Colorado State University Equine Veterinarians Release Results from Colorado, Nebraska West Nile Virus Study

Researchers at Colorado State University’s College of Veterinary Medicine and Biomedical Sciences today released results from a research project that summarizes the background characterization of last year’s outbreak of West Nile virus in equids from Colorado and Nebraska.

"The objective of the study was to describe the equine West Nile virus cases in Colorado and Nebraska in order to better understand the progression of clinical disease, clinical signs and clinical outcome," said Dr. Josie Traub-Dargatz, principal investigator of the project. "The study’s results give us a clearer picture of last summer’s outbreak, especially in terms of the numbers of horses affected that were unvaccinated or incompletely vaccinated as well as their survival rate."

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 state veterinarian’s offices had the client contact information on these cases but lacked the staff time to follow up with inquiries about areas such as vaccination status and treatment," said Dr. Tricia Salazar, lead author of the report. "We were able to enlist the assistance of about two dozen Colorado State veterinary students to help us draft a survey, make calls to horse owners and gather the details about their cases to analyze."

The entire report can be viewed and/or downloaded at or from the university’s Animal Population Health Institute Web site at A few of the study’s results are highlighted below.

  • 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.
  • The most common clinical sign associated with West Nile Virus infection identified in the study was an altered gait, including: reluctance to move; stumbling; perceived lameness; ataxia or weakness.
  • With the emergence of West Nile Virus in the United States, owners should immediately consult with a veterinarian if their horse develops any of these signs," said Traub-Dargatz. "Owners should also be alert to identify whenever their horse’s behavior changes, such as appearing lethargic, having a diminished appetite or developing a fever."

According to Salazar, the study suggests that vaccinating equids is one very important step toward preventing the disease in horses.

"There were only a small number of animals in the study that were fully vaccinated, and for those with known survival status, all of them lived," said Salazar. "Even horses that had not received both doses of vaccine several weeks prior to exposure to West Nile Virus were more likely to survive than those that were non-vaccinated."

The Colorado State researchers added that it is also important for equine owners to employ other strategies for prevention of disease including mosquito mitigation, and suggest that owners consult their veterinarians to develop a complete disease prevention plan for West Nile Virus.