Methods for the rapid appraisal of African animal trypanosomosis in the Gambia.

A technique for the rapid field assessment of African animal trypanosomosis (AAT) was developed during studies in the Gambia. This involved gathering indigenous information from rapid-appraisal questionnaires addressed to local informants, the results of single tsetse surveys and evaluations of the prevalence of trypanosome infections in village cattle. Local informants included livestock owners and herdsmen and trained personnel such as livestock assistants. The answers to the questionnaires were weighted in order to translate them into semi-quantitative ranked estimates (zero, low, medium, high or very severe) of the severity of AAT problems. A similar ranking was also defined for tsetse and prevalence data in the Gambia. The three assessment methods generally gave complementary results leading to similar conclusions about the severity of tsetse-trypanosomosis problems in a survey area; inconsistencies usually suggested that additional information was needed. The rankings of AAT intensity were used to develop management guidelines for minimising the impact of AAT at different levels through control interventions or improved livestock management. The methodology was designed to provide reliable, up-to-date and cost-effective assessments of AAT problems. Emphasis was placed on the importance of the involvement, priorities and perceptions of village livestock owners and herdsmen in making these assessments.

Culicoides in relation to transmission of African horse sickness virus in The Gambia.

Twelve light trap collections made near overnight shelters of horses and donkeys in four villages in the Central River Division of The Gambia captured fourteen species of biting midge of the genus Culicoides. Five species new to The Gambia were identified. This brought the number of recognized species of Culicoides (after a revision of C. schultzei) to twenty-nine in The Gambia. Species known or suspected as vectors of African horse sickness virus (AHSV) and bluetongue virus (BTV) comprised 83% of female captures, 65% of captures being C. imicola or its sibling species, C. miombo. Captures of female Culicoides in the late dry season were almost as large as in the early dry season, despite the extreme heat and dryness at this time of the year. Tests on batches of formalin-preserved female midges, using AHSV or BTV antigen capture ELISAs, did not show the presence of any virus amongst 2286 females in 240 aliquots. Nearly all Gambian equines are reportedly seropositive to AHSV and these results suggest that virus challenge from Culicoides vectors may be a factor in the health of Gambian horses and donkeys.

Observations on the prevalence of trypanosomosis in small ruminants, equines and cattle, in relation to tsetse challenge, in The Gambia.

The prevalence of trypanosome infections in Djallonké sheep and West African Dwarf goats at different sites in The Gambia showed a significant, positive correlation with contemporary assessments of tsetse challenge. A similar correlation was observed in village N'Dama cattle which showed comparable prevalence values in the same areas. Trypanosome prevalences also tended to be higher in horses and donkeys in areas with high tsetse challenge compared with sites with relatively few flies. A ranking of the numbers of tsetse blood-meals from cattle, small ruminants and equines (1:0.06: > 0.03) corresponded with the estimated biomass of these livestock groups (1:0.09:0.05). Observations on the grazing ranges of livestock showed that, while cattle foraged widely into tsetse-infested habitat, sheep, goats and donkeys remained closer to the villages. This difference indicated that, under the management system practised in The Gambia, small ruminants and equines were probably exposed to a lower level of tsetse attack than cattle.

Tsetse-trypanosomiasis challenge to village N'Dama cattle in The Gambia: field assessments of spatial and temporal patterns of tsetse-cattle contact and the risk of trypanosomiasis infection.

The severity of the trypanosomiasis problem in a particular location is traditionally assessed in terms of a challenge index-the product of some measure of tsetse abundance and infection-rate-which is assumed to be proportional to the force of infection. However, this index masks variation in the force of infection between herds and among individuals within herds. It is also not comparable between sites since the relative abundance of tsetse to hosts may vary. We have studied spatial distribution of herds of cattle in relation to tsetse in The Gambia and calculated an index of challenge based on the ratio of vectors to hosts over the livestock ranging area. This index is strongly correlated with estimates of the force of infection calculated from the incidence of infection in susceptible zebu; and it provides information on heterogeneity in exposure of different herds to tsetse.

Productivity of trypanotolerant cattle kept under traditional management conditions in The Gambia.

The productivity of trypanotolerant N'Dama cattle, kept under traditional management conditions in The Gambia, West Africa, was assessed by the regular, monthly collection of health and production parameters in two study areas. The study areas were selected because of differences in tsetse challenge. Performance traits were used to build up an index to estimate the productivity of village N'Dama cattle. The productivity index per 100 kg cow maintained per year varied from 37.2 kg in the study area of Keneba village (with a low tstse challenge) to 21.4 kg for cattle kept near the villages of Tuba and Sambelkunda, an area which had a high tsetse challenge. Average age at first calving was 4.5 or 5.0 years depending on the study area, calving intervals were 623 or 703 days and there was an average 12% loss of body weight in adult females during the dry season. The productivity indices of village N'Dama cattle in The Gambia compare favourably with similar indices for trypanotolerant and trypanosusceptible breeds elsewhere in Africa, and show that even under harsh conditions and with high tsetse challenge, they are able to effectively produce milk and meat for the rural population.

Cattle-tsetse contact in relation to the daily activity patterns of Glossina morsitans submorsitans in The Gambia.

The daily flight activity patterns of one of the main vectors of animal trypanosomiasis in West Africa, Glossina morsitans submorsitans, were assessed using four different methods. Results from all the methods showed that there was some flight activity nearly every hour in all seasons but they differed in the level of contact between grazing cattle herds and G.m.submorsitans. In the late dry season, trap data indicated that there was negligible activity from midday to late afternoon, whereas observations of tsetse contact with cattle herds or hand-net collections on herd followings showed no fall in attack rates, on the cattle by G.m.submorsitans. Differences between trap and animal-baited collection data may be attributable to the type of G.m.submorsitans sampled by each method. Male G.m.submorsitans captured by traps were more fat depleted than those caught on ox-baited flyrounds or by hand-net collections on herd followings. All methods showed that male G.m.submorsitans were most fat depleted in the late dry season and least in the early dry season. It was concluded that the traps were mainly sampling the spontaneous flights of G.m.submorsitans. Hunger and endogenous rhythms increase the likelihood of spontaneous flights towards dusk, particularly in conditions such as those at midday in the very hot, late dry season. However, the presence of cattle herds in infested habitats probably activated nearby G.m.submorsitans and the continual movement through the grazing areas ensured contact with tsetse throughout grazing.(ABSTRACT TRUNCATED AT 250 WORDS)

Cattle migration and stocking densities in relation to tsetse-trypanosomiasis challenge in The Gambia.

The local migration of village N'Dama cattle between two study sites, Niamina East and Bansang, 40 km apart in the inland region of The Gambia, West Africa, is described. The consequences of seasonal variations in local stocking densities on the epidemiology of African animal trypanosomiasis are reported. Tsetse abundance at each study site was monitored throughout the study period from trap catches, and cattle censuses at each site were carried out on a monthly basis. Detailed ecological, productivity and health data, including dietary intake and trypanosomiasis prevalence, were collected from selected study herds resident at the two sites and from a third group of (four) herds that migrated annually between the two areas to spend the late dry season period in Niamina East. It was shown that the migration strategy allowed migrants to maintain a high level of green grazing in the diet throughout the year. Cattle were moved to the area of highest tsetse density in the region to obtain this grazing, but it appeared that individual risk of trypanosome infection was diminished by a dilution effect created by locally high livestock densities. Trypanosomiasis prevalences in resident cattle at Niamina East were best correlated with the tsetse catch/trap/day 2 months previously, once this index of tsetse abundance had been corrected to allow for changes in relative stocking density.

Trypanosomiasis in cattle in Gambia: incidence, prevalence and tsetse challenge.

The incidence of trypanosome infections, measured by a Berenil Index in experimental herds of 10 Zebu and 10 N'Dama cattle, was compared with tsetse challenge and with the prevalence of parasitaemia in local N'Dama at three villages in Gambia. Tsetse challenge was more strongly correlated with the incidence of parasitaemia in the Zebu than in the N'Dama. There was a strong correlation between prevalence and incidence of infection in the N'Dama. There was no correlation, however, between prevalence of infection in cattle and tsetse challenge unless the data were offset by 3-5 months. The Berenil Index in the Zebu increased at about twice the rate as in the N'Dama under corresponding levels of challenge. It is concluded that whereas incidence of infection in susceptible animals is best measured independently, it can, under stable conditions, be inferred from an assessment of tsetse challenge.

An analysis of survey measurements of tsetse challenge to trypanotolerant cattle in relation to aspects of analytical models of trypanosomiasis.

The recent development of analytical models of trypanosomiasis has increased the general applicability of models to the strategic control of the disease. An analysis of detailed data on tsetse abundance and infection rates and trypanosome prevalence in village-based trypanotolerant cattle over 4 years in The Gambia showed that seasonal patterns of abundance in Glossina morsitans-infested areas were consistent, and that the rates of trypanosome infection remained relatively unchanging. However, there were two distinct seasonal trypanosome prevalence patterns in cattle, with peaks occurring either in May/June/July or November/December. The peaks of trypanosome prevalence therefore occurred either 4 months before or after the times of peak challenge from G. morsitans, not 1 or 2 months after as predicted by analytical models. In G. palpalis-infested areas there was little seasonal variation in abundance or trypanosome infections, but peak trypanosome prevalence still occurred mostly in June/July. Despite the incongruity between the months of peak prevalence and challenge, the overall annual prevalence rates and tsetse challenge indices showed a significant linear relationship. It is concluded that existing analytical models need to be refined to take into account trypanotolerance and the various influences on the expression of this trait.

Watering sites in Glossina fuscipes habitat as the major foci for the transmission of Gambiense sleeping sickness in an endemic area of southern Sudan.

In an area of endemic gambiense sleeping sickness in southern Sudan, the vegetation around 40 wells was categorised in terms of potential habitat for the vector, Glossina fuscipes, and the probability of repeated man/fly contact. These observations were related to the results of sleeping sickness surveys including the use of serodiagnostic (ICHA and CATT) tests which allowed the detection of the great majority of cases. Riverine woodland and gallery forest were the primary habitat of G. fuscipes and 1286 people using wells in this vegetation had an 11.1% infection rate including parasitological, clinical and serological cases. In contrast, 638 people using wells in open situations where the presence of G. fuscipes was unlikely, showed a significantly lower (4.5%) infection rate. These observations provide a basis for planning localised tsetse control, using, for example, insecticide impregnated targets, co-ordinated with mass survey and treatment of the human population.

Improved identification of Nannomonas infections in tsetse flies from The Gambia.

Trypanosomes from 36 midgut infections were isolated in procyclic culture from Glossina morsitans submorsitans and G. palpalis gambiensis in The Gambia. Twenty-eight stocks (78%) were identified using DNA probes specific for: (a) Trypanosoma (Nannomonas) congolense savannah type, (b) T. (N.) congolense riverine-forest type, (c) T. (N.) simiae and (d) Trypanozoon, T. simiae and savannah type T. congolense were found only in G.m. submorsitans while the riverine-forest type T. congolense was restricted to populations of G.p. gambiensis from two isolated areas of relict forest: one Trypanozoon stock was isolated from G.m. submorsitans. T. congolense accounted for only 17% of all Nannomonas infections, as identified by dissection, in G.m. submorsitans. This re-emphasises the importance of differentiating infections below the subgeneric level when estimating challenge to domestic animals. T. simiae could not be distinguished from T. congolense by the arrangement of trypanosomes in the fly proboscis. The 8 stocks which were not identified by DNA probes were separated into two groups on the basis of hybridization with total DNA probes and the cycle of development in experimental tsetse. One group of four isolates, all from G.m. submorsitans, was a new kind of Nannomonas which appeared to be common and widespread in The Gambia. The second group, which was found only in G.p. gambiensis, had a stercorarian cycle of development, maturing in the hindgut, and was morphologically similar to insect forms of the crocodile parasite T. grayi.

Isolation and cultivation in vitro to the infective, metacyclic stage of Trypanosoma (Nannomonas) simiae from Glossina morsitans submorsitans.

Two separate trypanosome isolations were made from a single Nannomonas-infected Glossina morsitans submorsitans from The Gambia. Inoculation of a piglet with the infected hypopharynx produced an infection with Trypanosoma simiae. DNA was isolated from the bloodstream forms to prepare a probe specific for this species. Trypanosomes isolated from the fly midgut were frozen in liquid nitrogen and then cultivated in vitro. Amplification of this population and elimination of a yeast contaminant were achieved by two passages through laboratory G. m. morsitans. Further cultivation in vitro resulted in the production of epimastigotes and, later, metacyclic forms. Two pigs inoculated with cultivated metacyclic forms developed infections with atypical, relapsing parasitaemias and extended survival time. Neither the metacyclic forms, nor bloodstream forms derived from them, infected calves. The identity of various stages of the in vitro cultivated, procyclic-derived stock was confirmed morphologically and with the T. simiae-specific DNA probe.

Use of DNA probes to identify Trypanosoma congolense and T. simiae in tsetse flies from The Gambia.

Species- and strain-specific DNA probes were used to identify patent midgut infections in Glossina morsitans submorsitans and G. palpalis gambiensis captured at four sites in The Gambia. 52% of mature Nannomonas infections and 12% of immature infections were identified. Trypanosoma (Nannomonas) simiae accounted for the majority of identified infections in G.m. submorsitans, indicating the importance of distinguishing this species from the closely related T.(N) congolense when assessing the trypanosomiasis challenge to cattle. Both the savannah and riverine-forest groups of T. congolense were present, although the riverine-forest form was found only in G.p. gambiensis at Pirang, an isolated area of forest. Two-thirds of the samples remain unidentified by probes specific for: Trypanozoon; T. congolense savannah, riverine-forest and Kenya coast forms; T. simiae; and T. vivax, probably owing in part to low numbers of trypanosomes. However, the failure to identify several heavy Nannomonas infections, strongly suggests the presence of a further, as yet unknown, kind of Nannomonas.

The feeding habits of the tsetse, Glossina pallidipes Austen on the south Kenya coast, in the context of its host range and trypanosome infection rates in other parts of east Africa.

The results of blood-meal identifications for 651 Glossina pallidipes from 5 subpopulations near the Kenya coast south of Mombasa, and one, 70 km inland, are presented. Bushpigs and/or warthogs were important hosts for G. pallidipes at all sites. Other major hosts included elephant, buffalo and bushbuck where they were present, and on a dairy ranch nearly 30% of feeds were taken from cattle. There was a general relationship between the numbers and diversity of wild herbivores and the abundance of G. pallidipes. These results are discussed in relation to published data on feeding patterns and trypanosome infection rates for G. pallidipes from other parts of East Africa. Overall, there are significant correlations between the proportions of bovid feeds and T. vivax infections. Bovid-feeding G. pallidipes populations with high T. vivax infection rates in south-east Uganda and western Kenya contrast with the coastal, suid-feeding populations with low T. vivax rates. These characteristics are presented as clines extending across East Africa.

Studies of house-entering habits of mosquitoes in The Gambia, West Africa: experiments with prefabricated huts with varied wall apertures.

The house-entering behaviour of nocturnal mosquitoes was studied in The Gambia. Mosquitoes were captured as they attacked man in the open and in experimental huts which comprised 1.8 m cube frames with corrugated iron roofs and plywood walls of various heights. Catches of all species were similar in the open and in a roofed, but unwalled, hut frame. The mosquitoes taken in catches in unwalled huts and others with wall heights of 0.6, 1.2 and 1.7 m (giving an 8 cm eaves-level entry slit) fell into two categories. The first group, which included the endophilic species Anopheles gambiae Giles s.l., An. melas Theobald and Mansonia spp. were only slightly affected by increasing wall height, but the second group, including the exophilic mosquitoes Aedes spp., An. pharoensis Theobald, Cx poicilipes (Theobald) and Cx thalassius Theobald showed a very marked progressive exclusion. In comparisons of catches in two huts with 8 cm entry slits at eaves or ground level, large numbers of An. pharoensis found access through the ground level entry but not at eaves level. No consistent difference could be demonstrated for other species. It is concluded that the house-entering behaviour which distinguishes endophagic mosquito species includes at least two distinct responses: flight upwards to eaves level and the passage from outside to indoors. It is also suggested that house entry as a component in host-seeking behaviour and indoor resting are distinct, but not necessarily exclusive, behavioural traits.

Observations on age-grouping the tsetse fly, Glossina pallidipes, by wing-fray and ovarian dissection, on the south Kenya coast.

Female Glossina pallidipes from biconical trap collections at five localities on the south Kenya coast were age-grouped by wing-fray and ovarian condition, and their trypanosome infection rates determined. From simultaneous application of the two age-grouping methods to 991 female tsetse, 17 reproductive age-categories were recognized. Separation into these categories was supported by a highly significant correlation between estimated age and proportion of tsetse uninfected by trypanosomes. Limitations of current age-grouping methods for tsetse are discussed.

The attractiveness of some birds and mammals for mosquitoes in The Gambia, West Africa.

In separate experiments in a savanna area of The Gambia, mosquitoes attacking chickens, pigeons, ducks, goats, a man and a calf were compared in baited traps. Anopheles melas and Culex thalassius were common at all baits, but An. melas predominated only in the catches from mammals, where it formed 60-75% of the total catch. Aedes spp., mainly Ae. albocephalus and Ae. punctothoracis, and C. tritaeniorhynchus were a significant component in the mammal-baited catches but were poorly represented at avian baits. In contrast, C. invidiosus, C. neavei, C. tigripes and C. weschei made up a major part (20-32%) of the catches only in the bird-baited traps. Freshwater An. gambiae, probably An. gambiae s.s., were separated from An. melas on palpal index measurements. An. gambiae was taken in significantly larger numbers at man compared with the other baits where few were identified, demonstrating the extreme anthropophily of this species.

A quantification of the risk of trypanosomiasis infection to cattle on the south Kenya coast.

The number of trypanosome-infected bites received by cattle grazed around an 8 ha area of forest harbouring a semi-isolated population of Glossina pallidipes Austen was estimated. The absolute size of the tsetse population was determined by mark-release-recapture techniques, the tsetse host range by the identification of blood-meals, and trypanosome infection rates by dissection of samples of tsetse. Feeding frequency was estimated and the number of cattle present was known. It was estimated that each cow received, from G. pallidipes, one infective inoculum of T. congolense every 5.8 days during the first experiment and 5.0 days in the second. For T. vivax results were 3.2 and 79.1 days, respectively.