West Nile Virus

Introduction

This is a relatively newly discovered virus, of the family flaviviridae, that is transmitted by mosquitoes. Reservior hosts are birds and the final hosts are horses and humans, hence this disease is zoonotic.

This virus originated in Africa as the name suggests, but it is now considered that two strains exist; the Old world, avirulent strain and the New world virulent mutant, which is mainly found in the USA. The virus originally entered the USA from Africa via mosquitoes or birds and then remained in the New York area as there is much stagnent water here for mosquitoes to breed. Over 200 people have died from the disease, as well as many birds. One in seven UK birds have the antibody to the virus but no clinical disease or deaths have been seen from it. This is presumed to be because the strain in the UK is currently avirulent and also mosquitoes are too uncommon in the UK for the disease to take hold and cause human illness.

Signalment

Horses and Man. Humans can be infected by the disease and show no clinical signs. Predisposing factors to making the disease much more severe include immunosupression e.g. HIV, AIDs or chemotherapy, old age or very young children and pregnant women.

Clinical Signs

The virus can cause no clinical signs whatsoever, or it can cause mild signs such as abdominal pain, pyrexia, diarrhoea, inappetance, lethargy, vomiting, enlarged lymph nodes and weakness. In humans it may cause the above, plus headaches, muscle aches, rash and nausea. In severe cases, the virus can cause West Nile Encephalitis, in which an aseptic encephalitis with concurrent fatalities may occur.

Diagnosis

Clinical signs plus time of year and signalment of affected animal or human may be indicative of the disese.

Control

Control measures include vector control (mosquitoes), such as draining away standing water, using ectoparasiticide sprays, fly rugs on horses and stabling horses at dawn and dusk (when mosquitoes most prevalent).

There is a vaccine being developed for horses, as the risk to the UK is more severe with introduction of passports and horses travelling all over the world. The disease is still currently notifiable in the UK.

References

Bridger, J and Russell, P (2007) Virolgy Study Guide, Royal Veterinary College
Knottenbelt, D.C. ,A Handbook of Equine Medicine for Final Year Students University of Liverpool
Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition), Merial
Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) Equine Internal Medicine (Third Edition), Saunders