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Animal Health Diagnostic Center -
Emerging Issues
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West Nile Virus (WNV)
Diagnostic Testing For Suspect Clinical Cases
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 Equine |
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Serology and Cerebrospinal Fluid (CSF) analysis: Natural infection causes
seroconversion with IgM. At this time, vaccination has not caused a detectable
IgM response in any animals, but does cause serum neutralizing antibodies
to develop. IgM antibody produced in response to WNV infection can be detected
during a relatively brief window which averages 2-3 weeks following onset
of clinical signs. Most clinically ill animals confirmed infected with WNV
have virus neutralizing antibodies against WNV, both in the serum and in
the CSF, early in the course of clinical illness. Failure to detect IgM
early in the clinical course may not rule out WNV. In some cases, serial
samples are required to detect WNV IgM and serum neutralizing antibodies.
Samples collected even 12 hours apart can be useful. Post mortem CSF should
be collected and submitted from animals that die or are euthanized, especially
if no brain or spinal chord samples will be obtained. There may be other
arbovirus infections, such as St. Louis Encephalitis virus, that can cross
react on the IgM Antigen Capture ELISA test and possibly the Serum Neutralization
test.
Virus isolation for WNV on whole blood or CSF is generally unrewarding,
due to the low level of viremia in infected horses. However, the differential
diagnosis will include other viral infections, such as Equine Herpes Virus
- Type 1, and should be attempted where appropriate.
PCR testing (polymerase chain reaction) can detect fairly low numbers of
viral particles. It can be performed on post mortem tissue specimens. The
diagnostic tissues for horses are central nervous system - brainstem, cerebellum
and spinal cord. PCR on CSF is usually unrewarding. Proper tissue handling
precautions should be taken, including gloves, masks, eye protection and
protective clothing. Remember that rabies should be included in the differential
diagnosis in all rabies endemic areas. |
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| Wild Birds |
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| These suspect cases must be handled through the Public Health Department.
If virus infection has already been demonstrated in a geographic area, further
testing may not be available for wild birds, as the risk has already been
established. |
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| Domestic Poultry |
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| For testing of suspect dead birds contact the Avian Diagnostics Program - Dr. Benjamin Lucio @ 607-253-4031. |
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| Pet and Zoo Birds |
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| Serum neutralization and PCR testing on blood, and PCR testing on tissue
samples, as well as virus isolation, are performed at our laboratory. Diagnostic
tissues requested for testing bird cases are blood, brain, heart and kidney. |
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| Companion Animals |
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| The intensity of the epizootic experienced in 2002 in some regions of
the country has led to the identification of several dogs with illness associated
with WNV infection. An IgM ELISA is now available for dogs that are clinical
WNV suspects. A positive WNV IgM result in combination with a positive WNV
serum neutralization test would indicate a recent WNV infection in a dog.
Serological testing for all other animals is limited to determining end-point
titers in acute and convalescent samples. Multiple serial samples may be
required to obtain a diagnosis and should certainly be collected if an animals
condition deteriorates. CSF samples are also very helpful. Submission of
tissue samples from animals that die or are euthanized for histology, and
for further diagnostic testing is encouraged. Please contact the laboratory
at 607-253 3900 for further information. |
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