Colorado State College of Veterinary Medicine and Biomedical
Sciences honors students contacted more than 500 owners of affected horses
in both states to gather information about their individual circumstances.
The study was a collaborative effort between the state veterinarians
in Colorado and Nebraska , the Veterinary Diagnostic Center in Nebraska
and Colorado State University 's College of Veterinary Medicine and
Biomedical Sciences. - The estimated case fatality rate for
affected horses in Colorado and Nebraska was 28.6 percent, similar to
reports from other regions of the United States . - All ages of equine
cases can apparently become diseased due to West Nile Virus infection.
Although the average age of equine cases affected in the study was nine
years, the age range for equine cases was from three months to 35
years. - Among the affected cases, 9.4 percent were intact males, 44.4
percent were castrated males and 46.2 percent were females. - Sixty-four
percent of West Nile Virus cases in Nebraska and Colorado were quarter
horses, which is likely because quarter horses are the most common breed of
horse in the Western states. However, many other horse breeds were also
affected. - It appears that both donkeys and mules can also develop disease
due to West Nile Virus. - Only 13 cases were fully vaccinated based on
current recommendations from the vaccine manufacturer, meaning the equines
had two vaccinations separated by three to six weeks followed by adequate
time for the vaccines to be fully effective before the arrival of vector
season. Of these, the survival status of 12 is known and all 12 lived. -
Where vaccination status was available, approximately 47 percent of
the affected cases in the study had received at least one dose of the
West Nile Virus vaccine before becoming infected. Three percent were
vaccinated only after clinical signs were displayed, and 50 percent
were non-vaccinated. - The case fatality rate among the West Nile Virus
cases that had been vaccinated a minimum of one time prior to onset of
signs was 20.3 percent, while 36.6 percent of unvaccinated animals had died
or were euthanized. - Of surviving animals, 82 percent were considered
fully recovered by their owners. In others still showing signs of
infection, decreased stamina was the most commonly reported residual sign,
followed by weight loss and/or loss of
condition.
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