10 December 2012

West Africa: Rain May Help Forecast Rift Valley Fever in West Africa

Dakar — Efforts are underway to forecast Rift Valley fever (RVF) outbreaks in West Africa, where such work has so far been challenging, as Mauritania suffers a deadly outbreak of the disease.

The outbreak has comprised more than 30 cases in six regions of the country and at least 17 deaths, according to a WHO release last month.

As well as being potentially fatal to humans, RVF can cause severe disease in animals, resulting in significant economic losses due to death and abortion among infected livestock. Outbreaks have previously also been reported in North Africa and the Gulf countries.

The Mauritanian outbreak appears to be have been caused by transmission from infected animals.

But the fever can also be transmitted by mosquito bites, and the abundance of those mosquitoes is sensitive to environmental conditions such as local weather patterns, according to a study published in PLOS Neglected Tropical Diseases.

However, despite outbreaks of the disease in East Africa having been directly correlated to the rainfall that is key to mosquito reproduction, the precise factors that cause West African outbreaks remain poorly understood, the paper says.

To try to rectify this, the study used a model to predict the abundance of the two main mosquito species involved in RVF transmission in Senegal: Aedes vexans and Culex poicilipes. The model did this by combining a mosquito population model with one that used rainfall to estimate the surface areas of ponds in which the insect breeds.

The modelling reveals that previous RVF outbreaks in Senegal occurred in years when both mosquito species were present simultaneously and in high densities because abundant, regular rainfall throughout the rainy season had made conditions favourable for their reproduction.

Yaya Thiongane, head of the National Laboratory of Livestock and Veterinary Research, Senegal, and one of the study authors, says the results show that an early-warning system based on real-time rainfall data could be developed to help predict outbreaks of the disease.

Kaba Cissé, researcher at the University of Nangui-Abrogoua, in Côte d'Ivoire, says the study is an important step towards controlling RVF in West Africa.

But he calls for greater efforts to encourage people to change how they farm livestock to minimise the risk of them catching the disease from infected animals.

"This is possible if people modify their rearing methods so they no longer live in a common environment with their animals," he tells SciDev.Net.

The last Mauritanian outbreak of RVF was in 2010, when it reportedly caused 68 serious cases and 13 deaths. Cheickn Ould Bakar, a resident of Graret Levrass region, tells SciDev.Net that, as well as human fatalities, the fever led to the deaths of tens of livestock animals, mainly goats and camels.

This article has been produced by SciDev.Net's Sub-Saharan Africa desk.

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