West Nile Virus (WNV)

A re-emerging insect-borne pathogen

West Nile Virus is the most widespread and well travelled insect-borne virus in the world. It is a small Flavivirus with two genetic lineages, I and II.

West Nile Virus is usually transmitted between wild birds by bird-feeding mosquitoes and spread by migratory birds. It can cause outbreaks with large numbers of avian deaths. Humans and horse are incidental hosts and acquire the infection from mosquitoes, mainly Culex univitattus, which have fed on infected birds.

Lineage II viruses are found in Africa and Madagascar. Historically lineage II isolates have been associated with mild disease, however during 2007/2008 new evidence emerged that lineage II viruses can cause neurologic disease in horses in South Africa.

In horses that develop WNV – medicated encephalitis the clinical signs may include fever, which is not a consistent sign. Other signs may include a staggering gait, weakness, paralysis of the hind legs, difficulty with eating and drinking and muscle fasciculations. The signs may progress to the point where the horse is unable to stand and may die. Outbreaks follow wet warm climatic conditions which favour breeding of mosquitoes. This disease typically occurs during summer.

Control of the disease is dependant on vaccination and standard mosquito control measures such as insecticides, repellents, vector-resistant housing and removal of stagnant water.

Vaccination Schedule
1st vaccination from 5 months of age
2nd vaccination 4 – 6 weeks later
Re-vaccination annually
48 hours rest is recommended after vaccination

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