West Nile Virus Case Definition

“Probable,” “presumptive,” “suspected,” and “confirmed.” All of these words are used to describe equine cases tested for West Nile virus (WNV). The cases might be “confirmed” on a local, state, or national level. What do these descriptions mean, and why don’t the totals reported by state and national agencies always match?

“It is easy to get confused about what different sources of information are calling ‘cases’ of West Nile virus infection in horses since they don’t always agree,” explained Eileen Ostlund, DVM, MS, PhD. Ostlund is head of the Equine and Ovine Viruses Section of the Diagnostic Virology Laboratory at the United States Department of Agriculture’s (USDA) National Veterinary Services Laboratories (NVSL). “The USDA Veterinary Services definition is quite stringent and was designed to minimize the possibility of reporting ‘false positives.’ Some states or other jurisdictions use slightly different criteria to classify cases,” she added.

Any NVSL-confirmed equine WNV case has clinical signs that include ataxia (inability to coordinate movement, stumbling, staggering, or wobbly gait) or at least two of the following: circling, hind limb weakness, inability to stand, multiple limb paralysis, muscle fasciculation (twitching), proprioceptive deficits (loss of position sense), blindness, lip droop/paralysis, teeth grinding, or sudden death.

Also, the case must meet at least one of a list of qualifications that involve detection of the virus or antibody in specific assays of serum, cerebrospinal fluid, or tissue. Cases can also be deemed “probable” if they meet specific qualifications set by the NVSL.

According to Randy Crom, DVM, Senior Staff Veterinarian for USDA’s Animal and Plant Health Inspection Service, there are additional reasons for differences between USDA and state reports of WNV cases.

“Some WNV specimens never get sent to USDA so we don’t know about them,” said Crom. “Also, states run different tests and/or get different results, and therefore classify cases differently (than the USDA, partly related to their different qualifications for a ‘case definition’). Additionally, data on many ‘cases’ aren’t specifically reported to USDA. For example, sometimes we aren’t provided specific clinical signs on a horse.”

The result of these discrepancies is an array of different figures for the confirmed case totals. Regardless of the numbers, if individuals are in path of the disease’s movement westward, West Nile virus is an imminent threat, and horse owners should be vigilant in following WNV-prevention strategies.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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