[Use of insecticide-treated cattle to control Rift Valley fever and West Nile virus vectors in Senegal]

Abstract

Rift Valley Fever (RVF) and West Nile fever (WN) viruses are transmitted by several mosquito species and share the same vectors in Northern Senegal (West Africa). In absence of an effective treatment and vaccines, vector control remains an alternative method of prevention and control of these vector-borne diseases. The methods targeting adults' pest mosquitoes and malaria vectors which are currently used by the population in the Barkedji area (insecticides treated nets, bombs and copper coil) would not be effective against these vectors because of their exophagic and zoophilic behavior. Thus, we decided to evaluate the effectiveness of insecticide-treated cattle as a method to control these vectors. We evaluated the effects of this treatment on the mortality and the behaviour (attractiveness and engorgement) of the main vectors and subsidiary the whole mosquito fauna. Our study was conducted during September 2005, and between July and November 2006, at Niakha pond located 4 km from the Barkedji village in the Sahelian region of Senegal. A bull-calf was treated with 25 mg/m2 of deltamethrin and compared to an untreated calf of the same weight used as a control. The assays were conducted using two net-traps placed at the edge of the pond from 19:00 PM to 22:00 PM each night for 4 nights per week for 4 consecutive weeks after each treatment. The risk that host- seeking mosquitoes that do not have possibility to feed on cattle might turn to men cohabiting with these cattle was evaluated simultaneously during the bioassay. The deltamethrin treatment led to a significant reduction in the average number of mosquitoes attracted by the treated-calf compared to the control during the first 2 weeks post-treatment both for all species and for the main vectors such as Ae. vexans, Ae. ochraceus, Cx. poicilipes, Cx. neavei and Ma. uniformis. However these means were comparable for the last two weeks post-treatment both for the whole mosquito fauna and the main vectors with the exception of Cx. neavei. The treatment resulted in a significant reduction in the rates of engorged females during the first two weeks post-treatment. Indeed, the females' engorgement rate on the treated-calf compared to the control has been reduced by 57.3% for Ae. vexans, 39.8% for Ae. ochraceus, 76.4% for Cx. poicilipes, 82.4% for Cx. neavei, 84.6% for Ma. uniformis and 64.1% for the whole mosquito fauna. This effect of the treatment was maintained in the second fortnight post-treatment only for Cx. neavei. Mortality rates were higher among mosquitoes collected with the treated-calf compared to the control in the first fortnight of the test. This impact of the insecticide has been maintained in the second fortnight post-treatment excepted for Cx. neavei. However a significant variation of the mortality of the mosquitoes collected with the treated-cattle was observed between the two fortnights. In fact, the mortality rate of the mosquitoes collected on the treated-cattle decreased by 43.8% for the mosquito fauna as a whole, 47.5% for Cx. poicilipes and 82.4% for Ae. vexans, 76% for Ae. ochraceus, and 63.3% for Ma. uniformis. No deviation of host-seeking mosquitoes, from treated calf to men, has been observed since the average number of host-seeking mosquitoes collected on men close to the two calves was statistically comparable. The insecticide-treated cattle may be a good method for controlling RVF and WN vectors and associated mosquitoes in Senegal. However there is a need to develop new formulations or new strategies of application that could improve the duration of the effectiveness of the treatment.