Airborne

As the plane takes off from Mali's capital city airport, Bamako, the cabin crew walk smartly down the aisles spraying insecticide as they go. Passengers avoid breathing in too deeply, despite the assurance that they, unlike any stowaway insects, will come to no harm. Despite the precautions, frequent, rapid transport of people and freight has multiplied by many times the risk of the spread of insect-borne diseases. Rift Valley fever is no exception.

First recognized in eastern Africa, in the region from which its name derives, Rift Valley fever is now endemic, in restricted sites, throughout much of sub-Saharan Africa. Alarmingly, for the first time, there is an outbreak outside Africa. In September, more than 30 people in Yemen were reported to have died, with a similar number dying from the disease in Saudi Arabia's Jizan province which borders Yemen. And authorities are remaining vigilant in the West African countries of Mauritania, Senegal and Mali following an outbreak in Mauritania two years ago.

Spread by mosquitoes, Rift Valley fever affects cattle, sheep, goats, pigs, camels, buffalo and humans. It can also be spread directly through aerosols, a route of transmission to which people working with, or slaughtering, animals are particularly susceptible. The symptoms are similar to flu and are usually non-fatal although the fatalities from the latest outbreaks are especially worrying. In livestock, the principal warning sign is mass abortions by pregnant sheep and an exceptionally high mortality rate in new-born sheep and goats. Older animals, especially cattle, generally recover.

Factors such as rainfall and wind direction, which affect the density and distribution of mosquitoes, also affect the risk factor from the disease. Veterinary authorities may therefore get some advance warning of when, and where, the risks are greatest. This is important because vigilance is the safest method of control. Vaccination is not recommended for livestock although both live and inactivated vaccine are available. This is for two reasons. Firstly, the live vaccine retains some pathogenicity and is a known cause of abortions in livestock. In addition, there is a real possibility that mosquitoes may transmit the vaccine virus from inoculated animals to other species, including man, and thereby cause disease. Secondly, the use of either live or inactivated vaccine could mask or confuse detection of disease presence and would also make 'freedom from disease' certification for trade purposes impossible.

Vigilance is therefore the key to early warning, early reaction and control. In a recently established technical cooperation programme with FAO, veterinary authorities in Mali, Senegal and Mauritania are increasing surveillance for early signs of Rift Valley fever by using sentinel herds where the risk is high. Every month, during the current rainy season, veterinary officers are visiting the selected sentinel herds of cattle and small ruminants to conduct clinical examination and take serum samples for laboratory diagnosis. Blood tests from animals in the Delta du Niger region of Mali have revealed that the disease is circulating within the sentinel herd. Although further investigation has so far not revealed any clinical signs among animals or humans, the presence of the disease at sub-clinical level reinforces the need for surveillance. If an outbreak occurs, early detection means earlier control of livestock movement, more effective quarantine and less risk of the disease spreading to other regions. But, as Karim Tounkara, Head of Diagnosis and Research at Mali's Animal Health Laboratory points out, "Zero risk does not exist. We now call our planet 'a global village' so I cannot say that all other countries or continents are safe. Can we be 100% sure that we will kill all the mosquitoes in the plane?"

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