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WEST NILE FEVER

West Nile fever is caused by the West Nile virus (WNV), a mosquito-borne flavivirus and a human, equine, and avian neuropathogen. Birds are the natural reservoir host, and the virus is maintained in nature in a mosquito-bird-mosquito transmission cycle. The virus has also been isolated from dogs.

A serological survey of dogs from the highveld region of South Africa showed that 37% (138 of 377) had neutralising antibodies to WNV (Blackburn et al., 1989). Two of three dogs infected with WNV, had a mild recurrent myopathy, but no other abnormalities were detected in the biochemical or haematological tests performed on any of the dogs. It was concluded that dogs do not play an important role in the epidemiology of WNV, but they possibly contribute to the maintenance of the virus in a region.

Epidemiology

West Nile virus has been detected in numerous countries of the Old World, including Africa, Near East and Asia. In Eurasia, WNV has been shown to cause meningoencephalitis in humans and horses (e.g., in Southern France, Romania, Italy and Russia). It first occurred in 1999 in New York City, but has extended its range throughout much of the eastern parts of the USA, and entered South-Central Canada and the Caribbean region.

Clinical signs

A study with experimentally infected dogs and cats in 2004 showed viraemia of different extent and no occurrence of clinical signs in dogs and mild, non-neurologic signs in some cats (Austgen et al., 2004). In another study, only serum creatine kinase was altered in infected dogs, suggesting a mild and recurrent myopathy (Blackburn et al., 1989). Generally, WNV infection is suggested not to be associated with clinical disease, especially in dogs, but single case reports of clinical signs and disease have been published (e.g., Lichtensteiger et al., 2003).

Human health risk

Most human infections with WNV are subclinical. If clinical signs occur, flu-like illness can be observed, which is characterized by high fever and chills, malaise, headache, backache and myalgia. In epidemics, fever, a flushed face, conjunctival injection and generalised lymphadenopathy were common. Severe cases exhibit neurological manifestations like meningitis, encephalitis and myelitis.

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