West Nile virus
West Nile virus (WNV) is an arbovirus similar to Everglades virus which rarely affects dogs but can cause progressive neurological disease and subclinical renal disease in dogs across the Americas and Asia.
West Nile Virus belongs to the genus Flavivirus and is maintained in an enzootic cycle involving birds and mosquitoes (Aedes albopictus, Culex pipiens, Culex quinquefasciatus), with humans, horses and dogs as 'dead-end' hosts.
Recent epidemiological surveys in the USA have shown a relatively high seroconversion rate for this virus, approaching 50% of the urban dog population and the USA strain is highly neuroinvasive compared with other European strains. Exposure to gaseous anesthetics increases WNV neuroinvasion in a concentration-and time-dependent manner, suggesting that other environmental or host factors may influence the development of neurologic disease in animals. The low incidence of clinical disease seen in dogs in the face of high seroprevalence suggests that dogs are more resistant to neuroinvasion than either humans or horses.
A number of diseases may mimic symptoms including tick-borne encephalitis virus, equine influenza A (H3N8), Aujeszky disease virus, Greyhound meningoencephalitis, Steroid-responsive meningitis-arteritis, rabies and canine distemper virus.
Clinically affected dogs may present with acute illness characterized by fever, lethargy, uncontrolled rolling, whole-body tremors, seizures and depression. Meningitis, myocarditis and polyarthritis are common sequelae in severe infections. Sudden deaths have been reported, possibly due to the fact that viral replication in extraneural tissues before neuroinvasion and that the kidney and heart are likely target tissues in the dog.
Blood results and CSF analysis are often unrewarding, although an electroencephalogram may reveal changes suggestive of encephalitis. Serology should be performed on CSF to rule out Toxoplasma gondii, Neospora caninum, Ehrlichia canis and Rickettsia rickettsii.
Caution must be exercised when attempting diagnosis as some dogs have reacted positively to immunohistochemical analysis of brain and kidney samples but were negative on PCR analyses or cross-reacted with tick-borne encephalitis virus.
Cases of non-suppurative meningoencephalitis, which shows histologically as a multifocal, nonsuppurative meningoencephalitis involving gray matter or mixed gray/white matter areas such as the brainstem, should be screened with PCR assays as a definitive diagnosis.
Treatment is relative limited to supportive intravenous fluids and anti-inflammatory drugs such as tolfedine or meloxicam. Use of prednisolone and other corticosteroids are contraindicated as their use has been shown to exacerbate clinical symptoms.
A recombinant canarypox vectored West Nile virus vaccine has been shown to protects dogs against WNV challenge and may be prudential with at-risk patients.
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