Epidemiology and population dynamics of Ascaris lumbricoides and Trichuris trichiura infection in the same community

Intestinal parasitic infections in children from marginalised Roma communities: prevalence and risk factors

BACKGROUND

Intestinal parasitic infections remain a significant global health issue, particularly affecting poor and marginalised populations. These infections significantly contribute to children's diseases, malnutrition, poor school performance, cognitive disorders, and future economic losses. This study aimed to explore and compare the occurrence of intestinal parasites in early childhood among the group of infants from the Slovak majority population and from marginalised Roma communities (MRCs). Furthermore, it aimed to explore the health complaints of children with and without intestinal parasitic infection in the past month and assess the effect of various risk factors on the occurrence of intestinal parasitic infection in infants from MRCs.

METHODS

We obtained cross-sectional data from mothers and stool samples of their children aged 13-21 months using the first wave of the longitudinal RomaREACH study. A total of 181 stools from infants were analysed: 105 infants from the Slovak majority population and 76 from MRCs.

RESULTS

Infants from MRCs are significantly more often infected by Ascaris lumbricoides, Trichuris trichiura and Giardia duodenalis than their better-off peers from the majority population. Infection rates are 30% in infants from MRCs vs. 0% in the majority population (p < 0.001). Single and mixed infections were observed in children from MRCs. Infants with intestinal parasitic infections suffer significantly more often from various health complaints, particularly cough, stomach ache, irritability, and diarrhoea. Within MRCs, the risk of parasitic infections in infants is significantly increased by risk factors such as the absence of flushing toilets in households (OR = 4.17, p < 0.05) and contact with un-dewormed animals (OR = 3.61, p < 0.05). Together with the absence of running water in the household, these three factors combined increase the risk more than ten times (p < 0.01).

CONCLUSION

Maintaining hygienic standards in conditions of socioeconomic deprivation in MRCs without running water and sewage in the presence of un-dewormed animals is problematic. These living conditions contribute to the higher prevalence of parasitic infections in children from MRCs, causing various health complaints and thus threatening their health and healthy development.

The adaptive immune response to Trichuris in wild versus laboratory mice: An established model system in context

Laboratory model organisms have provided a window into how the immune system functions. An increasing body of evidence, however, suggests that the immune responses of naive laboratory animals may differ substantially to those of their wild counterparts. Past exposure, environmental challenges and physiological condition may all impact on immune responsiveness. Chronic infections of soil-transmitted helminths, which we define as establishment of adult, fecund worms, impose significant health burdens on humans, livestock and wildlife, with limited treatment success. In laboratory mice, Th1 versus Th2 immune polarisation is the major determinant of helminth infection outcome. Here we compared antigen-specific immune responses to the soil-transmitted whipworm Trichuris muris between controlled laboratory and wild free-ranging populations of house mice (Mus musculus domesticus). Wild mice harbouring chronic, low-level infections produced lower levels of cytokines in response to Trichuris antigen than laboratory-housed C57BL/6 mice. Wild mouse effector/memory CD4+ T cell phenotype reflected the antigen-specific cytokine response across the Th1/Th2 spectrum. Increasing egg shedding was associated with body condition loss. However, local Trichuris-specific Th1/Th2 balance was positively associated with worm burden only in older wild mice. Thus, although the fundamental relationships between the CD4+ T helper cell response and resistance to T. muris infection are similar in both laboratory and wild M. m. domesticus, there are quantitative differences and age-specific effects that are analogous to human immune responses. These context-dependent immune responses demonstrate the fundamental importance of understanding the differences between model and natural systems for translating mechanistic models to 'real world' immune function.

Epidemiology of soil-transmitted helminth infections and the differential effect of treatment on the distribution of helminth species in rural areas of Gabon

BACKGROUND

Soil-transmitted helminth (STH) infections are a public health concern in endemic areas. For efficient control, the epidemiology of the disease needs to be monitored. This report assesses the prevalence, incidence, post-treatment infection (PTI) rate, and risk factors for STH infections in two rural areas of Gabon.

METHOD

In this longitudinal and prospective study, participants aged six to 30 years from the vicinity of Lambaréné and selected households using a simple randomization process were included and followed in two consecutive periods of six and nine months. Stool samples were obtained at the beginning and the end of each follow-up phase (FUP). The Kato-Katz technique was used for the detection of STH eggs, while the Harada-Mori technique and coproculture were used for the detection of larvae in stool processed within a maximum of four hours of collection. Prevalence was determined at the three main time points of the study, incidence was assessed during the two study phases, and PTI was defined as an infection detected nine months post-treatment.

RESULTS

A total of 262 participants were included. The overall prevalence of STH infections was 42% (95%CI: 34-50) and 44% (95%CI: 37-51) at baseline for the six and nine month FUPs, respectively. Trichuris trichiura was the most prevalent species at each time point of assessment. The cumulative incidence of STH at the 6- and 9-month follow-ups was 18% (95%CI: 12-27) and 35% (95%CI: 27-43), respectively, while the incidence rates were 41 (95%CI: 28-55) and 56 (95%CI: 46-67) per 100 person-years, respectively. The PTI rates at the 9-month follow-up for T. trichiura, hookworm, and Ascaris lumbricoides were 58% (95%CI: 41-74), 31% (95%CI: 11-59) and 18% (95%CI: 5-40), respectively. The STH infection intensity was generally light.

CONCLUSION

The prevalence level of STH infection is moderate in the vicinity of Lambaréné, with T. trichiura being the most prevalent species. Our results reveal a rapid spread of the disease in the population mainly following intervention, particularly for trichuriasis, and therefore call for the full implementation of the World Health Organization's recommendations in the area. Trial registration clinicaltrials.gov Identifier NCT02769013. Registered 21 April 2016, https://clinicaltrials.gov/study/NCT02769013.

Baseline soil-transmitted helminth and schistosome infection in the Geshiyaro project, Ethiopia: A unique transmission interruption project using biometric fingerprinting for longitudinal individual analysis

BACKGROUND

The Geshiyaro project aims to assess the feasibility of interrupting transmission of soil-transmitted helminths (STH) and schistosome (SCH) infection in the Wolaita zone of southern Ethiopia through high coverage community-wide mass drug administration (MDA), in combination with improved water, sanitation, and hygiene services and behaviour change communication delivered through the existing health care infrastructure. To accurately measure treatment coverage a population census was conducted enrolling individuals with biometric fingerprinting and barcoded ID cards. This paper details the baseline census and parasitology surveys conducted before the start of any interventions.

METHODS

The census was conducted in five of the 15 Wolaita districts between October 2018 and December 2019, enrolling all consenting participants from every household. Simultaneously, a cross-sectional parasitology survey was conducted in 130 out of 361 randomly selected communities from all 15 districts, with 100 individuals across all age groups (infant to adult) per community providing stool and urine for analysis by duplicate Kato-Katz and a point-of-care circulating cathodic antigen (POC-CCA) to test for Schistosoma mansoni and STH, and microhaematuria and urine filtration for Schistosoma haematobium. Of the 130 communities, 30 were randomly selected for annual, longitudinal parasitological monitoring, with 150 randomly selected individuals from infant to adult providing two days of stool and urine samples for analysis by the same diagnostic tests per community.

RESULTS

In total 97,919 households participated in the baseline census enrolling 466,071 individuals, with parasitological data obtained from 10,785 people. At baseline, 15.5% were infected with at least one STH species, with Ascaris lumbricoides (9.5%), followed by hookworm (7.2%) and Trichuris trichiura (1.8%). Substantial heterogeneity in STH prevalence was observed between communities ranging from 0% to 61% where most infections were low intensity. Schistosoma mansoni infection was the dominant schistosome infection (0.85% by Kato-Katz and 13.3% by POC-CCA trace negative and 21.5% trace positive), with few Schistosoma haematobium infections identified (2.77% haematuria positive and 0.13% positive by urine filtration).

CONCLUSIONS

While the national control program in Ethiopia has made good progress in reducing prevalence of STH and SCH in Wolaita since it was launched in 2015, there remain areas of persistent infection suggesting the existence of environmental or behavioural risk factors that contribute to ongoing transmission. This project aims to identify the most efficient intervention strategies to reduce community burden and reach interruption of transmission.

Un-‘Egg’-Plored: Characterisation of Embryonation in the Whipworm Model Organism Trichuris muris

Trichuris muris, is the murine parasite and widely deployed model for the human whipworm Trichuris trichiura, a parasite that infects around 500 million people globally. Trichuriasis is a classical disease of poverty with a cycle of re-infection due to the continual exposure of humans, particularly children, to infective eggs, which contaminate the soil in endemic areas. Indeed, modelling studies of trichuriasis have demonstrated that the low efficacy rate of current anthelmintics combined with the high possibility of re-infection from the reservoir of infective eggs within the environment, mean that the elimination of morbidity due to trichuriasis is unlikely to occur. Despite the importance of the infective egg stage in the perpetuation of infections, understanding the biology of the Trichuris ova has been neglected for decades. Here we perform experiments to assess the impact of temperature on the embryonation process of T. muris eggs and describe in detail the stages of larval development within these eggs. In keeping with the early works performed in the early 1900s, we show that the embryonation of T. muris is accelerated by an elevation in temperature, up to 37°C above which eggs do not fully develop and become degenerate. We extend these data to provide a detailed description of T. muris egg development with clear images depicting the various stages of development. To the best of our knowledge we have, for the first time, described the presence of birefringent granules within egg-stage larvae, as well as providing a qualitative and quantitative description of a motile larval stage prior to quiescence within the egg. These experiments are the first step towards a better understanding of the basic biology which underlies the process of egg embryonation. With the threat of elevation in global temperatures, the accelerated embryonation rate we observe at higher temperatures may have important consequences for parasite transmission rates and prospective modelling studies. In addition, a deeper understanding of the Trichuris ova may allow the development of novel control strategies targeting the egg stage of Trichuris in the environment as an adjunct to MDA.

Trichuris muris as a tool for holistic discovery research: from translational research to environmental bio-tagging

Trichuris spp. (whipworms) are intestinal nematode parasites which cause chronic infections associated with significant morbidities. Trichuris muris in a mouse is the most well studied of the whipworms and research on this species has been approached from a number of different disciplines. Research on T. muris in a laboratory mouse has provided vital insights into the host–parasite interaction through analyses of the immune responses to infection, identifying factors underpinning host susceptibility and resistance. Laboratory studies have also informed strategies for disease control through anthelmintics and vaccine research. On the contrary, research on naturally occurring infections with Trichuris spp. allows the analysis of the host–parasite co-evolutionary relationships and parasite genetic diversity. Furthermore, ecological studies utilizing Trichuris have aided our knowledge of the intricate relationships amongst parasite, host and environment. More recently, studies in wild and semi-wild settings have combined the strengths of the model organism of the house mouse with the complexities of context-dependent physiological responses to infection. This review celebrates the extraordinarily broad range of beneficiaries of whipworm research, from immunologists and parasitologists, through epidemiologists, ecologists and evolutionary biologists to the veterinary and medical communities.

Gastrointestinal Nematodes among Residents in Melong, Moungo Division, Littoral Region, Cameroon

BACKGROUND

Intestinal parasitic infections are among the most common infections worldwide. The present study was undertaken to provide baseline information on the status of gastrointestinal nematodes in Melong Subdivision, Moungo Division, Littoral Region, Cameroon. Material and Methods. Seven hundred and eighty-eight stool samples were collected in randomly selected quarters in the community of Melong. These stool samples were brought to the Laboratory of Applied Biology and Ecology in the University of Dschang for analysis using the qualitative (simple flotation) and quantitative (Mc Master count) technique.

RESULTS

The nematodes identified were Ascaris lumbricoides, hookworm, Trichuris trichiura, and Capillaria hepatica with respective prevalences and intensities of infection of 2.2% and 3691.12 ± 3898.47, 1.4% and 940.91 ± 1825.90, 1.0% and 193.75 ± 227.47, and 0.4%and 50 ± 00. The data on the prevalence of nematodes with respect to sex and age showed that females (6.0%) were more infected than males (2.76%) with no significant difference (P > 0.05). Furthermore, with respect to age, adults were more infected than children. The percentage of educational level showed a reduction in the number of parasites in the higher educational level. The prevalence of A. lumbricoides between localities showed a significant difference (P < 0.05) with "Quarter 1" harboring most of the nematodes. Cases of double (Ascaris lumbricoides + Trichuris trichiura) and triple (Ascaris lumbricoides + Trichuris trichiura + hookworm) parasitism were encountered with both having a prevalence of 0.3%. According to the fecal concentration of eggs, 63.89% of the infections were light, 5.56% moderate, and 30.56% heavy.

CONCLUSION

A relatively low overall prevalence was obtained in our study, showing that the national deworming campaign is proving effective, but more effort is needed to completely eradicate these parasites for a single infected individual can cause havoc.

Epidemiological insights from a large-scale investigation of intestinal helminths in Medieval Europe

Helminth infections are among the World Health Organization's top neglected diseases with significant impact in many Less Economically Developed Countries. Despite no longer being endemic in Europe, the widespread presence of helminth eggs in archaeological deposits indicates that helminths represented a considerable burden in past European populations. Prevalence of infection is a key epidemiological feature that would influence the elimination of endemic intestinal helminths, for example, low prevalence rates may have made it easier to eliminate these infections in Europe without the use of modern anthelminthic drugs. To determine historical prevalence rates we analysed 589 grave samples from 7 European sites dated between 680 and 1700 CE, identifying two soil transmitted nematodes (Ascaris spp. and Trichuris trichiura) at all locations, and two food derived cestodes (Diphyllobothrium latum and Taenia spp.) at 4 sites. The rates of nematode infection in the medieval populations (1.5 to 25.6% for T. trichiura; 9.3-42.9% for Ascaris spp.) were comparable to those reported within modern endemically infected populations. There was some evidence of higher levels of nematode infection in younger individuals but not at all sites. The genetic diversity of T. trichiura ITS-1 in single graves was variable but much lower than with communal medieval latrine deposits. The prevalence of food derived cestodes was much lower (1.0-9.9%) than the prevalence of nematodes. Interestingly, sites that contained Taenia spp. eggs also contained D. latum which may reflect local culinary practices. These data demonstrate the importance of helminth infections in Medieval Europe and provide a baseline for studies on the epidemiology of infection in historical and modern contexts. Since the prevalence of medieval STH infections mirror those in modern endemic countries the factors affecting STH decline in Europe may also inform modern intervention campaigns.

Whipworm and roundworm infections

Trichuriasis and ascariasis are neglected tropical diseases caused by the gastrointestinal dwelling nematodes Trichuris trichiura (a whipworm) and Ascaris lumbricoides (a roundworm), respectively. Both parasites are staggeringly prevalent, particularly in tropical and subtropical areas, and are associated with substantial morbidity. Infection is initiated by ingestion of infective eggs, which hatch in the intestine. Thereafter, T. trichiura larvae moult within intestinal epithelial cells, with adult worms embedded in a partially intracellular niche in the large intestine, whereas A. lumbricoides larvae penetrate the gut mucosa and migrate through the liver and lungs before returning to the lumen of the small intestine, where adult worms dwell. Both species elicit type 2 anti-parasite immunity. Diagnosis is typically based on clinical presentation (gastrointestinal symptoms and inflammation) and the detection of eggs or parasite DNA in the faeces. Prevention and treatment strategies rely on periodic mass drug administration (generally with albendazole or mebendazole) to at-risk populations and improvements in water, sanitation and hygiene. The effectiveness of drug treatment is very high for A. lumbricoides infections, whereas cure rates for T. trichiura infections are low. Novel anthelminthic drugs are needed, together with vaccine development and tools for diagnosis and assessment of parasite control in the field.

Can chemotherapy alone eliminate the transmission of soil transmitted helminths?

BACKGROUND

Amongst the world's poorest populations, availability of anthelmintic treatments for the control of soil transmitted helminths (STH) by mass or targeted chemotherapy has increased dramatically in recent years. However, the design of community based treatment programmes to achieve the greatest impact on transmission is still open to debate. Questions include: who should be treated, how often should they be treated, how long should treatment be continued for?

METHODS

Simulation and analysis of a dynamic transmission model and novel data analyses suggest refinements of the World Health Organization guidelines for the community based treatment of STH.

RESULTS

This analysis shows that treatment levels and frequency must be much higher, and the breadth of coverage across age classes broader than is typically the current practice, if transmission is to be interrupted by mass chemotherapy alone.

CONCLUSIONS

When planning interventions to reduce transmission, rather than purely to reduce morbidity, current school-based interventions are unlikely to be enough to achieve the desired results.

The coverage and frequency of mass drug administration required to eliminate persistent transmission of soil-transmitted helminths

A combination of methods, including mathematical model construction, demographic plus epidemiological data analysis and parameter estimation, are used to examine whether mass drug administration (MDA) alone can eliminate the transmission of soil-transmitted helminths (STHs). Numerical analyses suggest that in all but low transmission settings (as defined by the magnitude of the basic reproductive number, R₀), the treatment of pre-school-aged children (pre-SAC) and school-aged children (SAC) is unlikely to drive transmission to a level where the parasites cannot persist. High levels of coverage (defined as the fraction of an age group effectively treated) are required in pre-SAC, SAC and adults, if MDA is to drive the parasite below the breakpoint under which transmission is eliminated. Long-term solutions to controlling helminth infections lie in concomitantly improving the quality of the water supply, sanitation and hygiene (WASH). MDA, however, is a very cost-effective tool in long-term control given that most drugs are donated free by the pharmaceutical industry for poor regions of the world. WASH interventions, by lowering the basic reproductive number, can facilitate the ability of MDA to interrupt transmission.

How effective is school-based deworming for the community-wide control of soil-transmitted helminths?

Background

The London Declaration on neglected tropical diseases was based in part on a new World Health Organization roadmap to “sustain, expand and extend drug access programmes to ensure the necessary supply of drugs and other interventions to help control by 2020”. Large drug donations from the pharmaceutical industry form the backbone to this aim, especially for soil-transmitted helminths (STHs) raising the question of how best to use these resources. Deworming for STHs is often targeted at school children because they are at greatest risk of morbidity and because it is remarkably cost-effective. However, the impact of school-based deworming on transmission in the wider community remains unclear.

Methods

We first estimate the proportion of parasites targeted by school-based deworming using demography, school enrolment, and data from a small number of example settings where age-specific intensity of infection (either worms or eggs) has been measured for all ages. We also use transmission models to investigate the potential impact of this coverage on transmission for different mixing scenarios.

Principal Findings

In the example settings <30% of the population are 5 to <15 years old. Combining this demography with the infection age-intensity profile we estimate that in one setting school children output as little as 15% of hookworm eggs, whereas in another setting they harbour up to 50% of Ascaris lumbricoides worms (the highest proportion of parasites for our examples). In addition, it is estimated that from 40–70% of these children are enrolled at school.

Conclusions

These estimates suggest that, whilst school-based programmes have many important benefits, the proportion of infective stages targeted by school-based deworming may be limited, particularly where hookworm predominates. We discuss the consequences for transmission for a range of scenarios, including when infective stages deposited by children are more likely to contribute to transmission than those from adults.

Large donations of drugs to treat soil-transmitted helminths (STHs, intestinal worms) means that many more school-aged children will be treated, improving their well-being and development. These children will have to be repeatedly treated since reinfection will occur due to contaminated environments in the absence of improvements in hygiene and sanitation. Repeated treatment of school-aged children may have the added benefit of reductions in levels of infection for the whole community. This will in part be determined by the proportion of the total worms harboured or eggs output by school-aged children, a product of how heavily infected school-aged children are and how many school-aged children there are in the community. In one setting school-aged children output as little as 15% of hookworm eggs whereas in another setting they harbour up to 50% of roundworms. Thus, whilst school-based programmes may have important health benefits, the community-level impact on transmission could be limited unless school-aged children over-contribute to infection. We use mathematical models to show that if children contribute more infective stages to the environment which adults are exposed to than adults do, the reductions in transmission resulting from treating children will be larger, but may still be limited.

Regional, household and individual factors that influence soil transmitted helminth reinfection dynamics in preschool children from rural indigenous Panamá

Background

Few studies have investigated the relative influence of individual susceptibility versus household exposure factors versus regional clustering of infection on soil transmitted helminth (STH) transmission. The present study examined reinfection dynamics and spatial clustering of Ascaris lumbricoides, Trichuris trichiura and hookworm in an extremely impoverished indigenous setting in rural Panamá over a 16 month period that included two treatment and reinfection cycles in preschool children.

Methodology/Principle Findings

Spatial cluster analyses were used to identify high prevalence clusters for each nematode. Multivariate models were then used (1) to identify factors that differentiated households within and outside the cluster, and (2) to examine the relative contribution of regional (presence in a high prevalence cluster), household (household density, asset-based household wealth, household crowding, maternal education) and individual (age, sex, pre-treatment eggs per gram (epg) feces, height-for-age, latrine use) factors on preschool child reinfection epgs for each STH. High prevalence spatial clusters were detected for Trichuris and hookworm but not for Ascaris. These clusters were characterized by low household density and low household wealth indices (HWI). Reinfection epg of both hookworm and Ascaris was positively associated with pre-treatment epg and was higher in stunted children. Additional individual (latrine use) as well as household variables (HWI, maternal education) entered the reinfection models for Ascaris but not for hookworm.

Conclusions/Significance

Even within the context of extreme poverty in this remote rural setting, the distinct transmission patterns for hookworm, Trichuris and Ascaris highlight the need for multi-pronged intervention strategies. In addition to poverty reduction, improved sanitation and attention to chronic malnutrition will be key to reducing Ascaris and hookworm transmission.

Control of soil transmitted helminth (STH) infections is of central importance to improving preschool child health because these infections can have long lasting consequences on growth and development. Our study in indigenous Ngäbe preschool children in western Panama was conducted over a period of 16 months. We monitored reinfection dynamics of three STH infections (Ascaris, Trichuris and hookworm) over two reinfection cycles to gain an understanding of regional, household and individual factors that influenced transmission of these infections among preschool children. Despite the rural setting, where virtually all households live under conditions of extreme poverty, we identified spatial clusters of high prevalence of Trichuris and hookworm in the most remote and poorest area, whereas Ascaris was present throughout the study area. Preschool children who were chronically malnourished (low height-for-age) had a higher reinfection burden of Ascaris and hookworm. Household poverty (low relative household wealth and maternal education) and infrequent latrine use were also influential in Ascaris reinfection. This cross-disciplinary analysis of preschool child STH transmission in a poor rural setting provides pertinent information for STH control programs that aim to break the cycle of poverty and infection.

Soil-transmitted helminth reinfection after drug treatment: a systematic review and meta-analysis

Background

Soil-transmitted helminth (STH) infections (i.e., Ascaris lumbricoides, hookworm, and Trichuris trichiura) affect more than a billion people. Preventive chemotherapy (i.e., repeated administration of anthelmintic drugs to at-risk populations), is the mainstay of control. This strategy, however, does not prevent reinfection. We performed a systematic review and meta-analysis to assess patterns and dynamics of STH reinfection after drug treatment.

Methodology

We systematically searched PubMed, ISI Web of Science, EMBASE, Cochrane Database of Systematic Reviews, China National Knowledge Infrastructure, WanFang Database, Chinese Scientific Journal Database, and Google Scholar. Information on study year, country, sample size, age of participants, diagnostic method, drug administration strategy, prevalence and intensity of infection pre- and posttreatment, cure and egg reduction rate, evaluation period posttreatment, and adherence was extracted. Pooled risk ratios from random-effects models were used to assess the risk of STH reinfection after treatment. Our protocol is available on PROSPERO, registration number: CRD42011001678.

Principal Findings

From 154 studies identified, 51 were included and 24 provided STH infection rates pre- and posttreatment, whereas 42 reported determinants of predisposition to reinfection. At 3, 6, and 12 months posttreatment, A. lumbricoides prevalence reached 26% (95% confidence interval (CI): 16–43%), 68% (95% CI: 60–76%) and 94% (95% CI: 88–100%) of pretreatment levels, respectively. For T. trichiura, respective reinfection prevalence were 36% (95% CI: 28–47%), 67% (95% CI: 42–100%), and 82% (95% CI: 62–100%), and for hookworm, 30% (95% CI: 26–34%), 55% (95% CI: 34–87%), and 57% (95% CI: 49–67%). Prevalence and intensity of reinfection were positively correlated with pretreatment infection status.

Conclusion

STH reinfections occur rapidly after treatment, particularly for A. lumbricoides and T. trichiura. Hence, there is a need for frequent anthelmintic drug administrations to maximize the benefit of preventive chemotherapy. Integrated control approaches emphasizing health education and environmental sanitation are needed to interrupt transmission of STH.

Infections with soil-transmitted helminths (the roundworm Ascaris lumbricoides, the whipworm Trichuris trichiura, and hookworm) affect over 1 billion people, particularly rural communities in the developing world. The global strategy to control soil-transmitted helminth infections is ‘preventive chemotherapy’, which means large-scale administration of anthelmintic drugs to at-risk populations. However, because reinfection occurs after treatment, ‘preventive chemotherapy’ must be repeated regularly. Our systematic review and meta-analysis found that at 3, 6, and 12 months after treatment, A. lumbricoides prevalence reached 26% (95% confidence interval (CI): 16–43%), 68% (95% CI: 60–76%) and 94% (95% CI: 88–100%) of pretreatment levels, respectively. For T. trichiura, respective reinfection prevalence at these time points were 36% (95% CI: 28–47%), 67% (95% CI: 42–100%), and 82% (95% CI: 62–100%); and for hookworm, 30% (95% CI: 26–34%), 55% (95% CI: 34–87%), and 57% (95% CI: 49–67%). Prevalence and intensity of reinfection were positively correlated with pretreatment infection status. Our results suggest a frequent anthelmintic drug administration to maximize the benefit of preventive chemotherapy. Moreover, an integrated control strategy, consisting of preventive chemotherapy combined with health education and environmental sanitation is needed to interrupt transmission of soil-transmitted helminths.

Individual predisposition, household clustering and risk factors for human infection with Ascaris lumbricoides: new epidemiological insights

Background

Much of our current understanding of the epidemiology of Ascaris lumbricoides infections in humans has been acquired by analyzing worm count data. These data are collected by treating infected individuals with anthelmintics so that worms are expelled intact from the gastrointestinal tract. Analysis of such data established that individuals are predisposed to infection with few or many worms and members of the same household tend to harbor similar numbers of worms. These effects, known respectively as individual predisposition and household clustering, are considered characteristic of the epidemiology of ascariasis. The mechanisms behind these phenomena, however, remain unclear. In particular, the impact of heterogeneous individual exposures to infectious stages has not been thoroughly explored.

Methodology/Principal Findings

Bayesian methods were used to fit a three-level hierarchical statistical model to A. lumbricoides worm counts derived from a three-round chemo-expulsion study carried out in Dhaka, Bangladesh. The effects of individual predisposition, household clustering and household covariates of the numbers of worms per host (worm burden) were considered simultaneously. Individual predisposition was found to be of limited epidemiological significance once household clustering had been accounted for. The degree of intra-household variability among worm burdens was found to be reduced by approximately 58% when household covariates were included in the model. Covariates relating to decreased affluence and quality of housing construction were associated with a statistically significant increase in worm burden.

Conclusions/Significance

Heterogeneities in the exposure of individuals to infectious eggs have an important role in the epidemiology of A. lumbricoides infection. The household covariates identified as being associated with worm burden provide valuable insights into the source of these heterogeneities although above all emphasize and reiterate that infection with A. lumbricoides is inextricably associated with acute poverty.

Numerous analyses have found that people infected with roundworm (Ascaris lumbricoides) are predisposed to harbor either many or few worms. Members of the same household also tend to harbor similar numbers of worms. These phenomena are called individual predisposition and household clustering respectively. In this article, we use Bayesian methods to fit a statistical model to worm count data collected from a cohort of participants at baseline and after two rounds of re-infection following curative treatment. We show that individual predisposition is extremely weak once the clustering effect of the household has been accounted for. This suggests that predisposition is of limited importance to the epidemiology of roundworm infection. Further, we show that over half of the variability in average worm counts among households is explained by household risk factors. This implies that exposures to infectious roundworm eggs shared by household members are important determinants of household clustering. We argue that these results support the hypothesis proposed in the literature that the household is a key focus of roundworm transmission.

Multiple micronutrients including iron are not more effective than iron alone for improving hemoglobin and iron status of Malian school children

Iron deficiency and anemia remain among the most important global public health problems facing school children. Helminth infections often peak at school age and aggravate nutritional risks. We conducted a 12-wk randomized controlled trial in 406 Malian anemic schoolchildren infected with Schistosoma hematobium to examine the effects of 2 doses of praziquantel (P) (40 mg/kg body weight), P + 60 mg/d iron (Fe), and/or a multiple micronutrient supplement (MM) that included 18 mg/d Fe. Supplements were administered to the children each school day (5 d/wk) throughout the study. Changes in hemoglobin (Hb), serum ferritin (SF), and serum transferrin receptors (s-TfR) were followed. We also examined interactions between Fe and MM supplements on Hb and SF concentrations and malaria incidence. The effects of Fe on Hb and SF concentrations were greater than the effects of P alone and MM with or without added Fe at 6 and 12 wk (P < 0.001). In all groups, s-TfR decreased at 6 and 12 wk compared with baseline. The decrease was most pronounced in the P + Fe group compared with the other 3 groups at wk 6 (P = 0.05). Fe and MM interacted negatively at wk 6 and 12 to affect Hb (beta = -0.43, 95% CI = -0.77, -0.09; P = 0.01 and beta = -0.47, 95% CI = -0.83, -0.11; P = 0.01, respectively) and SF (beta = -0.42, 95% CI = -25.60, 12.31; P < 0.001, and beta = -0.37, 95% CI = -0.63, -0.12; P = 0.004, respectively). Malaria incidence was higher in the groups treated with added Fe (relative risk: 1.66; 95% CI: 0.75, 3.67). In this context, MM with added iron were not more effective than Fe without MM. Fe supplementation of schoolchildren with 60 mg/d for anemia control should be considered carefully.

Pulsed transmission of Pseudodiplorchis americanus (Monogenea) between desert hosts (Scaphiopus couchii)

Transmission of the monogenean Pseudodiplorchis americanus is restricted to the brief period when its host, the desert toad Scaphiopus couchii enters temporary water to breed. This study followed parasite recruitment into one host population in Arizona, USA, during the spawning season (July). Torrential rainfall triggered 3 successive assemblies. Amongst males, which may enter each assembly, the 3 pulses of invasion led to an approximate doubling of mean worm burdens at each exposure, culminating in 100% prevalence and a mean intensity of over 100 larvae/host. Females, which generally enter only one assembly, acquired a mean of about 40 larvae/host. Each exposure is limited to a maximum of 7 h by strictly nocturnal host activity, and the force of infection increased exponentially: around 10% of recruitment occurred in the first 3 h, and more than 30% in the last 1 h. Correlation of recruitment into males with parasite reproductive potential suggests that individual oncomiracidia had a 30% chance of invasion. Although invasion of the desert host relies on an aquatic infective stage, host behaviour ensures that targets are concentrated in space and time, and the parasite's annual reproductive output is synchronized with a period of host vulnerability totalling less than 24 h/year.

High latrine coverage is not reducing the prevalence of soil-transmitted helminthiasis in Hoa Binh province, Vietnam

A baseline epidemiological survey for parasite infections was conducted between December 2007 and January 2008 in 155 villagers in a rural commune in Hoa Binh province, Vietnam. The prevalence of Ascaris lumbricoides, Trichuris trichiura and hookworm infection was 13.5%, 45.2% and 58.1%, respectively. At least one of the parasites was detected in 72.3% of the samples. We found no association between infection with A. lumbricoides or T. trichiura and engagement in agriculture, while hookworm infection was more prevalent in populations having frequent contact with soil. Agricultural use of human faeces was not correlated with any of the infections. We suggest that the consumption of vegetables that are commonly fertilized with human faeces in the community has led to the high infection rates with A. lumbricoides and T. trichiura, rather than the manipulation of faeces in farming activity. This also explains the high infection prevalence, despite high latrine coverage (98.1%) in the study population. The presence of latrines alone is not sufficient to reduce the prevalence of helminthiasis in a rural agricultural community if fresh faeces are used as fertilizer.

Helminth infections among people using wastewater and human excreta in peri-urban agriculture and aquaculture in Hanoi, Vietnam

OBJECTIVE

To assess the prevalence of helminth infections and their associated risks in a community using both wastewater and human excreta in agriculture and aquaculture.

METHODS

A cross-sectional study was conducted in a peri-urban area in Hanoi, Vietnam. Data on the demography, socioeconomics and sanitation were collected from a survey of 400 agricultural households. Parasitological examination for the eggs of Ascaris sp., Trichuris sp. and hookworm was performed on single stool specimens obtained from study household members' 15-70 years and 0-72 months of age.

RESULTS

Of 807 stool samples collected from 620 adults and 187 children, 39% were infected with helminths. The prevalence of infections with Ascaris sp., Trichuris sp. and hookworm was 21.6%, 9.8% and 21.8%, respectively. Univariate and multivariate analyses showed that being an adult, female gender, living in a household without a latrine, excreta composted for less than 1 month and use of fresh human excreta were significantly associated with co-infection with all three helminths. Being an adult was an independent determinant for infections with individual helminths. The absence of a latrine and use of stored urine for irrigation were associated with an increased risk of Ascaris infection. Risk factors for Trichuris infection were inadequately composted excreta and year-round wastewater contact; risk factors for hookworm infection were female gender, household without a latrine and use of fresh human excreta.

CONCLUSION

Wastewater exposure did not pose a major risk for helminth infection in this community. Instead, lack of sanitation facilities and use of fresh or inadequately composted human excreta in agriculture were important risk factors.

Multiple infection diagnosis of intestinal helminthiasis in the assessment of health and environmental effect of development projects in Nigeria

Patterns of intestinal helminth infections among school-aged children have been assessed in Eko-ende and Ore, as part of an overall assessment of the public health impact of Erinle Dam Reservoir in Osun State, Nigeria. The investigation was carried out between January and May 2005 using the Kato Katz thick smear technique and simple questionnaire for information on the bio-data, knowledge, attitude and practice of individuals towards disease transmission and control.Ascaris lumbricoides,hookworm,Trichuris trichiura, Taeniaspp.,Strongyloides stercoralisandSchistosoma mansoniwere recovered at an overall prevalence of 78.3% among 309 children examined.Ascaris lumbricoides,hookworm,T. trichiuraandS. mansoniwere the most common infections whileS. stercoralisandTaeniaspp. were found only among a few children. Infection patterns of the common diseases were age-specific with peaks in the 11–15 age bracket. Children not attending school were significantly (P < 0.05) more at risk of infection than those attending school. Multiple infections were pronounced with over 54% double infections and 6% four parasites or more infections. The need for urgent intervention to arrest the obviously serious public health situation attributable to Erinle Dam is emphasized.

Human gastrointestinal nematode infections: are new control methods required?

Gastrointestinal (GI) nematode infections affect 50% of the human population worldwide, and cause great morbidity as well as hundreds of thousands of deaths. Despite modern medical practices, the proportion of the population infected with GI nematodes is not falling. This is due to a number of factors, the most important being the lack of good healthcare, sanitation and health education in many developing countries. A relatively new problem is the development of resistance to the small number of drugs available to treat GI nematode infections. Here we review the most important parasitic GI nematodes and the methods available to control them. In addition, we discuss the current status of new anthelmintic treatments, particularly the plant cysteine proteinases from various sources of latex-bearing plants and fruits.

Contrasting patterns in the small-scale heterogeneity of human helminth infections in urban and rural environments in Brazil

Marked heterogeneity exists in the patterns of parasitic infection between individuals, households and communities. Analysis of parasite distributions within populations is complicated by the fact that parasite distributions are highly aggregated and few studies have explicitly incorporated this distribution when investigating small-scale spatial heterogeneities. This study aimed to quantify the small-scale (within- and between-household) heterogeneity of helminth infection in an area of Minas Gerais State, Brazil, with rural and urban sectors. Parasitological data from a cross-sectional survey of 1,249 individuals aged 0-86 years from 242 households were analysed. Within-household clustering of infection was assessed using random effect logistic regression models and between-household spatial heterogeneity was assessed using a Bayesian negative binomial spatial model. The overall prevalence of hookworm (Necator americanus) was 66.9%, the prevalence of Schistosoma mansoni was 44.9% and the prevalence of Ascaris lumbricoides was 48.8%. Statistical analysis indicated significant (within) household and (between household) spatial clustering of hookworm in both rural and urban areas and of S. mansoni in rural areas. There was no evidence of either household or spatial clustering of S. mansoni in urban areas. The spatial correlation of S. mansoni was estimated to reduce by half over a distance of 700 m in the rural area. Rural hookworm had a much smaller half-distance (28 m) and urban hookworm showed an even smaller half-distance (12 m). We suggest that such species-specific differences in patterns of infection by environment are primarily due to variation in exposure and parasite life cycle, although host genetic factors cannot be ruled out.

Parasite risk factors for stunting in grade 5 students in a community of extreme poverty in Peru

Malnutrition in school-age children is common in developing countries and includes both stunting and underweight. Stunting, which represents a chronic state of nutritional stress, leads to adverse health, educational and cognitive effects. Although much research is focused on preschool-age children, recent studies show both the high prevalence of stunting and the effectiveness of interventions in school-age children. The objectives of the current study were to determine the risk factors for stunting only, and stunting and underweight. A survey was conducted in 1074 grade 5 children (mean age 10 years) from 17 schools in Belen, Peru, a community of extreme poverty. Prevalence of underweight and stunting were 10.5 and 34.5%, respectively, co-prevalence was 9.3%. Based on multivariable logistic regression analyses, significant independent risk factors (odds ratio: OR) for stunting and underweight were: age (per 1 year increment) (OR=1.55; 95% confidence interval (CI): 1.33, 1.81); diarrhoea in the last week (OR=1.96; 95% CI: 1.17, 3.29) and hookworm infection (OR=1.74; 95% CI: 1.05, 2.86). Significant independent risk factors for stunting only were: age (per 1 year increment) (OR=1.51; 95% CI: 1.35, 1.70); anaemia (OR=1.98; 95% CI: 1.26, 3.11); and moderate and heavy Trichuris and Ascaris co-infection (OR=1.95; 95% CI: 1.35, 2.82). Our results indicate a high prevalence of stunting, in addition to other adverse health indicators, in the study population. Due to the interrelation between many of these health and nutrition problems, interventions at both the school and community levels, including de-worming, feeding programs and health and hygiene education, are needed to reduce malnutrition in this and other similar populations living in conditions of extreme poverty.

AGE-DEPENDENT EPIDEMIOLOGICAL PATTERNS AND STRAIN DIVERSITY IN HELMINTH PARASITES

Field studies of schistosomes and the major intestinal nematodes Trichuris trichiura and Ascaris lumbricoides repeatedly demonstrate that the intensity and prevalence of infection exhibit marked dependency on host age. Peak levels of infection typically occur in hosts aged between 10 and 14 yr in endemically infected communities. It has widely been assumed that the slow acquisition of resistance in adults is caused by repeated exposure to the same antigenic repertoire of a single parasite strain. Consequently, these empirical patterns have previously been taken to suggest that human immunity to helminth parasites confers poor protection against reinfection. Here, an alternative explanation is suggested on the basis of results from a simplified model of helminth transmission. It is proposed that the empirical observations can be attributed to the circulation of multiple helminth strains that each elicit highly protective immunity. If this hypothesis is correct, estimates of epidemiological parameters from field data and the potential for control of helminth diseases might require reevaluation.

Impact of antihelminthic treatment on infection by Ascaris lumbricoides, Trichuris trichiura and hookworms in Covas, a rural community of Pernambuco, Brazil

This work aims to evaluate the impact of drug treatment on infection by Ascaris lumbricoides (Al), Trichuris trichiura (Tt) and hookworms (Hook) in a rural community from the sugar-cane zone of Pernambuco, Brazil. Four parasitological surveys were carried out from March 2001 to March 2002. Individual diagnosis was based on eight slides (four by the Kato-Katz method and four by the Hoffman method) per survey. Infected subjects were assigned to two groups for treatment with either albendazole (n = 62) or mebendazole (n = 57). Prevalence of infection fell significantly (p < 0.05) one month after treatment: Al (from 47.7% to 6.6%); Tt (from 45.7% to 31.8%) and Hook (from 47.7% to 24.5%). One year after treatment, infections by Tt and Hook remained significantly below pre-control levels. A substantial decrease in single-infection cases and multiple infections was found. Egg-negative rate was significant for Al (94.0%), Hook (68.3%) but not for Tt (45.5%), and did not differ significantly between subjects treated with mebendazole or albendazole. Egg counts fell significantly in the individuals remaining positive for Tt. It is recommended that antihelminthic treatment should be selective and given at yearly intervals preferably with albendazole, due to its cost-effectiveness.

Intestinal parasites and commensals among individuals from a landless camping in the rural area of Uberlândia, Minas Gerais, Brazil

We evaluated the occurrence of intestinal parasites and commensals among children and adults from a landless camping in the rural area of Uberlândia, State of Minas Gerais, Brazil, from October to November 2001. Stool samples from 78 individuals were examined by both the Baermann-Moraes and Lutz methods. Fifty-one (65.4%; CI 54.8 - 76.0) individuals were found to be infected, 23 (45.1%) children and 28 (54.9%) adults, of whom 34 (66.7%) were mono-infected, 9 (17.6%) bi-infected, and 8 (15.7%) poly-infected. In conclusion, the high prevalence of intestinal parasites and commensals suggests that parasitological exams should be periodically carried out in addition to the sanitation education and health special care in this population.

The role of health education and sanitation in the control of helminth infections

Health education and sanitation are two important components of primary health care system introduced by the World Health Organization (WHO) as a basis for the prevention and control of communicable diseases. However, the roles of health education and sanitation in disease control have been controversial, especially in the wake of recent advances in safe and effective oral drugs. This article has reviewed the various health education and sanitation interventions around the world to determine what roles they have played in the past relative to other intervention strategies and the role they have to play in future control efforts. It appears clear-cut from the review, that while chemotherapy has been and will remain the best option for morbidity control, sanitation has an important role to play not only to sustain the benefits of chemotherapy but also to protect the uninfected. Health education that is effective, simple and low-cost remains the only tool for creating the enabling environment for both chemotherapy and sanitation to thrive.

Prevalence and intensity of infections of Ascaris lumbricoides and Trichuris trichiura and associated socio-demographic variables in four rural Honduran communities

Between January and March 1998, a cross-sectional survey was carried out in four rural communities in Honduras, Central America. We examined the prevalence and intensity of Ascaris lumbricoides and Trichuris trichiura infections among 240 fecal specimens, and the association between selected socio-demographic variables and infection for 62 households. The overall prevalence of A. lumbricoides and T. trichiura was 45% (95% CI 39.0-51.9) and 38% (95% CI 31.8-44.4) respectively. The most intense infections for Ascaris and Trichuris were found in children aged 2-12 years old. By univariate analysis variables associated with infections of A. lumbricoides were: number of children 2-5 years old (p=0.001), level of formal education of respondents (p=0.01), reported site of defecation of children in households (p=0.02), households with children who had a recent history of diarrhea (p=0.002), and the location of households (p=0.03). Variables associated with both A. lumbricoides and T. trichiura infection included: number of children 6-14 years old (p=0.01, p=0.04, respectively), ownership of a latrine (p=0.04, p=0.03, respectively) and coinfection with either helminth (p=0.001, p=0.001, respectively). By multivariate analysis the number of children 2-5 years living in the household, (p=0.01, odds ratio (OR)=22.2), children with a recent history of diarrhea (p=0.0, OR=39.8), and infection of household members with T. trichiura (p=0.02, OR=16.0) were associated with A. lumbricoides infection. The number of children 6-14 years old in the household was associated with both A. lumbricoides and T. trichiura infection (p=0.04, p=0.01, OR=19.2, OR=5.2, respectively).

Parasitic infection with Trichuris trichiura influences plasma levels of soluble HLA class I

High levels of sHLA-I (soluble HLA--class I) have been correlated with rejection episodes in solid organ transplant recipients and with graft versus host disease in bone marrow recipients. Studies of human infection with parasitic worms of the gut have suggested that certain individuals may be genetically predisposed to intense infection. In this study, the influence of parasitic helminth infection on levels of sHLA-I in plasma was investigated in 155 HLA typed individuals from St. Lucia, exposed to the gut parasite Trichuris trichiura. The results confirmed previous findings showing increased levels of sHLA-I in HLA-A9, and in this case HLA-A23 positive individuals. However, HLA-A9 positive individuals with high worm burden had significantly lower levels of sHLA-I in their plasma compared with HLA-A9 positive subjects with low worm burden. These results suggest that the intensity of T. trichiura infection influences the ability of HLA-A9 positive subjects to maintain high levels of sHLA-I.

Epidemiology of hookworm infection in Itagua, Paraguay: a cross sectional study

A cross-sectional study in Itagua, Paraguay tested 192 people for the presence, intensity and species of hookworm infection. Fifty-nine percent of these individuals were found to be infected. Intensity of infection was determined on 92% of infected individuals by quantitative egg counts. The high intensity hookworm infections, which cause the greatest morbidity, were clustered between the ages of five and 14 years. No differences were seen between genders. The species of hookworm was determined for parasites reared from 72% of infected individuals. Both Necator americanus and Ancylostoma duodenale were identified, although the former species predominated. We conclude that hookworm infection continues to be a public health problem in Paraguay, particularly among children and adolescents who suffer from high intensity infections. A. duodenale continues to persist in the Western Hemisphere and has not been completely displaced by N. americanus.

The distribution of Ascaris lumbricoides in human hosts: a study of 1765 people in Bangladesh

The Ascaris lumbricoides expelled by 1765 people in a poor urban community in Bangladesh were recovered and counted after the subjects had been treated with pyrantel pamoate. The subjects were divided into 22 classes by age and sex (mean n = 80) to examine how prevalence, mean worm burdens and measures of aggregation of worms varied with age and between the sexes, and to see how a measure of aggregation, k, calculated in 3 ways (by maximum likelihood, from moments, or from the percentage uninfected) compared with an empirical aggregation index (the percentage of subjects who expelled an arbitrary 80% of all worms) and with the proportion who were moderately to heavily infected (defined as > or = 15 worms). The prevalence of infection ranged from 64% to 95%, mean worm burdens ranged from 7 to 23 worms, and k ranged from 0.3 to 1.2. There were significant differences between adult males and females in the prevalence of infection, mean worm burdens and measures of aggregation, differences which are probably driven more by behaviour than immunity. The parameter k was better described in terms of the proportion who were moderately to heavily infected (linear; range 0.15-0.58) than by the empirical aggregation index (non-linear; range 0.30-0.49).

Epidemiology of soil-transmitted nematode infections in Ha Nam Province, Vietnam

Baseline epidemiological data are presented from a parasitological survey conducted in Thuy Loi commune, Ha Nam province, Vietnam; a farming community where night soil is routinely used as fertilizer for crops. 177 households were visited and 543 individuals (aged 1-88 years) recruited to the study. Helminth infection intensity was assessed by Kato-Katz to determine the density of parasite eggs per gram of stool (epg). Ascaris lumbricoides, Trichuris trichiura and hookworm infections were the only species detected. 83% of individuals were infected with A. lumbricoides (mean epg = 11,971), 94% with T. trichiura (mean epg = 793) and 59% with hookworm (mean epg = 302). Age-dependent patterns of infection prevalence and intensity were similar for A. lumbricoides and T. trichiura, but markedly different for hookworm infection. Similarly, age-dependency in the k-values for the three infections was due to covariance with the respective mean intensities with age rather than to independent age effects, with similar patterns for A. lumbricoides and T. trichiura, and a different pattern for hookworm. Three major conclusions can be drawn from the multiple-species analyses: There is positive interaction between A. lumbricoides and T. trichiura infections; high-intensity A. lumbricoides infections are significantly associated with high-intensity T. trichiura infections; and there is positive interaction between these two species such that infection intensity of A. lumbricoides is higher in individuals with concurrent T. trichiura infection than in individuals without and vice versa.

Schools for health: Focus on health, education and the school-age child

Mortality in children under five years old has been dramatically reduced through successful programmes of immunization and control of diarrhoeal diseases. UNICEF estimates that some 90% of children in developing regions now survive to reach school age. These survivors face new and continuing threats to their health, which can affect their physical development and may also prevent them taking full advantage of their only opportunity for formal education. The physical and mental growth of the 1000 million school-age children today will influence how the world is shaped for coming generations. Yet the health problems of this age group have received little attention. Recognizing the importance of this age group, a workshop funded by the Edna McConnell Clark Foundation was held 10-13 November 1994 in Fort Mitchell, Kentucky, USA, to review what is known about the health of school-age children, what is or can be done to improve their health, and what steps must be taken to find ways to improve the health and educational achievement of this important segment of the world's population. Don Bundy and Helen Guyatt here report on the workshop, which had three major conclusions: (1) the school-age children of developing countries face health problems that remain neglected and poorly understood; (2) an important research need is to develop simpler means of monitoring the health status of school-age children and evaluating the impact of public health interventions in this age group; and (3) two strategies are available to address this public health problem. The first is to develop further and test programmes that appear, from available evidence and pilot studies, to offer effective means of improving the health of this age group at reasonable cost, and to be sustainable; and the second is, over a longer term, to develop the capacity within countries to assess the health problems of school-age children and devise cost-effective strategies to address these problems. This report attempts, in brief form, to survey what is known about the health status of school-age children, to discuss the possible benefits to health and learning that accrue from health interventions, and to suggest some avenues currently available for both research and application.

Ascariasis in the subdistrict of Cavacos, municipality of alterosa (MG), Brazil: effect of mass treatment with albendazole on the intensity of infection

The clinical and epidemiologic aspects of infection with Ascaris lumbricoides were studied in a random stratified sample of the population of the subdistrict of Cavacos, municipality of Alterosa (Minas Gerais, Brazil). The effect of mass treatment with a single dose of albendazole on the prevalence and intensity of infection was also studied six months later in the same population. During the first phase of the study, a questionnaire was applied to 248 individuals to obtain information about the socioeconomic, sanitary and clinical conditions of the population surveyed. A total of 230 fecal samples were also examined by the Kato-Katz technique in order to determine the intensity of A. lumbricoides infection. Two hundred and two individuals were simultaneously submitted to blood counts and 70 children aged 12 years or less were evaluated for nutritional status. The presence of A. lumbricoides and other helminth eggs was also determined in 22 soil samples collected in the urban zone of Cavacos. Infection with enteroparasitic helminths was detected in 29.1% of the sample, with a predominance of A. lumbricoides (23.9%). Parasitism and/or intensity of A. lumbricoides infection were significantly correlated with age range (15 years or less), social class, sanitary and living conditions (water, sewage and domiciliary area per person), and presence of abdominal pain. However, these parameters were not correlated with nutritional status or hematocrit levels. During the second phase of the study, a slight but not statistically significant decrease in the prevalence of A. lumbricoides infection was detected after treatment with albendazole. However, an important and significant reduction in the amount of A. lumbricoides eggs eliminated through the feces was detected, indicating that the intensity of A. lumbricoides infection was lower in all the age ranges of the Cavacos population, especially among younger individuals, even six months after administration of the anthelminthic agent.

Multiple infection with Plasmodium and helminths in communities of low and relatively high socio-economic status

A study was conducted in the city of Lubumbashi, Zaire: (1) to survey parasitic infections and clinical conditions in the local children and their mothers; (2) to identify combinations of parasites and clinical conditions that commonly occurred together in individuals; and (3) to determine whether single- and/or multiple-species infections were risk determinants of the observed clinical conditions. Overall, 1100 children and mothers from three subdivisions, two of low socio-economic status (LSES) and one of relatively high socio-economic status (HSES), provided stool and blood samples and were clinically examined. Plasmodium prevalence was higher in the two LSES subdivisions than in the HSES subdivision. Prevalence and intensity of Ascaris lumbricoides infection were low in the HSES subdivision and one of the two LSES subdivisions. In contrast, prevalence and intensity of Trichuris trichiura and of hookworms were similar in all subdivisions. Plasmodium and A. lumbricoides were the most frequently found single-species infections. The combination of A. lumbricoides and Plasmodium was the most frequent double-species infection and that of A. lumbricoides, Plasmodium and T. trichiura was the most frequent triple-species infection. Significant positive associations between parasite species were detected in the HSES subdivision, and in one of the two LSES subdivisions. Because the relationships were not consistently detected, it is hypothesized that the associations are determined by environmental conditions rather than synergy between the parasites in the host. The most commonly observed clinical conditions were abdominal pain, diarrhoea, fever, and low packed-cell volume (PCV). The occurrence of each was significantly lower in the HSES subdivision than in at least one of the two LSES subdivisions. Abdominal pain and low PCV were most common in individuals presenting with only a single clinical condition, and the combination of this symptom and sign was the most commonly observed pair of conditions. Abdominal pain, low PCV and diarrhoea was the most common combination in individuals with three clinical conditions. Logistic regression revealed that hookworm infection, T. trichiura infection, young age and residence in an LSES subdivision were determinants of diarrhoea. Trichuris trichiura infection, young age and living in an LSES subdivision were risk factors for abdominal pain. Plasmodium infection and young age were risk factors for fever. LSES was the only predictor of low PCV. Infection with A. lumbricoides did not enter any of the models. No significant interactions were detected among parasites, indicating that there was no synergism or antagonism among parasites in the induced disease.

Estimating the number of multiple-species geohelminth infections in human communities

Infections with Ascaris lumbricoides, Trichuris trichiura and the hookworm species are often found in the same communities and individuals. Hosts infected by more than one species are potentially at risk of morbidity associated with each infection. This paper describes the use of a probabilistic model to predict the prevalence of multiple-species infections in communities for which only overall prevalence data exist. The model is tested against field data, using log-linear analysis, and is found to be more effective at estimating the numbers of multiple infections involving hookworms than those involving only A. lumbicoides and T. trichiura. This latter combination of infections is found, in half the communities examined, to be more common than expected by chance. An age-stratified analysis reveals that the degree of interaction between these two infections does not alter significantly with age in the child age classes of a Malaysian population.

Antibody isotype responses to antigens of Ascaris lumbricoides in a case-control study of persistently heavily infected Bangladeshi children

Antibody responses to Ascaris lumbricoides worm antigens were examined by ELISA in a case-control study of 2 groups of Bangladeshi children, one of which had been shown over a period of 12 months to be consistently lightly infected (controls) and the other consistently heavily infected (cases). The children showed a wide range in intensity of infection; children identified as cases were on average 4 times more heavily infected than the controls. There were no significant differences in weight, height, mid-upper arm circumference and skinfold thickness between the case or control subjects at the time blood samples for analyses by ELISA were collected. Children with repeatedly heavy infections with A. lumbricoides had higher concentrations of antibody isotypes to the antigens of A. lumbricoides than children who are repeatedly lightly infected. IgG1, IgG4 and IgE to worm antigens occurred in significantly higher concentrations in heavily infected subjects. This suggests that these antibody responses simply reflect the intensity of infection and may not play a significant role in protecting against heavy infections.

Control of Ascaris infection by chemotherapy: which is the most cost-effective option?

Cost-effectiveness analysis is used to predict the optimal design of mass chemotherapy strategies in controlling Ascaris lumbricoides infection. The question of who to treat, how many to treat, and how often to treat are addressed using a population dynamic model of helminth transmission that assesses effectiveness in terms of disease reduction, combined with cost data from an actual control programme. Child-targeted treatment can be more cost-effective than population treatment in reducing the number of disease cases. The model also implies that, in the circumstances described here, enhancing coverage is a more cost-effective approach than increasing frequency of treatment.

The evaluation of potential global morbidity attributable to intestinal nematode infections

This paper presents a method of estimating the potential global morbidity due to human intestinal nematode infections (Ascaris lumbricoides, Trichuris trichiura and hookworms), based on the observed prevalence of infection. The method relies on the observed relationships between prevalence and intensity of infection, and between worm burden and potential morbidity. This approach is shown to be sensitive to the precision of the original prevalence estimates and, in particular, to the degree of spatial heterogeneity in levels of infection. The estimates presented here indicate that several tens of millions of children are likely to suffer developmental consequences from infection, and suggest that the global disease burden of geohelminthiasis may be significantly greater than was supposed.

A population dynamic approach to the cost-effectiveness analysis of mass anthelmintic treatment: effects of treatment frequency on Ascaris infection

This study examines the cost-effectiveness of different frequencies of mass chemotherapy in reducing helminth infection and morbidity at the community level. The cost-effectiveness framework utilities a dynamic model of helminth transmission which permits the effectiveness of treatment to be expressed as some long-term reduction in infection and disease. Cost analysis is incorporated into this framework using an itemized cost menu and cost data from an actual control programme. The cost-effectiveness of mass treatment is investigated using epidemiological measures considered appropriate to Ascaris lumbricoides infection in both a high and low transmission area. The optimal strategy for control is shown to be dependent on the control objectives, the rate of transmission, and existing budget constraints. The analysis suggests that if the aim of control is to reduce morbidity due to ascariasis and mass treatment is the approach selected, then it is more cost-effective to intervene in a high transmission area than in a low transmission area, and that relatively long intervals between treatments offer the most cost-effective strategy.

A quantitative framework for evaluating the effect of community treatment on the morbidity due to ascariasis

There is growing emphasis on the use of community treatment to reduce the level of morbidity caused by helminth infection. The design of chemotherapy programmes, in terms of frequency of treatment and proportion of the community treated, would be assisted by a quantitative framework which enabled the morbidity reduction achieved by different approaches to be compared. The present study describes a model developed for this purpose which embodies two innovative features. First, a quantitative score of morbidity (the proportion of individuals harbouring an intense infection) is used to rate the success of a programme and, second, the distribution of helminths in the host population is generated by a mechanism that allows the distribution to change dynamically as a function of both treatment and reinfection. The model behaviour, using values typical of Ascaris lumbricoides, indicates that the benefit derived from community chemotherapy increases non-linearly with the coverage and efficacy of treatment.

Comparative prevalences of Ascaris lumbricoides, Trichuris trichiura and hookworm infections and the prospects for combined control

Programmes to control Ascaris lumbricoides, Trichuris trichiura and hookworm infections have often been targeted at each infection separately, but the advent of benign and broad-spectrum anthelmintics suggests that combined control may be feasible. The extent to which the infections co-occur in communities will determine the need for, and potential benefits of, such a combined approach. This paper examines the comparative distribution of the three geohelminths in different geographical areas and shows that A. lumbricoides and T. trichiura have closely related distributions, while hookworm infection is largely independent of the other two. These results indicate that many communities are at risk of disease from infection by more than one species of helminth. The similar distributions and epidemiological characteristics of A. lumbricoides and T. trichiura suggest that simultaneous control of these two parasites by the same strategy would be feasible and highly beneficial to communities. Multiple species control strategies which aim to control hookworm infection may require a more complicated protocol with more precise locality targeting.

Prevalence of Entamoeba histolytica and other intestinal parasites in a community from Maracaibo, Venezuela

The prevalences of Entamoeba histolytica and other intestinal parasites were assessed in a suburban community of Maracaibo, Venezuela, by examination of a stool specimen from each of 342 individuals, using iron-haematoxylin stained faecal smears and formalin-ether concentration. The overall parasitic infection rate was 80.4%, and 65.8% of the population had multiple infections. The overall amoebic infection rate, which was highest in female adults, averaged 39.7%. The E. histolytica infection rate was 8.7% and most of those infected were passing cysts. Entamoeba polecki was observed in two samples. Amongst the protozoa, Entamoeba coli was observed most frequently (24.8%) and Giardia lamblia was the predominant pathogen (13.0%). Trichuris trichiura (71.9%) and Ascaris lumbricoides (54%) were the most common parasites, particularly in school-children. The high rates of parasitic and multiple infections reflect the low socio-economic status of the community studied.

The prevalence and intensity of soil-transmitted helminthiases in rural communities in southern Nigeria

An investigation of the prevalence and intensity of soil-transmitted helminthiases was undertaken in four villages in Oyo state, Nigeria. Diagnosis was based on examination of stool samples for the presence of helminth ova in all age classes. The prevalence of Ascaris lumbriocoides ranged from 61.5-72.2%, of Trichuris trichiura from 65-74%, and of hookworm from 52.4-63% depending on the village concerned. Intensity, assessed indirectly as the mean eggs per gram of faeces (epg), varied with regard to the four villages. The mean egg count of A. lumbricoides varied from 6815-10,823 per gram, of T. trichiura from 127-246, and of hookworm from 122-267. Host age, sex and household size were found to be significant factors influencing faecal egg output, and indirect measure of the intensity of the helminth infections.

The effect of repeated chemotherapy on age-related predisposition to Ascaris lumbricoides and Trichuris trichiura

This study examines the persistence of predisposition to Ascaris lumbricoides and Trichuris trichiura during repeated chemotherapy in an urban community in Kuala Lumpur, Malaysia. Significant predisposition was observed over 2 periods of reinfection with and without age-standardization of data. Analysis of different age groups indicated that predisposition was most strongly detectable in the younger age classes. The intensities of infection with both parasites were strongly correlated at each cycle of intervention, suggesting that individuals were similarly predisposed to both species.