

Peridomestic distribution of Triatoma garciabesi and Triatoma guasayana in north-west Argentina

The Reduviid bugs Triatoma garciabesi Carcavallo et al. (Hemiptera: Reduviidae: Triatominae), previously known as T. sordida Ståhl in the semi-arid chaco region, and T. guasayana Wygodzinsky & Abalos, vectors of Trypanosoma cruzi Chagas (Kinetoplastida: Trypanosomatidae), were found to occupy wide but different ranges of ecotopes in the peridomestic environment. At Amamá and nearby rural villages in north-western Argentina, a combined total of 1233 specimens were collected from 325/2314 (14%) sites surveyed at 6-monthly intervals from November 1994 to November 1996. Triatoma garciabesi and T. guasayana displayed a significantly different distribution among peridomestic ecotopes. Triatoma garciabesi predominated over T. guasayana in the prevalence of infested sites, the number of colonies and the number of bugs collected. For T. garciabesi, the predominant ecotopes most likely to yield T. garciabesi repeatedly were the rugged bark of Prosopis alba or P. nigra (Fabaceae) trees, where chickens roosted, and chicken coops. For T. guasayana the main ecotopes were goat or sheep corrals, piled materials and orchard fences. Triatoma garciabesi and T. guasayana were collected concurrently from the same site on 9/2314 (0.4%) occasions, and on different dates at the same site on 12 (0.5%) occasions. The observed low frequency of mixed populations (< 1%) was not significantly different from that expected from a hypothesis of independence. Triatoma garciabesi clearly outnumbered T. guasayana in four of the nine mixed populations, none of which persisted as such. Neither T. garciabesi nor T. guasayana colonized human habitations, even in the absence of T. infestans (formerly the predominant domestic vector of T. cruzi in this area), a situation that apparently has not changed in the last 50 years in northern Argentina.

Congenital transmission of Trypanosoma cruzi: an operational outline for detecting and treating infected infants in north-western Argentina

We designed a set of procedures for first-line local health services to detect and treat the congenital transmission of Trypanosoma cruzi at a province-wide scale, and field-tested the programme in the province of Tucumán, northwestern Argentina, from 1992 to 1994. The programme consists of routine screening of pregnant women for seroreactivity to T. cruzi, serological and parasitological follow-up of the newborn at least twice during the first year of age, treatment of the infected infants, and evaluation of the outcome. 927 (5.5%) of 16 842 pregnant women were seroreactive to T. cruzi by indirect haemagglutination assay and ELISA. Twenty-one (6.7%) of 315 newborns to seroreactive mothers were diagnosed as infected with T. cruzi parasites microhaematocrit concentration before 30 days of age. Five newborns who initially tested negative had a T. cruzi infection detected by microhaematocrit and/or serological techniques at 3 or 6 months of age. Thus, congenital infection was diagnosed in 26 (7.1%) infants born to seroreactive women and residing in houses free of triatomine bugs. Four of 6 infants born to seroreactive mothers died during the first year of age and had some evidence of T. cruzi infection; one of the deaths was attributed to T. cruzi based on clinical evidence. After specific treatment with nifurtimox or benznidazole, 30 of 32 infants remained parasitologically and serologically negative. This study shows the feasibility of controlling the incidence of congenitally acquired T. cruzi infections at a province-wide scale by means of a specific screening programme at first-line health services level.

[Control campaigns against Triatoma infestans in a rural community of northwestern Argentina]

Control campaigns against Triatoma infestans, the principal vector of Trypanosoma cruzi, have relied on the application of residual insecticides and have regarded the system as homogeneous. Reinfestation generally starts in peri-domestic residual foci or in preexisting foci that escaped spraying for diverse reasons. From these foci T. infestans adults actively invade other sites, or they are transported passively within objects or goods from infested communities. In the absence of additional spraying of insecticides after the attack phase, domiciliary reinfestation expanded exponentially to return to pre-spraying levels in 3-4 years in Amamá, Santiago del Estero. However, the rate of recovery of T. infestans abundance was much lower than that predicted by a simple mathematical model or in experimental field studies. Reinfestation did not progress homogeneously within the village. Two longitudinal studies in Santiago del Estero revealed that the presence of T. infestans-infested peri-domestic sites increased the risk of domiciliary reinfestation. "Key" sites where reinfestation started early were goat or sheep corrals, pig corrals, chicken houses, and store-rooms. Peri-domestic reinfestation is likely the result of multiple factors, such as insecticide breakdown by climatic agents, and the greater surface and availability of refuge and hosts in peri-domestic rather than domiciliary sites. Such environmental heterogeneity generates insecticidal effects heterogeneity and increases the probability of persistence of T. infestans even under control pressures. Several studies showed that sustained surveillance after the attack phase decreased sharply the domiciliary colonization by T. infestans and the percentage of bugs infected with T. cruzi, and the local incidence of infected children. The sustained elimination of T. infestans demands a scientific approach that is not only centered in insecticide use but that also includes the environment and house-holders in their specific social and political scenario.

[Control of congenital transmission of Trypanosoma cruzi in Argentina]

The vertical transmission of Trypanosoma cruzi has been augmenting its relative importance as vector and transfusion-mediated transmission routes have been, and continue to be, increasingly controlled. The vertical transmission of T. cruzi cannot be prevented; but early detection and treatment of congenital infection achieve cure rates close to 100%. In Argentina, the Subprogram of Control of Pregnant Women examined 58,196 women from 13 provinces in 1997 and found a 9% seropositivity to T. cruzi. In spite of such high maternal prevalence rates of T. cruzi, only a small proportion of live newborns to infected mothers acquires the infection. The probability of vertical transmission was 1.9% (range: 0.1% to 3.5%) in surveys carried out in the '70s, and 2.5% (range: 0.7% to 10.4%) in others conducted in the '70-'80s. Other more recent studies in Argentina estimated the probability of transmission in 2.6%-6.7%, but studies from Paraguay estimated 10.5% by PCR or serodiagnosis. The microhematocrit technique is the recommended parasitologic method to detect congenital infection. Routine serodiagnosis that detects IgG against T. cruzi is only helpful after the newborn reaches 6 months of age. Detection of specific IgM using recombinant antigens and PCR constitute excellent alternatives, but their feasibility from operational and cost-effective viewpoints in affected endemic areas remains to be considered. In a longitudinal project carried out in Maternidad Nuestra Señora de la Merced in the city of Tucumán between 1992-1994, the majority of congenital cases were asymptomatic. They were diagnosed through the microhematocrit technique, but a number of cases could only be detected later as a result of the parasitological and/or serological follow-up. Of a total of 32 newborns infected with T. cruzi who were treated with nifurtimox or benznidazole, 30 had a negative microhematocrit and serodiagnosis between 6 months and 2 years post-treatment. The magnitude of congenital transmission, and its associated morbidity and mortalidad, largely justify the efforts needed to detect T. cruzi infection in the mothers and newborns. This project demonstrated that the transmission of T. cruzi can be successfully controlled at a provincial scale through a specific program inserted in the primary health care system or at the first level of attention. The congenital transmission of T. cruzi clearly represents a public health problem in areas that in the past were of active transmission, even years after being under entomologic surveillance.

Long-term effects of a nationwide control program on the seropositivity for Trypanosoma cruzi infection in young men from Argentina

Unselected nationwide cohorts of Argentine men 18 years of age summoned for military service were tested for antibodies to Trypanosoma cruzi each year from 1981 to 1993. After an initial screening using indirect hemagglutination test, the positive sera were retested by titrated indirect hemagglutination and immunofluorescence antibody tests at 39 laboratories or at the national reference center in Buenos Aires. Nearly 1.8 million men were examined for T. cruzi antibodies using the same standardized procedures under a quality assurance program. The prevalence of seropositivity for T. cruzi decreased significantly from 5.8% in 1981 to 1.9% in 1993, but the decrease was not homogeneous among provinces within each region or constant over time. Prior to the nationwide control campaign initiated in 1961-1962, 15 provinces had high (> 20%) percentages of houses with domiciliary infestation by Triatoma infestans bugs, which decreased to nine provinces in 1982, and four provinces in 1992. The observed decrease in the prevalence of seropositivity for T. cruzi may be mostly attributed to the spraying with insecticides to eliminate the domiciliary populations of Triatoma infestans. The lack of a sustainable triatomine surveillance program set a limit to the decrease of seropositivity rates and prompted a revised strategy based on community participation.

Small-scale field trial of a sensing device for detecting peridomestic populations of Triatoma infestans (Hemiptera: Reduviidae) in northwestern Argentina

Two prototypes of sensing devices for detecting peridomestic populations of Triatoma infestans Klug were tested in paired trials with bamboo canes in Amamá and nearby rural villages under triatomine surveillance. In infested peridomestic structures housing domestic animals, 1-2 pairs of numbered devices were placed per test site, left for 3-9 nights, and inspected for evidence of infestation. Prototype A was a black plastic cylinder 19 cm high and 10 cm diameter, with a screw cap on the top, 2 openings in the bottom, and a removable central structure made of resistant plastic coated with leather. Prototype B had square leather pieces rolled into cylinders instead of the central structure. Prototype A was significantly more sensitive than the bamboo cane with pleated paper inside in 13 test sites in which 20 pairs were tried. In a smaller series involving 7 pairs, prototype B also detected infestations more frequently than the cane. Triatomine feces were the signs most frequently recorded by both prototypes, whereas the bamboo canes recorded no feces. Ten T. infestans and 1 Triatoma guasayana Wygodzinsky & Avalos were collected from the prototypes placed on the ground or walls, not beneath the thatched roofs of the animal shelters, whereas only 3 T. infestans were collected from the canes. This study describes an effective sensing device for detecting T. infestans populations in outdoor animal shelters and provides quantitative field data on its performance.

Trypanosoma cruzi infection in Triatoma infestans and other triatomines: long-term effects of a control program in rural northwestern Argentina

The prevalence of Trypanosoma cruzi infection in Triatoma infestans, Triatoma guasayana, and Triatoma sordida was evaluated in Amamá and other neighboring rural villages in northwestern Argentina for five years after massive spraying with deltamethrin in 1992 and selective sprays thereafter. Local residents and expert staff collected triatomines in domiciliary and peridomestic sites. During 1993-1997, the prevalence of T. cruzi was 2.4% in 664 T. infestans, 0.7% in 268 T. guasayana, and 0.2% in 832 T. sordida. T. cruzi infection was more frequently detected in adult bugs and in triatomines collected at domiciliary sites. The infected T. guasayana and T. sordida were nymphs and adults, respectively, captured at peridomestic sites. The prevalence of T. cruzi infection in T. infestans decreased from 7.7% to 1.5% during the surveillance period, although that change was not statistically significant. Comparison of T. infestans infection rates before the control program and during surveillance showed a highly significant decrease from 49% to 4.6% in bedrooms, as well as a fall from 6% to 1.8% in peridomestic sites. Because of its infection with T. cruzi and frequent invasion of domiciliary areas and attacks on humans and dogs, T. guasayana appeared implicated as a putative secondary vector of T. cruzi in domestic and peridomestic sites during the surveillance period. T. sordida was the most abundant species, but it was strongly associated with chickens and showed little tendency to invade bedrooms.

Effects of non-susceptible hosts on the infection with Trypanosoma cruzi of the vector Triatoma infestans: an experimental model

We tested experimentally the effects of the presence of non-susceptible hosts on the infection with Trypanosoma cruzi of the vector Triatoma infestans. The experiment consisted in two treatments: with chickens, including two chickens (non-susceptible hosts) and two infected guinea pigs (susceptible hosts), and without chickens, including only two infected guinea pigs. The hosts were held unrestrained in individual metal cages inside a closed tulle chamber. A total of 200 uninfected T. infestans third instar nymphs were liberated in each replica, collected on day 14, and examined for infection and blood meal sources on day 32-36. The additional presence of chickens relative to infected guinea pigs: (a) significantly modified the spatial distribution of bugs; (b) increased significantly the likelihoods of having a detectable blood meal on any host and molting to the next instar; (c) did not affect the bugs' probability of death by predation; and (d) decreased significantly the overall percentage of T. infestans infected with T. cruzi. The bugs collected from inside or close to the guinea pigs' cages showed a higher infection rate (71-88%) than those collected from the chickens' cages (22-32%). Mixed blood meals on chickens and guinea pigs were detected in 12-21% of bugs. Although the presence of chickens would decrease the overall percentage of infected bugs in short term experiments, the high rate of host change of T. infestans would make this difference fade out if longer exposure times had been provided.

Monitoring house reinfestation by vectors of Chagas disease: a comparative trial of detection methods during a four-year follow-up

Domestic reinfestations by triatomine bugs were monitored after application of deltamethrin and apparent elimination of Triatoma infestans in Amama and other nearby rural villages, north-west Argentina, from 1992 to 1996. The five methods used were sensor boxes, sheets of pink typing-paper, timed manual catches by a skilled three-person team aided by a flushing-out agent, collections by house-dwellers, and knockdown using insecticide fumigant canisters. In bedrooms, house-dwellers collected T. infestans significantly more frequently than the flushing-out method, but the reverse occurred in peridomestic sites. Both methods and sensor boxes revealed the frequent invasion of adult Triatoma guasayana and T. infestans, but neither T. guasayana nor Triatoma sordida colonized bedroom areas in spite of their rising abundance in nearby peridomestic sites. Sensor boxes were significantly more sensitive than the matched paper-sheets in three of five cross-sectional surveys. On average, each box recorded 2.0-3.2 times more triatomine fecal smears than each paper sheet. The frequency of dejecta in sensor boxes correlated positively with the proportion of houses where T. infestans, T. guasayana or T. sordida were captured by any method in bedroom areas. Triatomine fecal smears in sensor boxes were the earliest and most frequent sign of domiciliary infestation, followed by dwellers' collections of adult bugs. Analyzing the data prospectively, we provide a quantitative, predictive understanding of detection methods and review the validity and interpretation of the different signs of infestation obtained. The most sensitive and cost-effective combination of detection methods for vector surveillance in domestic areas was the use of sensor boxes and house-dwellers collections.

Household prevalence of seropositivity for Trypanosoma cruzi in three rural villages in northwest Argentina: environmental, demographic, and entomologic associations

Environmental, demographic, and entomologic variables were analyzed by logistic multiple regression analysis for their association with the likelihood of being seropositive for Trypanosoma cruzi in three highly infested rural villages of northwest Argentina. The prevalence of seropositivity for T. cruzi, as determined by the composite results of three serologic tests, was 34% among 338 persons in 1992. The strongest positive predictors of the adjusted odds of being infected were the household number of dogs, the density of T. cruzi-infected Triatoma infestans in bedroom areas, and each person's age. Dwellers from houses with roofs made completely or partly with a grass called simbol, or which used insecticides rudimentarily and nonsystematically, had a significantly lower odds of being seropositive for T. cruzi than residents from other types of dwellings. The adjusted odds of infection also increased with the number of T. cruzi-infected dogs or cats and the presence of chickens in bedroom areas. No significant effects on the adjusted odds of infection of a community-wide deltamethrin spraying carried out in one of the villages seven years before were detected. Socioeconomic indicators, such as domiciliary area, and numbers of corrals and livestock, were inversely related to being infected. Our study identified several manageable variables suitable for control actions, most of them not examined before in univariate or multivariate analyses. Environmental management based on low-cost housing with appropriate local materials and removal of domestic animals from domiciliary areas have a crucial role to play in the control of Chagas' disease in rural areas.

Evaluation of dogs as sentinels of the transmission of Trypanosoma cruzi in a rural area of north-western Argentina

Dogs, the main domestic reservoirs of Trypanosoma cruzi in the Argentine chaco, may be useful as sentinels of vector-mediated transmission of Trypanosoma cruzi in control programmes if canine infections acquired by all other routes could be excluded. In the present study, in the province of Santiago del Estero, Argentina, the dog populations in the neighbouring villages of Amamá, Trinidad, Mercedes, Villa Matilde and Pampa Pozo were studied immediately before spraying of residual insecticides (in 1992 in Amamá, Trinidad and Mercedes and 1993-1994 in the other villages) and twice during a subsequent programme of triatomine surveillance (in 1994 and 1996). Overall seropositivity for Trypanosoma cruzi infection (i.e. the proportion of dogs found positive by at least two different serological tests in any one survey) steadily declined from 65% (54/83) in 1992 to 39% (70/182) in 1994 and 15% (36/237) in 1996. No seroconversion was detected between the 1994 and 1996 surveys in any dog aged > 2 years. However, among the dogs aged < 2 years (which were born after virtual elimination of Triatoma infestans from the villages), seropositivity fell from 15% (12/81) in 1994 to 5% (6/131) in 1996. Thirteen demographic, behavioural and entomological variables were analysed by logistic multiple-regression analysis for their association with seropositivity in dogs born locally during the surveillance programme. The total number of Triatoma guasayana collected in the bedroom areas of the owner's house, having a mother which was seropositive for Trypanosoma cruzi, and the number of seropositive dogs with which it cohabited were all found to be significant predictors of seropositivity in any dog. The results are the first indication that Triatoma guasayana is the vector responsible for the new cases of Trypanosoma cruzi infection seen in dogs in domestic or peridomestic sites during the surveillance programme.

Immunodiagnosis of Trypanosoma cruzi (Chagas' disease) infection in naturally infected dogs

This study reports on the standardization of an enzyme-linked immunosorbent assay (ELISA) for detecting specific antibodies anti-Trypanosoma cruzi in naturally infected dogs. Sera from 182 mongrel dogs of all ages residing in four rural villages in Santiago del Estero, Argentina, were collected in November 1994 and preserved in buffered neutral glycerin. All sera were tested by indirect hemagglutination test (IHAT), indirect immunofluorescence test (IFAT), and ELISA using the flagellar fraction of T. cruzi as antigen. Dog sera from an area without vectorial transmission were used to calculate ELISA specificity and cut-off value. Eighty-six percent of sera had concordant results for all tests. All sera reactive for IHAT and IFAT were also reactive for ELISA, except in one case. Sera tested by ELISA when diluted 1:200 allowed a clearer division between non-reactive and reactive sera than when 1:100 with greater agreement among serologic techniques. The specificity of ELISA was 96.2%. Among 34 adult dogs with a positive xenodiagnosis, sensitivity was 94% both for ELISA and IFAT. ELISA is the first choice for screening purposes and one of the pair of techniques recommended for diagnostic studies in dog populations.

Factors limiting the domestic density of Triatoma infestans in north-west Argentina: a longitudinal study

Reported are the environmental and demographic risk factors associated with the domestic infestation and density of Triatoma infestans in three heavily infested rural villages in Santiago del Estero Province, Argentina. In a one-factor unadjusted analysis, the number of T. infestans captured per person-hour was associated significantly and negatively with the use of domestic insecticides by householders, type of thatch used in the roofs and the age of the house; and positively with the following: degree of cracking of the indoor walls and presence of hens nesting indoors. In one model, using multiple linear regression and a backward stepwise elimination procedure, most of the variation in the overall abundance of T. infestans was explained by insecticide use and the presence of hens nesting indoors; in another model using the same procedure it was explained by insecticide use, bug density in 1988 and previous spraying with deltamethrin in 1985. Variations in bug density per capture stratum (household goods, beds, walls and roof) were explained by the bug density in other strata and by one or two of the following risk factors: hens nesting indoors, type of roof, presence of cracks in the walls and number of people living in the house. Bug density might be locally controlled by the availability of refuges in the roofs and walls, by the presence of hens nesting indoors and by the use of domestic insecticides. Certain local materials, such as a grass known as simbol, could be successfully used in rural housing improvement programmes aimed at reducing the availability of refuges for insects in the roof.

Effects of chickens on the prevalence of infestation and population density of Triatoma infestans in rural houses of north-west Argentina

Effects of the presence of chickens on population density of the bug Triatoma infestans, principal vector of Chagas disease, were investigated by standardized sampling (indoors and peridomestically) from sixty-eight houses of three rural communities in north-west Argentina, during March 1992. The domiciliary density of T. infestans increased linearly with the percentage of bugs that fed on chickens, as identified by agar double-diffusion tests. Bug density was significantly higher in houses where hens usually nested indoors than in those where they did not, as determined by concurrent direct observations and interviews of householders. Multiple linear regression analysis of domiciliary bug density on (a) the total number of people, dogs and cats per house; (b) the percentage of domiciliary bugs that fed on chickens, or (c) the indoor-brooding habit of hens, showed the two variables (b,c) related to chickens as significant predictors in each regression model. Inclusion both variables representing chickens increased the fit significantly. Addition of other potentially confounding factors (domestic insecticide use, type of roof and walls of house) did not affect the significant variables retained in the best-fitting regression model. Peridomestic infestation was positively associated with the household number of fowls. Chickens were the main bloodmeal source of peridomestic T. infestans populations. Human-fed bugs were detected in peridomestic sites of sixteen houses, indicating active dispersal of adults and large nymphal instars from bedroom areas. Exclusion of hens from domiciliary areas and promotion of chicken sheds, of an appropriate design that would not harbour bug populations, should limit the triatomine population growth rate and reduce the risk of infestation.

The role of the peridomiciliary area in the elimination of Triatoma infestans from rural Argentine communities

The purpose of this study was to identify the origin of Triatoma infestans reinfestation and study its dynamics following spraying with deltamethrin inside and around 94 houses in three rural communities in northwestern Argentina. The effectiveness of the spraying was evaluated immediately after the houses were sprayed and two months later. In addition, five residual peridomiciliary foci were found and sprayed, as well as three preexisting ones that had not been sprayed. To monitor reinfestation, biosensors were placed in the houses and each family was also asked to capture triatomines and keep them in plastic bags; in addition, triatomines were searched for in and around houses, using an aerosol that dislodged them from their hiding places. Selective sprayings were carried out only where a colony of T. infestans was found. During the 30 months of follow-up, the percentage of houses in which any T. infestans were captured varied between 3% and 9%. In six houses, T. infestans were captured during more than one evaluation. The number of peridomiciliary areas found to be infested (19) was double the number of infested houses (9). Colonies of T. infestans were found only in the peridomiciliary areas, where the number of T. infestans captured was six times higher than in the houses. Chickens were the host most frequently associated with peridomiciliary foci. This area was the origin and principal source of reinfestation. To reduce the speed of reinfestation and the frequency with which sprayings are needed, the following environmental and chemical control methods must be combined in the peridomiciliary area: reduce the number of hiding places of triatomines; restrict the raising of birds to structures that cannot be colonized by triatomines; apply an insecticide that is less likely to be degraded by exposure to the elements, or perform a second spraying 6 to 12 months after the first; and employ a device for early detection of the presence of T. infestans around houses.

Density estimates of the domestic vector of Chagas disease, Rhodnius prolixus Stål (Hemiptera: Reduviidae), in rural houses in Venezuela

We reported the use of the timed manual method, routinely employed as an indicator to the relative abundance of domestic triatomine bugs, to estimate their absolute density in houses. A team of six people collected Rhodnius prolixus Stål bugs from the walls and roofs of 14 typical palm-leaf rural houses located in Cojedes, Venezuela, spending 40 minutes searching in each house. One day after these manual collections, all the houses were demolished and the number of triatomine bugs were identified by instar and counted. Linear regression analyses of the number of R. prolixus collected over 4 man-hours and the census counts obtained by house demolition indicated that the fit of the data by instar (stage II--adult) and place of capture (roof versus palm walls versus mud walls) was satisfactory. The slopes of the regressions were interpreted as a measure of "catchability" (probability of capture). Catchability increased with developmental stage (ranging from 11.2% in stage II to 38.7% in adults), probably reflecting the increasing size and visibility of bugs as they evolved. The catchability on palm wall was higher than that for roofs or mud walls, increasing form 1.3% and 3.0% in stage II to 13.4% and 14.0% in adults, respectively. We reported, also, regression equations for converting field estimates of timed manual collections of R. prolixus into absolute density estimates.

Detecting domestic vectors of Chagas disease: a comparative trial of six methods in north-west Argentina

Six methods for detecting domestic infestations by triatomine bugs were compared in the rural community of Amamá, north-west Argentina. An average of three pairs (range, 2-5 pairs) of sensor boxes and sheets of pink typing-paper were tacked to the walls of human sleeping areas in 45 houses for 30 days and then inspected by a two-man team. Triatoma infestans bugs were collected in bedrooms by a different two-man team aided by a flushing-out agent both before and after application of sensing devices. Finally, knockdown collections of bugs after application of one insecticide fumigant canister per bedroom were also made. The proportion of houses with evidence of current domestic bug infestations that were detected by the various methods were as follows: sensor boxes (95.3%), reports of householders (88.4%), knockdown (87.8%), paper-sheets (86.0%), and flushing-out (69.8-76.7%). The detectability of infestations, irrespective of the method used, increased with the density of the bugs. At low or intermediate bug densities, individual sensor boxes were more sensitive than their matched paper-sheets, but at any bug density there were no significant differences between the pooled results for all the boxes and for all the paper-sheets in the house. On average, each sensor box recorded 2.25 times more triatomine faecal smears than its matched paper-sheet, and this relation increased with the density of bugs in the house. Both sensing devices were effective at monitoring unsuccessful attempts of peridomestic triatomine populations to colonize houses.

Chagas disease in north-west Argentina: risk of domestic reinfestation by Triatoma infestans after a single community-wide application of deltamethrin

The risk of domestic reinfestation by Triatoma infestans after a single community-wide application of delta-methrin (2.5% suspension concentrate at 25 mg active ingredient/m2) was studied in Amamá, north-west Argentina, where no insecticide spraying had been done by official control services. The percentage of infested houses fell from 88% before spraying in 1985 to nil during the 6 months after spraying, and thereafter increased from 5% in 1986 to 96% before a second treatment in 1992, fitting closely to a logistic model (r2 = 0.997). Significant risk factors associated with domestic reinfestation determined from stepwise logistic regression and one-factor analysis were the density of T. infestans in bedrooms just before spraying and the surface structure of indoor walls. Peak densities of bugs in 1988-1989 significantly differed between levels of both risk factors. Our study suggests the existence of stable determinants of infestation linked to the household which, in the absence of effective control measures, would also determine the speed of house recolonization and the ensuing bug densities. Plastering of mud walls before application of insecticides to all domestic and peridomestic structures is supported by the study.

Comparison of two sampling methods for domestic populations of Triatoma infestans in north-west Argentina

In mud-and-thatch houses of Santiago del Estero Province, north-west Argentina, where no insecticides had been sprayed officially to control domestic infestations of the Chagas disease vector Triatoma infestans (Klug) (Hemiptera: Reduviidae), two knockdown (KD) insecticidal collection procedures were compared with the standard flush-out (FO) method for sampling T. infestans. Bugs were collected by FO using 0.2% tetramethrin in bedrooms of (1) 41 houses of Amamá village employing 1 man-hour of capture effort per house, and (2) 19 houses of Trinidad and Mercedes villages employing 4 man-hours/house. From the same houses, 2-5 days after the manual FO collection, bugs were collected by KD indoor-spraying of deltamethrin 25 mg a.i./m2 in Amamá, or burning of one gamma-HCH (= gamma-BHC) fumigant tablet 3.1 g of gamma-isomer) per bedroom in Trinidad and Mercedes. The majority of infestations were detected by both methods, the proportion of positive houses being 81% at Amamá and 95% at the other villages. Although the FO method was more sensitive, at least because it was applied first, the KD method detected infestations in 25% of houses where bugs were not found by FO. Bug densities estimated by FO or by subsequent KD in each house were significantly correlated: r = 0.795 for deltamethrin; r = 0.882 for gamma-HCH. Compared with FO collections of T. infestans large stages, i.e. adults plus fourth and fifth instar nymphs, the KD catch averaged 0.88 x with deltamethrin and 0.57 x with gamma-HCH, regardless of the apparent population density of bugs per house. However, the KD method has practical advantages of speed and standardization.

Chagas disease in north-west Argentina: association between Trypanosoma cruzi parasitaemia in dogs and cats and infection rates in domestic Triatoma infestans

The association between Trypanosoma cruzi parasitaemia in dogs and cats and Tryp. cruzi infection rates in domestic Triatoma infestans was studied in a cross-sectional survey of 31 houses (89%) in the rural villages of Trinidad and Mercedes, north-west Argentina, where no spraying of insecticides had ever been done. Similar prevalence rates of parasitaemia, determined by xenodiagnosis, were recorded among 68 dogs (41.2%) and 28 cats (39.3%). Bug infection rates were significantly associated with the presence of infected cats (those with positive xenodiagnosis) stratified by the number of infected dogs (relative risk = RR = 1.90; 95% confidence interval = CI = 1.51-2.38), and with the number of infected dogs stratified by the presence of infected cats (RR = 2.71; CI = 1.81-4.07). The percentage of infected bugs in houses with and without children stratified by the presence of infected dogs or cats was not significantly different (RR = 0.69; CI = 0.45-1.05). The combined effect of infected dogs and infected cats on bug infection rates fitted closely with an additive transmission model. Bug infection rates were significantly higher when infected dogs shared the sleeping areas of people than when they did not (RR = 1.79; CI = 1.1-2.91). Our study showed that infected dogs and infected cats increase the risk of domestic transmission of Tryp. cruzi to T. infestans.

Dispersive flight and house invasion by Triatoma guasayana and Triatoma sordida in Argentina

Flight activity and invasion of houses by Triatoma sordida and T. guasayana were studied in the Province of Santiago del Estero, Argentina. Spontaneous findings of both species in houses were recorded from 1982 to 1989. Light trap collections were performed in 1982, 1983 and 1984, at the woods surrounding the settlements of Amamá (43 houses) and Trinidad (19 houses). Most of the 101 triatomines collected, were unfed and negative for Trypanosoma cruzi. T. guasayana predominated over T. sordida, and both appeared on the lighted screens between 19-31 min (mean 24) after dusk and the catch time was 30-45 min. Although entomological evaluation of 41 houses at Amamá performed in September 1985, just before insecticidal spraying, showed that Triatoma infestans predominated, adults of T. guasayana were collected in sleeping places, in 7 houses (17%). Most triatomines invading houses from then up to 1990 were flying T. guasayana (20/27) and females outnumbered males. Three non-infected T. guasayana females were fed on man and two T. guasayana males positive for "T. cruzi like" trypanosomes were unfed. Therefore, visiting hungry adults could transmit T. cruzi to people and introduce wild parasites to the domestic cycle. T. guasayana stands as the main potential substitute of T. infestans in the studied area, and it might play there the same role as T. sordida in Brazil.

Infectivity to the vector Triatoma infestans of dogs infected with Trypanosoma cruzi in north-west Argentina

The influence of reinfections on the infectivity to Triatoma infestans of dogs seropositive to Trypanosoma cruzi was investigated in Amamá, north-west Argentina, before (1984) and after (1986-1987) elimination of domestic bugs with residual insecticides in 1985. Infectivity was quantified by the percentage of infected bugs in each xenodiagnosis of a seropositive dog, i.e. the infective force (IF). Exposure to reinfection was represented by the domiciliary density of infected bugs and by an index of the potentially-infective contacts (IPIC) experienced by each dog (calculated as the product of the relative density of bugs per dog, the proportion of infected bugs, and the proportion of bugs taking blood meals from the dog). IF was unrelated to the dog's age or sex, or the level of specific antibodies to T. cruzi. Before vector elimination, the IF of seropositive dogs was significantly associated with both the density of infected bugs and the IPIC. After vector elimination, seropositive dogs resident at houses which had 10-70 infected bugs before spraying showed a higher median IF (67-75%) than those living in houses with zero to eight infected bugs before spraying (47-55%). The age-specific IF of seropositive dogs aged one year or older, recorded before (45-64%) and after (52-56%) the elimination of domestic bugs, did not differ significantly. Our data suggest that the infectivity of seropositive dogs to bugs is modified by previous or current exposure to bug-mediated reinfections, and that this relationship might be irreversible in the short term.