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R W Ashford

Publications and source records attributed to R W Ashford.

At least 19 recordsLinked to original sources
PubMed2004

Leishmania donovani is the only cause of visceral leishmaniasis in East Africa; previous descriptions of L. infantum and "L. archibaldi" from this region are a consequence of convergent evolution in the isoenzyme data.

Isoenzyme-based studies have identified 3 taxa/species/'phylogenetic complexes' as agents of visceral leishmaniasis in Sudan: L. donovani, L. infantum and "L. archibaldi". However, these observations remain controversial. A new chitinase gene phylogeny was constructed in which stocks of all 3 putative species isolated in Sudan formed a monophyletic clade. In order to construct a more robust classification of the L. donovani complex, a panel of 16 microsatellite markers was used to describe 39 stocks of these 3 species. All "L. donovani complex" stocks from Sudan were again found to form a single monophyletic clade. L. donovani ss stocks from India and Kenya were found to form 2 region-specific clades. The partial sequence of the glutamate oxaloacetate transaminase (GOT) gene of 17 L. donovani complex stocks was obtained. A single nucleotide polymorphism in the GOT gene appeared to underlie the isoenzyme classification. It was concluded that isoenzyme-based identification is unsafe for stocks isolated in L. donovani endemic areas and identified as L. infantum. It was also concluded that the name L. archibaldi is invalid and that only a single visceralizing species, Leishmania donovani, is found in East Africa.

Africa, Eastern↗
PubMed2004

Frenkelia parasites in a small mammal community. Dynamics of infection and effect on the host.

A community of small mammals, Clethrionomys glareolus, Arvicola terrestris, Microtus arvalis, M. agrestis, M. subterraneus, Apodemus spp. and Sorex spp., was studied as hosts of Frenkelia glareoli and F. microti in Fronche-Comté (France). They were monitored in spring, summer and autumn on an area of about 1,350 ha comprising open field, hedgerow network and forest. Among 1,714 small mammals examined between July 1992 and October 1993, 47% (178/376) of C. glareolus, 9.9% (14/139) of A. terrestris and 1.3% (4/311) of Apodemus spp. were infected by F. glareoli. The prevalence of infection with F. microti was 9.2% (66/716) in M. arvalis and 8.2% (6/73) in M. agrestis. M. subterraneus and Sorex spp. were not infected. The maintenance of each parasite in a rural landscape is assured both by a forest and a grassland host. Multiple logistic regression showed that prevalence was highly age-dependent, with an apparent seasonal pattern. Prevalence varied between 30% in summer and 60% in early spring for F. glareoli in C. glareolus and between 3% in autumn to 30% in early spring for F. microti in M. arvalis. The year, habitat, host sex, relative density had no impact on prevalence. In M. arvalis only, sexually active voles were preferentially uninfected, indicating a possible impact of this parasitism on fertility.

Age Factors↗
PubMed2003

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.

Aging↗
PubMed2003

Leishmania major infection in the fat sand rat Psammomys obesus in Tunisia: interaction of host and parasite populations.

The causative agent of cutaneous leishmaniasis, Leishmania major, was studied in a Tunisian population of the fat sand rat, Psammomys obesus. Seasonal changes in the abundances of parasite and host were monitored in a longitudinal field survey lasting 21 months. Overall, 566 P. obesus, collected during 10 trapping sessions between May 1995 and January 1997, were examined. Analysis of prevalence, using logistic regression, revealed that extrinsic factors, such as season and climatic conditions, and intrinsic factors, such as host age, have a combined effect. Leishmania major showed a seasonal pattern, with most transmission occurring in late summer and in autumn, when prevalences were 7.5- and 6.6-fold higher, respectively, than in spring. Prevalence peaked, at 70%, in September 1995 and then decreased to 0% in June 1996. The same temporal fluctuation was observed during the second study year, albeit among prevalences that were relatively low because of unusually dry conditions. Prevalence was highly dependent on the age of the P. obesus, and season and age acted in synergy so that the rodents were highly infected in late summer and in autumn. Prevalence was not correlated with the relative density of the P. obesus and also appeared independent of gender. Although the ear lesions observed on 378 sand rats during a 1-year survey were closely associated with Leishmania infection, such lesions were not good predictors of infection, as 35% of the rodents found to be infected had no visible lesions on their ears. The prevalences of Leishmania infection observed in this study, among P. obesus living in monospecific colonies, were generally lower than those observed in other studies of P. obesus. It seems possible that P. obesus living in monospecific colonies could have a lesser role in propagating the parasite than those living in plurispecific colonies of rodents, and act as an 'epidemiological sink'.

Animals↗
PubMed2002

Towards a standard battery of microsatellite markers for the analysis of the Leishmania donovani complex.

The investigation of microsatellite markers has recently superseded that of isoenzymes for many population-biology applications. Microsatellites have the advantages of being dominant, neutral, highly polymorphic and easily scored by high-throughput methods. However, it is necessary to develop a new panel of markers for each group of organisms of interest. Previously, only about 5% of the markers that amplify Leishmania major microsatellite loci were also found to amplify L. donovani loci. A panel of 20 microsatellite markers that are polymorphic in L. donovani and L. infantum has now been developed, using a rapid-enrichment method that will be suitable for developing libraries of markers for other trypanosomatid species. This is the first panel of polymorphic microsatellite markers, to be isolated de novo from any species of Leishmania, that is large enough for population-biology applications.

Alleles↗
PubMed2002

Zoonotic species of Cryptosporidium are as prevalent as the anthroponotic in HIV-infected patients in Thailand.

The epidemiology of chronic diarrhoea in adults with late-stage HIV infection was investigated in a prospective study in Bangkok, Thailand. During this investigation, 34 Cryptosporidium isolates were obtained from the faeces of 36 patients, with mean CD4(+) counts of only 14 x 10(6) CD4(+) cells/litre (range = 2 x 10(6) - 53 x 10(6)/litre), who had symptomatic cryptosporidiosis. Genotyping of these isolates, by RFLP analysis and DNA sequencing of the hypervariable region of the 18S rRNA gene, indicated that only 17 (50%) were of the C. parvum human genotype. The rest were of C. meleagridis (seven), the C. parvum 'bovine' genotype (five), C. felis (three) and C. canis (two). Extensive genotypic heterogeneity was observed among the C. parvum isolates, and two other isolates, one of C. meleagridis and the other of C. felis, produced atypical restriction patterns and were only identified by sequencing. This appears to represent the first report of C. canis and the 'bovine' genotype of C. parvum in HIV-infected Thai patients.

Adult↗
PubMed2001

Current usage of nomenclature for parasitic diseases, with special reference to those involving arthropods.

Terminological confusion has been aggravated by efforts to develop a standardized nomenclature for parasitic diseases (SNOPAD) arising from the proposal by Kassai et al., 1988) for a standardized nomenclature of animal diseases (SNOAPAD). To restabilize international nomenclature of parasitic diseases it is recommended that, whenever appropriate, names should follow the 'International Nomenclature of Diseases' (IND) compiled by the Council for International Organizations for Medical Sciences (CIOMS/WHO, 1987). For diseases not included in IND, familiarity should guide the choice of name: traditional English language names of diseases should be preferred, e.g. 'malaria', 'scabies' or, for parasitic diseases having no traditional name, the taxonomic name of the causative organism should be applied, e.g. 'Brugia timori microfilaraemia'; 'Plasmodium malariae infection'; 'Simulium allergy'--instead of the generic derivatives proposed by SNOPAD, i.e. brugiosis, plasmodiosis and simuliidosis, respectively. For names of new diseases or those rarely mentioned, the suffix -osis would normally take precedence. Generally, the name of choice for any disease in any language should be the vernacular term, with commonest English usage preferred for international communication, and publications should include synonyms in the list of keywords.

Animals↗
PubMed2000

The leishmaniases as emerging and reemerging zoonoses.

The 20 or so species of Leishmania which have been recorded as human infections are all either zoonotic, or have recent zoonotic origins. Their distribution is determined by that of their vector, their reservoir host, or both, so is dependent on precise environmental features. This concatenation of limiting factors leads to specific environmental requirements and focal distribution of zoonotic or anthroponotic sources. Human infection is dependent on the ecological relationship between human activity and reservoir systems. Examples are available of the emergence of leishmaniasis from the distant past to the present, and can be postulated for the future. These emergences have been provoked by the adoption of new, secondary reservoir hosts, the adoption of new vector species, transport of infection in humans or domestic animals, invasion by humans of zoonotic foci, and irruption of reservoir hosts beyond their normal range. The leishmaniases therefore present an excellent model for emerging disease in general, and for the generation of the principles governing emergence. The model is, however, limited by gaps in our knowledge, usually quantitative, sometimes qualitative, of the structure of reservoir systems.

Animals↗
PubMed2000

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.

Animals↗
PubMed2000

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.

Animals↗
PubMed1999

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

The need to define the geographical distribution of Phlebotomus orientalis results from its importance as the dominant vector of kala azar (visceral Iceishmaniasis) in Sudan. Recent epidermics 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.

Animals↗
PubMed1998

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 Administration's 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.

Animals↗
PubMed1997

The leishmaniases as model zoonoses.

Of the 374 species of parasite recorded as naturally infecting Homo sapiens, all but some 40 are zoonotic. A classification of human parasites is proposed, based on the role of H. sapiens in the maintenance of suprapopulations. Categories range from 1, for which man is irrelevant to the persistence of the species, to 4, which includes the small number of host-specific forms. The classification is illustrated by various species of Leishmania, and these are used to illustrate various host-parasite systems. The 'Wells effect' is proposed, to describe the protection of host populations by their parasites.

Animals↗
PubMed1994

Visceral leishmaniasis in Ethiopia. III. The magnitude and annual incidence of infection, as measured by serology in an endemic area.

A serological study of visceral leishmaniasis was carried out in a cohort of people in an endemic area of Ethiopia. The people were tested three times, at 6-monthly intervals. Seroprevalence at the start was 112 per 1000, and the annual rate of conversion from negative to positive was estimated as 106 per 1000. Both seroprevalence and incidence of seroconversion increased with age, and males showed higher rates than females, with a male:female ratio of seroconversion of 2:1. Accumulation of 6-monthly seroconversion rates was considered to give a better estimate of the annual rate itself, as the half-life of positive serology was only around 3 months.

Adolescent↗
PubMed1994

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.

Adolescent↗