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Testing of Live Equine Suspects
Diagnosis of WNV infection in live horses with neurologic signs is made by testing blood or cerebrospinal fluid for antibodies against the virus at the Michigan State University (MSU) Diagnostic Center for Population and Animal Health (DCPAH). Both DCPAH’s Immunodiagnostics/Parasitology and Virology sections perform diagnostics for WNV. Testing for acutely infected and non-vaccinated horses is done by West Nile Virus IgM Capture ELISA (enzyme-linked immunosorbent assay). Positive results by this assay, in conjunction with compatible clinical signs, is strong support for WNV as the causative agent. For vaccinated horses, the West Nile Virus Neutralization (VN) test is preferred. It is important that vaccination history for WNV is provided. Either of those tests is performed as part of the Equine Neurologic Disease Core Panel.
Horse owners should contact their veterinarian immediately if a horse is observed to have neurologic signs. Veterinarians seeking additional details on WNV testing, including collection protocol and shipping requirements, should visit DCPAH’s test catalog available at animalhealth.msu.edu and search “West Nile” or call 517.353.1683.
Testing of Dead Equine Suspects
In horses which die or are euthanized with WNV infection, necropsy findings indicate inflammation in the brain (encephalitis) and/or spinal cord (myelitis). Cranial and peripheral nerves also may be involved. Immunohistochemical (IHC) staining of the nervous tissues for WNV antigen, however, generally yields minimal reaction. For this reason, IHC has not been a useful confirmatory test for pathologic diagnosis of WNV infection in horses. Rather, fresh brain or spinal cord tissue is tested by RT-PCR or virus isolation to confirm a clinical or necropsy diagnosis.
Horses and other equidae with acute neurologic signs that die or are euthanized are tested not only for WNV infection, but also for rabies virus, and eastern equine encephalitis (EEE) virus, and possibly other diseases. At DCPAH, brain tissue is first collected and submitted to the MDCH for rabies testing. At the same time, brain and possibly spinal cord tissues are examined microscopically for evidence of disease. If rabies results are negative, and arbovirus encephalitis (caused by WNV or EEE) is suspected following microscopic examination, then the nervous tissue is tested by reverse transcription-polymerase chain reaction (RT-PCR) for these viruses.
The Michigan Department of Agriculture and Rural Development may facilitate testing of suspect cases by providing transportation of specimens to the MSU DCPAH, and also covers test expenses.