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Individuals with symptoms consistent with West Nile virus (high fever, confusion, muscles weakness, severe headache) should contact their physician. The physician may suspect WNV infection based on clinical signs, travel history, age and medical history, however a cerebral spinal fluid (CSF) sample submitted to the Michigan Department of Community Health (MDCH), Bureau of Laboratories (BOL) is required for confirmation.
The MDCH, BOL conducts testing for the 4 mosquito-borne viruses that can cause illness in humans in Michigan. This Arbovirus Serology panel includes West Nile virus (WNV), St. Louis encephalitis virus (SLE), La Crosse encephalitis virus (LC), and Eastern Equine encephalitis virus (EEE).
The laboratory test most commonly used for WNV measures antibodies (IgM antibodies) that are produced at an early stage in a person infected with WNV. These IgM antibodies can be measured in blood serum or in cerebral spinal fluid (the fluid surrounding the brain and spinal cord). Because IgM antibodies have been detected in serum for up to 500 days, cerebral spinal fluid is the preferred specimen for testing. The initial testing takes two days to perform and is repeated in duplicate if a positive result is obtained.
Additional testing on an IgM positive sample may be performed by Plaque Reduction Neutralization Test (PRNT) to rule out cross reactions between WNV and other arboviruses, to confirm a state's first positive human case, or if IgM results cannot be determined based on the sample.
Diagnosis of WNV infection in live horses with neurologic signs is made by testing blood or cerebrospinal fluid for antibodies against the virus at MSU DCPAH. The most widely used test is the 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. Laboratories which are equipped to handle live virus may also perform a PRNT, in which paired serum samples (taken 2-3 weeks apart) are measured for WNV antibodies; a 4-fold or greater rise in titer is indicative of active infection.
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, eastern equine encephalitis (EEE) virus, and western equine encephalitis (WEE) virus infections, and possibly other diseases. The head of the animal should be submitted to the MSU 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, EEE, or WEE) is suspected following microscopic examination, then the nervous tissue is tested by reverse transcription-polymerase chain reaction (RT-PCR) for these viruses.
Currently, testing of samples from living non-equine species for evidence of WNV infection is limited. At the DCPAH, two tests are available for canine serum and/or CSF. The IgM capture ELISA can be run for dogs showing acute neurological signs.
Necropsy of non-equine WNV suspects will be similar to the protocol described above for horses. Tissues will be examined microscopically, and if indicated, specific tests for rabies virus and other arboviruses will be performed.
Birds and Mammals
Corvids (crows, blue jays and ravens) are reported by the public, local health departments and local DNR offices if found dead or observed displaying symptoms consistent with WNV infection. Trained personnel from local health departments or other state or local agencies conducting WNV surveillance may collect an oral swab from the bird, which can be submitted to Michigan State University, Diagnostic Center for Population and Animal Health (DCPAH) where the oral swabs are tested for the presence of West Nile virus (WNV) via PCR.
Non-corvid birds and mammals observed alive prior to death with signs of a central nervous system disorder can be reported to the Michigan Department of Natural Resources via the "How to Report a Dead Bird or Mammal" link on this site.
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