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From The
State Vet’s Office
by Dr. Terry Slaten


West Nile Virus 101

Spring is just around the corner and I am about ready for it. When I think of spring, I think of everything waking up from the winter months. I think of new calves, new foals, green grass, spring rains, mud puddles, mosquitoes, and West Nile Virus. While it is very difficult to predict whether this year will be a very active year for the virus, you do not want to wait to find out before planning your prevention program.

Our world keeps changing and occasionally a new disease moves in and establishes itself. That is the case with the West Nile Virus that we have heard quite a bit about in the press. It had never been diagnosed in the United States before 1999. In 

just over six years it has made itself at home here in the U. S. In 2005, reports of human cases of WNV came from all but six of the continental United States, with birds, horses, or mosquitoes being found infected in those six states.

West Nile Virus (WNV) is a virus that is maintained in a transmission cycle between wild birds and the Culex mosquito. It is what is considered to be a mosquito borne virus. The antibodies to the virus have been found in over 150 species of birds. It does not treat all birds equally. WNV infections are usually fatal among corvids which include crows and blue jays. Other birds, however, may have high and sustained periods of viremia (virus circulating in the blood) yet show little or no signs of clinical disease.

The disease becomes a problem outside the normal cycle between birds and mosquitoes when an infected mosquito bites another human or animal that is susceptible to the virus. That has resulted in antibodies being found in a large array of mammals, a few amphibians, and even an alligator or two. The most common recipients of this virus outside of birds and mosquitoes are humans and equidae (horses, mules, zebras, etc.).

By far the majority of the humans and horses exposed to the virus never develop the encephalitis (inflammation and swelling of the brain) that is associated with the WNV infection. Some research places the rate of infection by the virus at between 10 and 30 per cent in horses. Once a horse develops the disease, about 30 per cent of those cases will be fatal.

In the State of Alabama in 2005 there were 12 horses diagnosed positive for WNV out of 165 samples submitted to our laboratory. You may think that in a state the size of Alabama with the number of horses we have, that 12 is an extremely small number. However, it is quite reasonable to believe that the number of positive animals diagnosed at out laboratory does not represent all of the positive cases. Often horses become extremely sick and the owner chooses to have the horse euthanized and no samples taken or the horse may be found dead and is subsequently buried by the owner.

The clinical signs of WNV are extremely variable. Most often the presenting complaint is neurological abnormality. This may include incoordination, staggering, weakness, paralysis of the facial muscles, depression, sleepiness, or even hyperexcitability. Other common presenting signs that are not neurological in nature are colic, lameness, loss of appetite, and fever. In severe cases the disease progresses to complete paralysis in 7-10 days, and the most common cause of death is euthanasia for humane reasons.

It is worth remembering that another mosquito borne virus, Eastern Equine Encephalitis (EEE), mimics the WNV. EEE is usually more virulent and the case fatality rate is much higher. In Alabama in 2005 there were 37 positive cases of EEE diagnosed in horses. Horses that have EEE infections and begin showing clinical signs have a 70-90 per cent chance that the disease will be fatal.

The simple two word solution for these viruses is simply to AVOID MOSQUITOES. Unfortunately, that is not an option for most of our equine friends. Therefore, it is vitally important to vaccinate your horses for these diseases. And while you’re at it, make sure you vaccinate for tetanus. I have seen too many horses suffering from painful, tragic diseases when vaccinations were and continue to be available. At this point, we continue to recommend vaccination twice yearly for the encephalitis viruses.

Dr. Terry Slaten is Alabama’s Associate State Veterinarian.

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Date Last Updated January, 2006