R. W. Ashford

Towards a kala azar risk map for Sudan: mapping the potential distribution of Phlebotomus orientalis using digital data of environmental variables.

Summary The need to define the geographical distribution of Phlebotomus orientalis results from its importance as the dominant vector of kala azar (visceral leishmaniasis) in Sudan. Recent epidemics of this disease in southern and eastern Sudan caused an estimated 100000 deaths and have renewed the impetus for defining the ecological boundaries of the vector. This information is an essential prerequisite to the production of a risk map for kala azar. This study uses data on the presence and absence of P. orientalis from 44 collecting sites across the central belt of Sudan. A logistic regression model was used to estimate the probability of the presence of P. orientalis at each collecting site as a function of climatic and environmental variables (rainfall; temperature; altitude; soil type and the satellite‐derived environmental proxies – Normalized Difference Vegetation Index and Land Surface Temperature). The logistic regression model indicates mean annual maximum daily temperature and soil type as the most important ecological determinants of P. orientalis distribution. An initial risk map was created in a raster‐based geographical information system which delineates the area where P. orientalis may occur. This map was then refined using a mask layer indicating the known rainfall‐based boundaries of the distribution of Acacia‐Balanites woodland – a woodland type known to be associated with the distribution of this vector. The predictive performance of the risk map is discussed.

Intestinal parasites of the chimpanzee Pan troglodytes in Kibale Forest, Uganda

One-hundred-and-twenty-three stool samples were examined from 45 chimpanzees in a natural population in western Uganda. Comparison with previous studies is complicated by the diversity of techniques used and interpretations. The Ugandan population had relatively many intestinal protozoa, including the probably beneficial entodiniomorph ciliates. Strongyloid nematodes are universal among chimpanzees, but were surprisingly absent from those on Mount Assirik in Senegal. Railletina tapeworms are sporadic in their occurrence. The absence of spiruroid nematodes in the Ugandan population reflects the absence of insectivory in this population. There was little evidence of seasonal difference in prevalences.

Environmental determinants of the distribution of Phlebotomus orientalis in Sudan.

Despite its importance as a vector of visceral leishmaniasis in Sudan, the ecology of Phlebotomus orientalis is still poorly understood. The results of a ground-based survey and a geographical-information-system (GIS) study, carried out to investigate the environmental determinants of the distribution of P. orientalis in the wooded areas of the central savannah belt of Sudan, are described here. The survey, carried out in April-June 1996, consisted of a collection of sandflies over two consecutive nights at each of 44 study sites, using three CDC, miniature, light traps at each site. During the survey, the ecology of each site was described. Phlebotomus orientalis was caught at 17 of the sites. Environmental data on the collection sites (rainfall, minimum and maximum temperatures, soil class, vegetation and land-surface-temperature indices) were extracted from a range of sources of digital data collected by satellites in the National Ocean and Atmospheric Administrations series. These data were then analysed, to ascertain which variables were significantly associated with sites positive for P. orientalis. In line with the results of previous studies, P. orientalis was found to have a significant association with the presence of the tree species Acacia seyal and Balanites aegyptiaca and with the black cotton (vertisolic) soils of eastern Sudan. The positive sites were found to have significantly higher annual mean maximum and minimum daily temperatures than the negative sites and the annual mean maximum normalized-difference vegetation index (NDVI) value was also found to be significantly higher in these sites than in sites where no P. orientalis were found.

Visceral leishmaniasis in Ethiopia. IV: Prevalence, incidence and relation of infection to disease in an endemic area

In a population of some 4600 people in southern Ethiopia, in which visceral leishmaniasis is endemic, 142 cases were recorded in an 8-year period to 1990. The cases were very unequally distributed between the six villages studied, with more than 90% in the four which were closest to the uninhabited valley of the Segen River. It was deduced that transmission occurs in the villages at lowest altitude, as well as in the Segen Valley. The youngest children were rarely affected and half as many cases were females as males. In a year-long intensive study, annual incidence of disease was estimated at 6.9/1000 in the whole population. Incorporating the results of previously published immuno-epidemiological studies, the annual incidence of disease in susceptible people was calculated as 1.9% while the rate of immunoconversion was 5.6 times greater. This indicates a high incidence of abortive or cryptic cases, but it remains to be demonstrated whether or not these cases are sources of infection.

Patterns of infection of haemoparasites in the fat sand rat, Psammomys obesus, in Tunisia, and effect on the host.

Two bacterial and one protozoan blood parasite, belonging to the genera Bartonella, Borrelia and Babesia, were studied in a Tunisian population of Psammomys obesus. Seasonal changes in the abundance of the parasites and host were monitored in a longitudinal field survey lasting 17 months. Blood samples collected during eight rodent-trapping sessions, between September 1995 and January 1997, were examined microscopically. Bartonella sp. showed a seasonal pattern, with most transmission occurring in summer and autumn; most rodents (90%) were infected in August-September, when they were at low density and adult. Borrelia sp. showed low prevalences, with few seasonal fluctuations, and Babesia sp. showed an intermediate pattern, differing from one year to another. In the cohort of adult rats, infections with Bartonella sp. and Babesia sp. were less prevalent in winter than in the previous summer. Single and mixed infections were equally prevalent in females and males, and in sexually active and inactive adults. In addition, infection had no apparent effect on the weight of adult P. obesus. The observation that the proportion of erythrocytes infected with Bartonella sp. decreased with increasing host age is probably indicative of some acquired immunity to this micro-organism. The absence of detectable infections with Borrelia sp. in old rats indicates that the prevalence and/or intensity of infection declines with host age or that infected animals die selectively. However, there was no indication that any of these parasites combined sufficient pathogenicity and abundance to have any measurable effect on the rodent population.

Strongyloides fuelleborni kellyi: Infection and disease in Papua New Guinea

A series o f unexplained deaths of two-month-old babies at on isolated mission station in Papua New Guinea led to the description o f a parasitic disease with unique and intriguing features. The parasite was named Strongyloides fuelleborni kellyi; S. fuelleborni is normally a parasite of non-human primates in Africa and Asia and of humans in Africa. How did the new subspecies reach non-human primate-free New Guinea? How do infants become infected in the first few days after birth? Why is the fatal disease restricted to two small areas while the infection itself is widespread? Ten years of study have only begun to indicate answers, as discussed here by Dick Ashford, Guy Barnish and Mark Viney.

Visceral leishmaniasis in Ethiopia. I: Cross-sectional leishmanin skin test in an endemic locality

A cross-sectional leishmanin skin test was carried out on a sample population of 730 individuals. The overall prevalence of leishmanin positivity was 36.4%; 50.9% of males and 23.2% of females showed positive skin test reaction. The skin-test profile in the study area is typical of an endemic focus of visceral leishmaniasis. The increase in leishmanin positivity with age parallels the age-specific disease profile and indicates an outdoor exposure to infection.

Occasional parasitic infections of man in Papua New Guinea and Irian Jaya (New Guinea)

Most parasites of man in New Guinea are cosmopolitan species, widely distributed and highly prevalent in the island. Reports of other parasite infections are reviewed. This impoverished fauna includes anthroponotic species normally occurring at low prevalences, e.g. Isospora belli, Dientamoeba fragilis, Trichomonas hominis and a few zoonotic forms with pigs as reservoir hosts, notably Balantidium coli and Entamoeba polecki. Other zoonoses are very few and extremely rare. This may be due to the reduced placental mammal fauna. The potential for the spread of Taenia solium infection is highlighted, as is the unique zoogeographical distribution of the Strongloides species resembling S. fuelleborni. Enterobius gregorii is recorded for the first time from Papua New Guinea.

Patterns of the tapeworm Raillietina trapezoides infection in the fat sand rat Psammomys obesus in Tunisia: season, climatic conditions, host age and crowding effects

The tapeworm Raillietina trapezoides was studied in a Tunisian population of the fat sand rat Psammomys obesus. Seasonal changes in the abundance of parasite and host were monitored in a longitudinal field survey lasting 20 months. In total 582 intestinal samples were collected during 10 trapping sessions between May 1995 and January 1997 and examined. The impact of abiotic and biotic factors on the prevalence and parasite burden were explored, using generalized linear models. R. trapezoides showed a seasonal pattern with most transmission occurring in summer and autumn. In August-September when the rodents were at low density and most adult (90-100%) were infected. In winter, infections with R. trapezoides were highly prevalent in the cohort of adults only. The worm burden by number was also high in late summer and autumn. Prevalence and parasite burden by number or by biomass were highly age dependent. Season and age act in synergy so that the rodents were highly infected in late summer and autumn. Unusually dry conditions in winter seemed to increase prevalence and to reduce the number of worms. There was no difference between males and females in prevalence or parasite burden (by number or by biomass). Prevalence was not correlated with the relative density of the hosts whereas parasite burden depended upon their densities. The more numerous the worms were, the smaller they were individually, reflecting a crowding effect. The possibility of an immune response regulating P. obesus is discussed, in regard to the very high natural prevalence and the variation in worm burden, and compared with other parasite-host systems.

Visceral leishmaniasis in Ethiopia. II. Annual leishmanin transformation in a population. Is positive leishmanin reaction a life-long phenomenon?

A one-year follow-up leishmanin skin test was undertaken in 605 subjects aged five years and above. The point prevalence of leishmanin positivity in the selected age group was 46.8%; 62.5% of the males and 32.1% of the females showed positive leishmanin reactions. The annual incidence of skin test transformation was 14.8%. In this study, positive to negative leishmanin conversion was noted in 9.3% of study subjects with initial positive leishmanin reaction.