West Nile encephalitis: A new differential for neurological illness in dogs and cats - DVM
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West Nile encephalitis: A new differential for neurological illness in dogs and cats
As virus gains momentum, researchers still have incomplete picture on its impact on companion animals

DVM Best Practices

The level of the WNV transmission between all species in the Midwest states was exceedingly high as evidenced by the enormous numbers of equine and human cases occurring in those regions. Our experience last year is supportive of a possible role for WNV in neurological illness in dogs, and less definitively supportive of a role for WNV in neurological illness in cats in regions of high epizootic transmission. However, the data is incomplete as IgM capture ELISA for dogs and cats were not been widely available.

Necropsies and additional testing have rarely been pursued due to prohibitive costs. While dogs and cats are not considered at risk for development of clinical illness following WNV infection, it is probable that the infection rate was so high in these regions that a low rate of clinical illness in a percentage of infected animals became visible.

It is likely still true that cats and dogs will not commonly develop WNV-associated clinical illness following infection, but in areas with confirmed active WNV transmission, WNV infection should be included in the differential diagnosis in animals presenting with a sudden onset of neurologic signs which include weakness or ataxia. Collection of more information from laboratory confirmed clinical WNV-associated illness in dogs and cats is required to define this infection in these populations.

Diagnosis The key to diagnosing WNV-associated illness in companion animals is first to include it on the differential and then to test for it. Because evidence suggests that dogs and cats can be infected with WNV with no clinical signs of illness, multiple testing strategies are required to achieve the highest probability of diagnostic accuracy.

Testing strategies will depend on the samples available. Testing in animals with mild signs of illness will rely heavily on serology. Animals in WNV-endemic areas may be antibody positive so this alone should not be considered proof of causality in an animal with a neurologic illness. A minimum standard of association of WNV with a clinical illness in a companion animal when serology is the only available test should be the presence of IgM antibody with a positive virus neutralizing antibody titer in serum and cerebral spinal fluid (CSF) or demonstration of a four-fold or higher rise in antibody titer in paired serum samples.

Paired serum samples should be submitted routinely and serial samples taken days apart may also be helpful. All antibody capture ELISAs, whether for IgM or IgG should initially be confirmed by serum neutralization, at least until the sensitivity and specificity of the ELISA tests can be determined. EDTA blood can also be submitted for reverse transcription – polymerase chain reaction (RT-PCR) detection of viral nucleic acid and for virus isolation. It is currently unknown how useful this will be for the detection of WNV as titers of virus in blood have generally been described as low and transiently present.

If an animal should die or be euthanized as a result of disease progression, a post mortem CSF sample should be collected and held pending rabies testing.

Ideally, the body should be submitted to a veterinary diagnostic laboratory for a full necropsy, histopathology and ancillary testing. Once rabies is ruled out, fresh tissues samples should be tested for WNV by RT-PCR and virus isolation should be attempted from kidney, brain and heart. Histopathology and immunohistochemical staining should also be pursued whenever possible.

Safety There are several challenges facing individuals at all levels within the diagnostic process. While it is believed that cats and dogs with WNV infections represent no risk of horizontal transmission to humans or other animals, there is theoretically an increased risk of exposure to virus as a result of the necropsy procedure. Where rabies is a realistic possibility, minimal handling of the carcass is encouraged until a negative rabies result is obtained though routine public health laboratory testing.

Necropsy of an animal with a neurologic illness should always be approached with personal safety issues foremost in mind. Individuals electing to do necropsies in-house are encouraged to wear appropriate personal protective clothing. This would include at a minimum two pairs of impermeable gloves, a disposable gown, face-mask and a face-shield or goggles. Power saws should not be used at any time during the procedure. All surfaces should be thoroughly chemically disinfected. Most commonly used disinfectants are capable of inactivating WNV.

Challenges of responding Challenges within the veterinary diagnostic and academic communities make rapid response to a virus infection of this nature difficult. Resources sufficient to provide the testing services required are limited. Very few laboratories have the ability to provide all of the testing services needed to obtain a definitive diagnosis. This is due in large part to the requirement for high security biocontainment laboratories (biosafety level 3) to perform virus isolation and serum neutralization testing and to a shortage of reagents required for ELISA-based test development.

In every year since 1999, testing capacity has been maximally challenged in both the human and animal diagnostic communities. More veterinary diagnostic laboratories are acquiring the ability to do some WNV serology, generally IgM and IgG antibody capture ELISAs.


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