The mosquito Aedes (Aedimorphus) vexans arabiensis as a probable vector bridging the West Nile virus between birds and horses in Barkedji (Ferlo, Senegal).

Active catches of adult females of Aedes vexans arabiensis Patton, (Diptera: Culicidae) Patton by nets or aspirator, were conducted in 2003 and 2004 in the vegetation at the edge of temporary ponds in Barkedji, Senegalese Ferlo area. Two hundred and forty-one engorged females were captured, dissected and the gut content adsorbed on a Whatman filter paper and analysed using the enzyme-linked immunosorbent assay (ELISA) technique to determinate the bloodmeal origin. Results indicated that Ae v. arabiensis fed primarily on mammals, including horses (35.7% of the bloodmeals), but also on birds (10%). Moreover, associations between horses and birds accounted for 42% of the mixed bloodmeals. These results show an opportunistic feeding behaviour and suggest that Ae v. arabiensis is a probable vector bridging the West Nile virus between horses and birds hosts in the Ferlo area.

Use of sentinel chickens to study the transmission dynamics of West Nile virus in a sahelian ecosystem.

During the 2003 rainy season, a follow-up survey in sentinel chickens was undertaken to assess the seasonal transmission of West Nile virus (WNV) in a sahelian ecosystem: the Ferlo (Senegal). The estimated incidence rate in chickens was 14% (95% CI 7-29), with a very low level of transmission between July and September, and a transmission peak occurring between September and October. Comparing these results with the estimate obtained from a previous transversal serological study performed on horses the same year in the same area, the relevance of sentinel chickens in estimating the WNV transmission rate is highlighted. Conventionally adult mosquito populations disappear during the dry season but WN disease was shown to be endemic in the study area. The mechanisms of virus maintenance are discussed.

Exposure of sheep to mosquito bites: possible consequences for the transmission risk of Rift Valley Fever in Senegal.

Rift Valley Fever (RVF) is a growing health problem in West Africa. In northern Senegal, the candidate vectors of this arbovirosis are Aedes (Aedimorphus) vexans Meigen and Culex (Culex) poicilipes Theobald (Diptera: Culicidae). Domestic ruminants are the reservoirs of the virus. A study was undertaken during the 2002 rainy season to assess spatial and temporal variations in exposure to mosquito bites in sheep herds, and to evaluate the possible consequences on the risk of RVF transmission to sheep. Mosquitoes were collected with sheep-baited traps. The number of Ae. vexans females (the predominant species during the 2002 rainy season) trapped per trap-night was the dependent variable in statistical analyses. The trapping periods were divided into six series of two to five consecutive days, from July to November 2002. Three temporary ponds were selected according to their ecological features: depth, bank slope, size and vegetation cover. Traps were laid on the pond bank and in the nearest available compound, close to the sheep night pen. Data were analysed using mixed-effects Poisson models. The explanatory variables were the trapping period, the pond, and the capture site. The exposure to mosquito bites varied according to the pond type, suggesting that the risk of transmission was spatially heterogeneous. However, there was no obvious trend in transmission risk due to the effect of the distance from the compound to the pond. The period with the highest exposure was in October, i.e. when transhumant herds left the Ferlo to relocate to their dry-season settlement. It is thus hypothesized that transhumance, the seasonal movements of herds, plays a significant role in the dissemination of RVF virus in the region.

Tsetse challenge, trypanosome and helminth infection in relation to productivity of village Ndama cattle in Senegal.

Data on tsetse fly, and on village Ndama cattle collected over a 4-year period in southern Senegal, were analysed. A total of 431 Ndama cattle in four herds of three villages in the Upper Casamance area of southern Senegal were monitored monthly. Glossina morsitans submorsitans and Glossina palpalis gambiensis are present in the study area. Mean tsetse apparent density was 5.4 flies/trap/day. Trypanosome (Trypanosoma congonlense and Trypanosoma vivax) infection rate in flies was 2.4 (s.e. 0.37)%. Tsetse challenge index was 17.3 (s.e. 4.18). Mean monthly trypanosome prevalence in cattle was 2.5 (s.e. 0.51)%. Highest trypanosome prevalence occurred during the dry season, and animals less than 1-year old were more frequently infected than older animals. The linear relationship between the log10+1 tsetse challenge and the arcsine of the trypanosome prevalence was significant only when mean monthly values of these variables over the 4-year period were used with tsetse challenge preceding infection rate by 3 months. Mean monthly prevalence of strongyle, Strongyloides spp., Toxocara spp. and coccidia were 34.4 (s.e. 0.60), 2.1 (s.e. 0.18), 1.2 (s.e. 0.45) and 15.6 (s.e. 0.47)%, respectively. Calf mortality rate at 1,6 and 12 months of age was 2.1 (s.e. 2.1), 5.2 (s.e. 2.8) and 12.2 (s.e. 3.3)%, respectively. Calving interval (584 s.e. 58 days) was not influenced by trypanosome status of the cow during lactation. Calving interval was shorter by 167 days when the calf died before 1 year of age in comparison to calving intervals for which the calf survived beyond one year. Live weight at birth, 6 and 12 months of age were 15.8 (s.e. 0.54), 48.1 (s.e. 2.56) and 71.1 (s.e. 5.44) kg, respectively. Mean lactation length, total and daily milk offtake were 389 (s.e. 16) days, 231 (s.e. 15) litres and 0.69 (s.e. 0.037) litres, respectively. Trypanosome infection during lactation did not have a significant effect on the amount of milk extracted for human consumption nor did trypanosome status affect calf growth.