Landscape epidemiology of vector-borne diseases

Landscape epidemiology describes how the temporal dynamics of host, vector, and pathogen populations interact spatially within a permissive environment to enable transmission. The spatially defined focus, or nidus, of transmission may be characterized by vegetation as well as by climate, latitude, elevation, and geology. The ecological complexity, dimensions, and temporal stability of the nidus are determined largely by pathogen natural history and vector bionomics. Host populations, transmission efficiency, and therefore pathogen amplification vary spatially, thereby creating a heterogeneous surface that may be defined by remote sensing and statistical tools. The current review describes the evolution of landscape epidemiology as a science and exemplifies selected aspects by contrasting the ecology of two different recent disease outbreaks in North America caused by West Nile virus, an explosive, highly virulent mosquito-borne virus producing ephemeral nidi, and Borrelia burgdorferi, a slowly amplifying chronic pathogen producing semipermanent nidi.

Anaplasmosis and ehrlichiosis - Maine, 2008

Anaplasmosis and ehrlichiosis are rickettsial tickborne diseases that have had at least a twofold increase in prevalence in the United States since 2000. Despite similar clinical presentations, the causative organisms are carried by different ticks with distinct geographic and ecologic associations. Surveillance efforts are complicated by ambiguous terminology and serologic testing with antibody cross-reactivity. Although anaplasmosis historically has been reported in Maine, ehrlichiosis has been reported infrequently. During 2007-2008, the number of physician-reported anaplasmosis cases nearly doubled in Maine, and ehrlichiosis cases increased more than fourfold. To examine this increase, the Maine Department of Health and Human Services (MDHHS) analyzed available data on tick burden and physician-reported cases of anaplasmosis and ehrlichiosis during 2000-2008. This report describes the results of that analysis, which indicated that Ixodes scapularis (the tick vector for Anaplasma phagocytophilum) was broadly distributed in Maine, whereas Amblyomma americanum (the tick vector for Erhlichia chaffeenisis) was scarce. Moreover, 95% of physician-reported ehrlichiosis cases lacked a concurrent serologic assessment to exclude anaplasmosis, suggesting that antibody cross-reactivity might have resulted in misclassification. In 2008, Maine modified case classification to enhance specificity; ehrlichiosis cases that lack a concurrent test for anaplasmosis are now classified as suspect rather than probable and therefore are not included in national surveillance summaries. The accuracy of case classification and surveillance can be improved by educating health-care providers regarding 1) the expected geographic distribution of tick vectors and 2) recommendations for confirmatory testing to distinguish between the causative organisms of anaplasmosis and ehrlichiosis.

Operational vector-borne disease surveillance and control: closing the capabilities gap through research at overseas military laboratories

Malaria, dengue fever, chikungunya virus, leishmaniasis, and a myriad of other vector-borne diseases pose significant threats to the warfighter and to the overall combat effectiveness of units. Military preventive medicine (PM) assets must accurately evaluate the vector-borne disease threat and then implement and/or advise the commander on countermeasures to reduce a particular threat. The success of these measures is contingent upon the biology of the disease vector and on the tools or methods used to conduct vector/pathogen surveillance and vector control. There is a significant gap between the tools available and those required for operational PM assets to provide real-time, effective surveillance and control. A network of US Army and US Navy overseas laboratories is focused on closing the current capabilities gap. Their mission is to develop and field test tools and methods to enhance the combatant commander's ability to identify and mitigate the threat posed by these vector-borne diseases.

Babesia bigemina in ticks grown on non-bovine hosts and its transmission to these hosts

The failure of Boophilus microplus to be cleansed of Babesia bigemina while the ticks were growing on non-bovine hosts, as well as the infection of the non-bovine hosts, was demonstrated in experiments in which infected ticks were released on a number of sheep, a goat and a horse.

Seven of ten batches of ticks, collected from six non-bovine hosts, retained their babesia-infection.

Blood subinoculated into cattle from seven of nine sheep and from the horse showed that these animals carried B. bigemina whilst the ticks were feeding on them. This suggested that ticks may have become reinfected from the non-bovine hosts, a possibility supported by the finding of B. bigemina in blood smears from two of the sheep.

This work was performed at the Animal Research Institute, Yeerongpilly, and in the Department of Parasitology, University of Queensland, and supported by funds from both organizations. I am indebted to Mr G. D. Daly for technical assistance, to Professor J. F. A. Sprent and Dr H. M. D. Hoyte for their interest and help.

International network for capacity building for the control of emerging viral vector-borne zoonotic diseases: ARBO-ZOONET

Arboviruses are arthropod-borne viruses, which include West Nile fever virus (WNFV), a mosquito-borne virus, Rift Valley fever virus (RVFV), a mosquito-borne virus, and Crimean-Congo haemorrhagic fever virus (CCHFV), a tick-borne virus. These arthropod-borne viruses can cause disease in different domestic and wild animals and in humans, posing a threat to public health because of their epidemic and zoonotic potential. In recent decades, the geographical distribution of these diseases has expanded. Outbreaks of WNF have already occurred in Europe, especially in the Mediterranean basin. Moreover, CCHF is endemic in many European countries and serious outbreaks have occurred, particularly in the Balkans, Turkey and Southern Federal Districts of Russia. In 2000, RVF was reported for the first time outside the African continent, with cases being confirmed in Saudi Arabia and Yemen. This spread was probably caused by ruminant trade and highlights that there is a threat of expansion of the virus into other parts of Asia and Europe. In the light of global warming and globalisation of trade and travel, public interest in emerging zoonotic diseases has increased. This is especially evident regarding the geographical spread of vector-borne diseases. A multi-disciplinary approach is now imperative, and groups need to collaborate in an integrated manner that includes vector control, vaccination programmes, improved therapy strategies, diagnostic tools and surveillance, public awareness, capacity building and improvement of infrastructure in endemic regions.

[Neglected arthropod-borne viral infections in the Czech Republic]

Central European encephalitis is the most common arthropod-borne virus disease in the Czech Republic, with the mean annual incidence of 6 cases per 100 000 population. However, seven less known arboviruses (Flavivirus West Nile, Bunyavirus Tahyna, Bunyavirus Batai, Bunyavirus Sedlec, Bunyavirus Lednice, Orbivirus Tribee, Uukuvirus Uukuniemi) also circulate in this country, of which West Nile, Tahyna, Tribec and possibly Batai have been reported to cause human disease. Moreover, antibodies against two other pathogenic viruses found in Europe, i.e. Alphavirus Sindbis and Coltivirus Eyach, have been detected in the Czech Republic. The aim of this study is to review briefly the less known arboviruses found in the Czech Republic with emphasis on the taxonomic status, identification of their hosts and vectors, and pathogenicity to humans. These arboviruses can cause febrile illness to aseptic (meningo)encephalitis of unclear etiology. The review points out the possible emergence of these neglected arboviruses in the foreseeable future and provides diagnostic guidance.

[Ecology of vector systems: a tangle of complexity]

The long co-evolutionary process between arthropods and microorganisms has resulted in a wide variety of relationships. One such relationship involves a wide range of infectious agents (virus, bacteria, protozoa, helminthes) that use blood-feeding arthropods (insects and mites) as vectors for transmission from one vertebrate to another. Transmission involves three components, i.e., microorganism, vector(s), and vertebrate host(s). Study under natural conditions has shown that the underlying mechanisms are extremely complex with circulation of the infectious agents depending on numerous conditions linked not only to bioecology but also to genetic factors in all three component populations. The role of arthropods sometimes goes beyond that of a transmitter of disease. In some cases they also serve as reservoirs or disseminators. In addition changes in the environment whether due to natural causes or human activities (e.g. pollution, agropastoralism, urbanization, transportation network development, and climate change) can have profound and rapid effects on the mechanisms underlying these vector systems. In short the ecology of vector systems closely reflects the extreme complexity of epidemiological studies on diseases caused by infectious agents depending on this type of transmission. As a result prediction of infectious risks and planning of preventive action are difficult. It appears obvious that a good understanding of vector systems in their natural context will require a truly ecological approach to the diseases that must be the focus of extremely close epidemiologic surveillance. Achieving this goal will necessitate more than the skills of physicians and veterinarians. It will require the contribution of specialists from a variety of fields such as microbiology, entomology, systematics, climatology, ecology, urbanism, social sciences, economic development, and many others.

Origin of the old testament plagues: explications and implications

Analyses of past disasters may supply insights to mitigate the impact of recurrences. In this context, we offer a unifying causative theory of Old Testament plagues, which has present day public health implications. We propose the root cause to have been an aberrant El Niño-Southern Oscillation teleconnection that brought unseasonable and progressive climate warming along the ancient Mediterranean littoral, including the coast of biblical Egypt, which, in turn, initiated the serial catastrophes of biblical sequence - in particular arthropod-borne and arthropod-caused diseases. Located beyond the boundary of focal climate change, inland Goshen would not have been similarly affected. Implicit in this analysis is a framework to consider a possibility of present day recurrence of similar catastrophes and their impact upon essential public services.

Bartonella species in blood of immunocompetent persons with animal and arthropod contact

Using PCR in conjunction with pre-enrichment culture, we detected Bartonella henselae and B. vinsonii subspecies berkhoffii in the blood of 14 immunocompetent persons who had frequent animal contact and arthropod exposure.

Arthropod-borne diseases in homeless

Homeless people are particularly exposed to ectoparasite. The living conditions and the crowded shelters provide ideal conditions for the spread of lice, fleas, ticks, and mites. Body lice have long been recognized as human parasites and although typically prevalent in rural communities in upland areas of countries close to the equator, it is now increasingly encountered in developed countries especially in homeless people or inner city economically deprived population. Fleas are widespread but are not adapted to a specific host and may occasionally bite humans. Most common fleas that parasite humans are the cat, the rat, and the human fleas, Ctenocephalides felis, Xenopsylla cheopis, and Pulex irritans, respectively. Ticks belonging to the family Ixodidae, in particular, the genera Dermacentor, Rhipicephalus, and Ixodes, are frequent parasites in humans. Sarcoptes scabiei var. hominis is a mite (Arachnida class) responsible for scabies. It is an obligate parasite of human skin. The hematophagic-biting mite, Liponyssoides sanguineus, is a mite of the rat, mouse, and other domestic rodents but can also bite humans. Finally, the incidence of skin disease secondary to infestation with the human bedbug, Cimex lectularius, has increased recently. Bacteria, such as Wolbacchia spp. have been detected in bedbug. The threat posed by the ectoparasite in homeless is not the ectoparasite themselves but the associated infectious diseases that they may transmit to humans. Except for scabies all these ectoparasites are potential vectors for infectious agents. Three louse-borne diseases are known at this time. Trench fever caused by Bartonella quintana (B. quintana), epidemic typhus caused by Rickettsia prowazekii, and relapsing fever caused by the spirochete Borrelia recurrentis. Fleas transmit plague (Xenopsylla cheopis and Pulex irritans), murine typhus (Xenopsylla cheopis), flea-borne spotted rickettsiosis on account of the recently described species Rickettsia felis (C. felis), and occasionally cat scratch disease on account of Bartonella henselae (C. felis). The role of fleas as potential vector of B. quintana has recently been suggested. Among the hematophagic-biting mites, L. sanguineus, is responsible for the transmission of Rickettsia akari, the etiologic agent of rickettsialpox. Virtually, no data are available on tick-borne disease in this population. This article will deal with epidemiology, diagnosis, prevention, and treatment of these ectoparasite and the infectious diseases they transmit to the homeless people.

Seroprevalence study of Tick-borne encephalitis, Borrelia burgdorferi, Dengue and Toscana virus in Turin Province

Tick borne encephalitis virus (TBEV) is present in some European countries and it is transmitted by a tick bite. Ixodes ricinus is the main vector of the infection in Italy, where fortunately clinical neurological manifestations, typical of the more serious phase of the disease, are very rarely observed. This behaviour is different from other endemic Euroasiatic areas where numerous cases of encephalitis are described. However TBE transmission has not been widely investigated in Italy and available epidemiological data have been obtained only by studies performed in Central and Northern Regions of the country. In addition seroepidemiological researches were made prevalently on subjects at high risk of tick bite, such as hunters or forest guards from Trentin and Central Italy. No precise information about TBE virus diffusion was available in the Piedmont before our investigations. We found that hunters and wild boar breeders seem to be particularly exposed to the risk of TBE virus infection in Turin Province and in particular in the Susa valley, although no neurological involvement was observed in our population. In particular a seroprevalence of about 5% was detected by the use of purified antigens ELISA test, amongst the subjects at high risk of tick bite. Moreover low risk individuals showed a seroprevalence of below 2%. In addition a parallel seroepidemiological study was performed in Turin Province for Borrelia burgdorferi, the aetiological agent of Lyme disease, also transmitted by tick bite (e.g. Ixodes ricinus), for Dengue and Toscana (TOS) arboviruses, respectively transmitted by Aedes mosquitoes and phlebotomes. Data reported here demonstrate only a sporadic presence in our population of antibodies against Borrelia and Dengue infection. Moreover using an ELISA test performed with viral purified nucleoprotein, we reported a total percentage of about 3% of subjects positive for TOSV.

[Babesiosis of human and domestic dog; ethiology, pathogenesis, diagnostics]

This article presents the current state of our knowledge on babesiosis (piroplasmosis), one of the dangerous, invasive disease of humans and animals, transmitted by ticks. It is included among emerging diseases because its spread and significance have increased in recent years. This sickness is caused by intraerythrocytic parasites belonging to the Babesia species and it is a well-known zoonosis occurring in animals; as a human disease it was unknown almost till the first half of the last century. The intensified migration of human population and human interference in a forest biotope caused that number of recognized cases has grown considerably in recent years. Piroplasmosis in dogs is widely spread all over the world and it is caused by several Babesia species. The principal etiological factor of babesiosis in dogs is B. canis, which turned out to be a collective species represented by three subspecies for which the vectors are three different species of ticks. Their geographical extent indicates the endemic areas for this often fatal disease. A technique, the most often applied in the detection of Babesia is a full blood smear stained with Giemsa or Wright method. However, the estimation of the specimen depends to a large extent on the experience of the diagnostician. The immunological and serological methods are characterized with a high specificity and sensitivity but there are patients in which the false negative results have been obtained. Therefore, the traditional methods have been complemented or even ousted by the molecular methods, in which polymerase chain reaction (PCR) brings the biggest profits. However, the standardization of this technique still remains under elaboration. The usefulness of the PCR protocol has been tested with different molecular destinations from which sequences of genes encoding rRNA for small ribosomal subunit are taken into consideration. Within ribosome, the evolutionally conservative areas can be distinguished, i.e. having the nucleotide sequences similar to the majority or all Babesia species and to others closely related to them. Such construction of gene enables designing of starters complementary to conservative sites to PCR, detecting a large group of related organisms. Another molecular marker allowing on the accurate identification of Babesia is gene encoding the beta-tubuline protein. There are two introns within this gene, from which the first one shows a big variability with regard to the length as well as to the nucleotide sequence, therefore, the PCR products show a diverse length depending on the Babesia species. But these differences are too small for some species and, confirming methods that extend time of diagnostics are essential. The other genes which sequences can be used as molecular aim to the detection of DNA and Babesia species diversification are genes encoding the Heat Shock Proteins HSP 70. However, the gene hsp 70 shows a big conservatism of the nucleotide sequence even between the non related organisms; therefore, this method, based on the amplification of whole genome or its fragments, applies mainly in analysis of molecular phylogenetic.

Phortica variegata as an intermediate host of Thelazia callipaeda under natural conditions: evidence for pathogen transmission by a male arthropod vector

Knowledge about Phortica variegata (Drosophilidae, Steganinae), the intermediate host of the eyeworm Thelazia callipaeda (Spirurida, Thelaziidae), is confined to experimental studies. To investigate the role P. variegata plays in the transmission of T. callipaeda under natural conditions, the population dynamics of these flies in the natural environment and their feeding preferences (on vegetables and/or animal lachrymal secretions) were examined. From April to November 2005, a total number of 969 (557 males and 412 females) P. variegata flies were collected weekly in a region of southern Italy with a history of canine thelaziosis. The flies were identified and dissected or subjected to a PCR assay specific for a region within the ribosomal ITS-1 DNA of T. callipaeda. The zoophilic preferences of P. variegata were assessed by collecting flies around the eyes of a person or around a fruit bait. Seven hundred and twenty flies (398 males and 322 females) were dissected under a stereomicroscope; 249 flies (158 males and 91 females) that died prior to the dissection were subjected to molecular investigation. Only P. variegata males were infected with larval T. callipaeda both at dissection (six, 0.83%) and with the specific PCR (seven, 2.81%), representing a total percentage of 1.34% flies infected. Interestingly, only males were collected around the eyes, compared with a male/female ratio of 1:4 around the fruit. This survey indicated that P. variegata males act as intermediate hosts of T. callipaeda under natural conditions in Europe. Both the zoophilic behaviour of P. variegata males on lachrymal secretions and their role as vector of T. callipaeda have been discussed as they represent a peculiarity in medical and veterinary entomology. The synchrony between the fly population dynamics and the biology of the nematode in the definitive host provides an interesting model for exploring the co-evolution of Thelazia spp. with their hosts.

Épidémie de leishmaniose cutanée chez des militaires de retour de mission en Guyane

BACKGROUND

An outbreak of cutaneous leishmaniasis occurred among 71 soldiers who had participated in various missions during a 4-month's period in French Guiana. The aims of this study were (i) to describe outbreak and (ii) to determine risk factors of cutaneous leishmaniasis.

METHODS

All patients were hospitalised. Cutaneous lesions were biopsied and cultured for species identification. Individual information was collected by a physician or a nurse, using on a standardised, anonymous chart. Data were processed with EpiInfo 6.04 and SAS.

RESULTS

Mean age of the 71 soldiers was about 25.9 years (19-37 years). Twelve soldiers presented 56 lesions due to Leishmania (Viannia) guyanensis (attack rate = 16.9 for 100). Among 56 lesions, 13 lesions were localized on the trunk, usually an unexposed body area. Logistic regression highlighted military exercises in the forest during a high risk period of leishmaniasis transmission (OR = 11.2; p < 0.01), and the young age (OR = 1.33; p = 0.04). Vector control measures were not statistically significant.

CONCLUSION

Military authorities should restrict deep forest activities during periods of high risk transmission. Vector control measures are essential. Officers should motivate their soldiers and supervise vector control measures. As ecotourism is developing, tourists as well as workers staying in deep forest must be informed of the risk and about vector control measures.

Important experimental parameters for determining infection rates in arthropod vectors using pool screening approaches

Measuring transmission of a vector-borne infection is essential to understanding infection dynamics. When infection prevalence in the vector population is low, transmission is often measured by pool screening (also referred to as group testing). Several investigators have developed statistical methods to recover infection prevalence estimates from pool screen data. These are based on models that contain certain assumptions, and a pool screening approach must be designed to take these into account if accurate estimates of infection prevalence are to be obtained. Here we describe these assumptions and discuss appropriate sampling protocols. The sources of error inherent in pool screening are described, and we show that, under most conditions in which one would want to use group testing, most of the error results from sampling and not the pooling process. Issues involved in developing a sampling protocol, including the total number of insects to be screened and optimal pool size, are explored. The meaning of confidence intervals associated with prevalence estimates and the appropriate interpretation of these intervals are discussed.

Laboratory containment practices for arthropod vectors of human and animal pathogens

Arthropod-borne pathogens have an impact on the health and well-being of humans and animals throughout the world. Research involving arthropod vectors of disease is often dependent on the ability to maintain the specific arthropod species in laboratory colonies. The author reviews current arthropod containment practices and discusses their importance from public health and ecological perspectives.

Role of the Yersinia pestis plasminogen activator in the incidence of distinct septicemic and bubonic forms of flea-borne plague

Yersinia pestis is transmitted by fleas and causes bubonic plague, characterized by severe local lymphadenitis that progresses rapidly to systemic infection and life-threatening septicemia. Here, we show that although flea-borne transmission usually leads to bubonic plague in mice, it can also lead to primary septicemic plague. However, intradermal injection of Y. pestis, commonly used to mimic transmission by fleabite, leads only to bubonic plague. A Y. pestis strain lacking the plasmid-encoded cell-surface plasminogen activator, which is avirulent by intradermal or s.c. injection, was able to cause fatal primary septicemic plague at low incidence, but not bubonic plague, when transmitted by fleas. The results clarify a long-standing uncertainty about the etiology of primary septicemic plague and support an evolutionary scenario in which plague first emerged as a flea-borne septicemic disease of limited transmissibility. Subsequent acquisition of the plasminogen activator gene by horizontal transfer enabled the bubonic form of disease and increased the potential for epidemic spread.

Field-user acceptability of new camouflage face paint formulations in the Republic of Korea

New formulations of camouflage face paint (CFP), one with 30% N,N-diethyl-3-methylbenzamide (DEET) and the other without DEET, were evaluated for soldier-user acceptability during a military field-training exercise in the Republic of Korea. Soldiers testing the CFP formulations were members of one of four U.S. Army infantry companies (A, B, C, or D). The formulations were evaluated while soldiers participated in simulated combat exercises for 5 days during hot, humid summer weather in Korea. Results showed that soldiers found both of the new formulations easier to apply (91.3% of respondents who used CFP without DEET and 87.9% of respondents who used CFP with DEET) and remove (82.6% without DEET and 81.2% with DEET) than the previous standard military-issue CFP. Soldier acceptability was higher for the new CFP formulation with 30% DEET (70.5%) than for the formulation without 30% DEET (52.9%). Soldiers recommended it more frequently (70.5%) than the formulation without 30% DEET (50.0%). The new CFP formulation with 30% DEET was rated more often (79.5%) as either good or excellent than the new formulation without 30% DEET (67.4%). Soldiers reported that the CFP formulation with 30% DEET more successfully camouflaged the face (92.7%) than the formulation without 30% DEET (80.0%).

[Bacterial zoonoses: emerging concepts]

Bacterial zoonoses are evolving with changes in society, climate and lifestyles. A hierarchy of non food-borne zoonoses was recently proposed in France, and includes characteristics such as severity criteria and bioterrorism potential. The creation of specific networks and reference centers has provided the means to monitor the emergence (or re-emergence) of zoonoses such as brucellosis and Q fever. Molecular tools have facilitated the detection of bacteria that are transmitted by arthropod vectors (ticks, fleas, etc.) and that cause diseases such as Lyme borreliosis, bartonellosis and ehrlichiosis.