West Nile Virus: From Africa to America

In 1999, West Nile virus was first detected in the New York area of the USA. Several species of birds in the local zoo succumbed to the virus. There were also 59 cases of meningoencephalitis disease diagnosed among New York residents, and seven people died. Since then, the virus has spread right across the USA. In 2002, around 4,000 Americans were diagnosed as having contracted the virus and 259 died.

Origins

West Nile Virus was first isolated in 1937 from a woman in Uganda who was suffering from a high fever, although it is now known to be endemic in much of Africa as well as the Middle East and central and southern Europe. Recent genetic analyses have shown that there are actually two forms of the virus. Isolates from southern Africa tend to cause a more mild disease in human patients compared to isolates from the genetically distinct strains found further north. It was a strain of this second form of the virus that was introduced into the USA. Over 40 mosquito species can transmit WNV, including its main vector, Culex univittatus. This species breeds in shallow water and often feeds on birds' blood.

Birds, such as geese and storks, are the main hosts of the virus and most transmission to humans is thought to be caused by mosquitoes that have previously fed on an infected avian host. In the USA, many species of bird are susceptible to the disease and crows, in particular, are often killed by the virus. Horses can also become infected and could act as reservoirs of infection. In human hosts, however, the virus does not reach sufficiently high levels in the blood for feeding mosquitoes to acquire an infective dose. Human to human transmission via mosquitoes, therefore, is unlikely. For these reasons, most cases of West Nile fever have been associated with rural populations living close to lakes which lie on waterfowl migration routes.

In southern Africa, WNV is widely endemic in areas where Culex univittatus and bird hosts are present. Even so, human infections tend to be sporadic, with between five and fifteen cases reported each year in South Africa. However, after heavy rains and warm temperatures have boosted vector populations, large epidemics have occurred. In 1974, for example, several thousand people over a 2,500 km2 area of the Karoo and Northern Cape Provinces became infected and, in one town, 1,700 people sought medical attention. Luckily, because the southern African strain is comparatively mild, no human deaths were recorded. Tests on wild birds, though, detected antibodies against WNV in 53% of samples.

Impact on livestock

Credit: Wendy Craig

As far as the agricultural effects of the virus are concerned, perhaps its biggest effect to date has been on Israeli geese flocks. In 1997, WNV was found infecting commercial geese flocks for the first time. In both 1998 and 1999, about 20% of the countries 60 flocks were affected and around 5000 birds either died or were put down. The Carmargue region of southern France, well known for its wetlands, migrating birds and semi-wild white horses, has also seen the recent re-emergence of WNV. Between 1962 and 1966 about 50 horses became infected, although the disease seemed to die out naturally thereafter. However, 76 more cases were confirmed in 2000. In Morocco, too, 42 of 94 infected horses died during a 1996 epidemic, and in the USA, as of December 1, 2002, the Department of Agriculture had reported a total of 14,358 equine cases of WNV-illness. Experimental studies have shown that the virus could be pathogenic to sheep, and in South Africa, WNV has also been implicated in the premature death of young ostriches on ostrich farms, although it is just one of a number of possible pathogens being investigated.

Globalisation

Genetic analyses have shown that the WNV introduced into the USA in 1999 is very similar to that which caused the disease in Israel's geese flock. It is likely, therefore, that an infected person travelled from the Middle East to the USA and, once there, was bitten by a mosquito which subsequently transmitted the virus into the local bird population. WNV has since been recorded from a patient on the Cayman Islands, south of Cuba, and from several bird samples collected in Mexico. Given the propensity of the virus to be spread by migrating birds and its ability to adapt to new mosquito hosts, it is probably only a matter of time before it is also discovered in South America. Although the disease causes few problems for livestock farmers (except perhaps those rearing horses, geese and possibly ostriches), resource-poor farmers, especially those living near lakes on bird migratory routes, have cause to fear the debilitating effects of several days' illness.

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