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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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