“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