About West Nile Virus

About West Nile Virus

We have all heard the shocking news about our recently crowned Champion stallion, for the 5th year in a row, Jet Master suffering from this virus.

Here are some facts, with the assistance of Dr Sarah Seitz (Bsc DVM) about what the virus is. West Nile Virus or WNV is a mosquito borne RNA virus that affects the central nervous system and can cause severe neurological disease (meningitis & encephalitis).

The disease is naturally transmitted between birds via mosquitoes. Humans and horses are “dead end hosts”. This means that humans and horses can get the disease if bitten by a mosquito that previously bit an infected bird, however, the levels of virus in the blood of humans and horses does not reach high enough levels to be able to be transmitted by mosquitoes. In other words, if a mosquito bites an infected horse or human, the mosquito cannot transmit the disease. It can only transmit the disease if it bites an infected BIRD. Person to person or horse to horse transmission through contact does NOT occur. (Need a mosquito that has bitten an infected bird).

WNV was first discovered in a febrile woman in the West Nile district of Uganda in 1937, and was first discovered in the western hemisphere in 1999 in the Bronx zoo in New York.

99% of infections caused by WNV are asymptomatic – the body’s immune system quickly clears the virus. Only in 1% of cases, does neurologic disease occur. WNV is found throughout most of the world including Africa, Asia, Europe, the Mediterranean, the Middle East, Australia and the Americas. The disease is carried by migratory birds who in turn carry WNV into new areas.

WNV can be destroyed by many disinfectants including sodium hypochlorite solutions (bleach dilutions), 3% hydrogen peroxide, and 8% formaldehyde. It is also inactivated by UV light, gamma irradiation and heat. Reducing the risk of exposure to the mosquitoes that may carry the virus is also an important part of prevention. It involves eliminating mosquito breeding sites to reduce the number of hatching mosquitoes, and to reduce exposure to adult mosquitoes. Mosquitoes breed in stagnant water, so reduction of breeding sites involves eliminating stagnant water sources.

Human illness can be classified into 2 forms: West Nile Fever (mild and flu-like) and West Nile Neuroinvasive disease (severe neurologic disease).

The incubation period in horses is 3-15 days (time from infection to manifestation of signs). Infection with West Nile Virus does not always lead to signs of illness in people or animals. Horses appear to be a species that is susceptible to infection with the virus. In horses that do become clinically ill, the virus infects the central nervous system and may cause symptoms of encephalitis. Clinical signs of encephalitis in horses may include a general loss of appetite and depression. Most infection in horses is subclinical (asymptomatic). In symptomatic cases, horses may display anorexia, & neurologic signs.

West Nile encephalitis describes an inflammation of the central nervous system. Neurologic signs include ataxia (stumbling, falling), weakness, paralysis of one or more limbs, impaired vision, inability to swallow, teeth grinding, aimless wandering, circling, facial paralysis and convulsions. Horses may have one or more of these signs of varying severity.

Fever may be present but not in all cases. If a stallion does develop fever, fertility for the following 60 days may be compromised.

It is important to note that not all horses with clinical signs of encephalitis have West Nile encephalitis. Certain other diseases can cause a horse to have symptoms similar to those resulting from infection with West Nile Virus. If you are concerned that your horse may be exhibiting signs of encephalitis, please contact your veterinarian. Laboratory tests are necessary to confirm a diagnosis. It is important to diagnose WNV because infection is an indication that mosquitoes carrying the virus are in the area and need to be eliminated.

Some horses do not survive, but horses that do recover may not necessarily return to full function. 10-20% may have residual defects such as weakness, exercise intolerance, muscle atrophy or behavioral changes. There is no cure for WNV, and treatment consists of supportive care.

A WNV vaccine for horses is available, whilst vaccination of horses is not a guarantee of protection against infection it is the best method of prevention.

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