West Nile virus infection: Essential facts for small-animal practitioners - Veterinary Medicine
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West Nile virus infection: Essential facts for small-animal practitioners
Your clients may be concerned about this virus in their pets. Are you prepared to answer their questions?


VETERINARY MEDICINE SUPPLEMENT


Diagnosis

West Nile virus-induced disease may resemble many other conditions in dogs and cats, including bacterial meningitis, distemper, rabies, fungal or rickettsial infections, systemic lupus erythematosus, or primary neurologic disease. Rank West Nile virus infection in your differential diagnoses based not only on clinical signs, but in light of the intensity of West Nile virus activity in your region. Among other resources, local information compiled by the U.S. Geological Survey and the CDC can be easily accessed at http://westnilemaps.usgs.gov/. At this site, the avian, veterinary, and human maps will probably be most helpful, because many regions lack good mosquito and sentinel surveillance programs. Counts of West Nile virus-positive avian fatalities have been shown to provide the best estimator of the intensity of viral activity. However, many county health departments have elected to cease testing birds after the first positive result each year, so cumulative avian fatalities are unknown in these regions.


Table 1: Antemortem Diagnostic Testing for West Nile Virus Infection in Small Animals
Antemortem diagnosis (Table 1) of West Nile virus infection can be challenging. West Nile virus is typically already cleared from the bloodstream by the time horses and people present for medical care. So if this is also true in dogs and cats, serum virus isolation is not likely to be effective. Single-sample tests of neutralizing antibody are rarely meaningful because of the high incidence of seroconversions secondary to subclinical West Nile virus infections in pets.22,23 Thus, paired acute and convalescent serologic testing for neutralizing antibody is currently considered the gold standard for antemortem diagnosis of West Nile virus infection. The neutralizing antibody test can be performed in any species, but it requires special facilities and, thus, is not offered by most laboratories. Data on the clinical utility of single-sample cerebrospinal fluid testing in horses24 and people2 conflict, and little is known about this method of West Nile virus infection diagnosis in other animals. Therefore, cerebrospinal fluid samples should be considered adjunctive to tests for serum antibodies in dogs and cats.

The species of origin should be emphasized on the sample label so that serum is not mistakenly subjected to another species' IgM antibody testing, which is species-specific. IgM antibody testing is widely available for horses and people, but canine IgM assays are currently offered only by the Veterinary Medical Diagnostic Laboratory at the University of Missouri College of Veterinary Medicine and by the Michigan State University Diagnostic Center for Population and Animal Health (Virology/Serology section).

The presence of IgM in the serum indicates recent exposure to West Nile virus. But because of the commonality of subclinical West Nile virus infections, IgM in the serum is not diagnostic unless it is accompanied by correlative clinical signs. The incubation period for West Nile virus-induced disease and the timing of the appearance of serum antibody are variable,2,16 so IgM measurements taken early in the course of disease may be negative or inconclusive, requiring repeat testing.


Table 2: Postmortem Diagnostic Testing for West Nile Virus Infection in Small Animals
Diagnosing West Nile virus infection postmortem is comparatively straightforward since more definitive testing modalities can be used on tissue samples harvested at necropsy (Table 2). Because West Nile virus can reach high concentrations in some tissues and exposure to the virus during postmortem examination has resulted in human West Nile virus infection, necropsy of animals suspected of having West Nile virus infection should be performed in a laminar flow cabinet or at least in a well-ventilated area. Be sure to use a mask, gloves, disinfectants, and protective clothing and equipment. If possible, have a veterinary diagnostic laboratory perform the necropsy.


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