West Nile virus

From Dog
Histological appearance of WNV-induced meningoencephalitis, showing scattered small perivascular cuffs of mononuclear inflammatory cells and mild microgliosis in the pyriform lobe[1]

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[2].

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[3][4]), with humans, horses and dogs as 'dead-end' hosts.

Evidence is accumulating that the virus is moving southwards across the Americas and across Western Europe[5][6][7].

Recent epidemiological surveys in the USA have shown a relatively high seroconversion rate for this virus, approaching 50% of the urban dog population[8] and the USA strain is highly neuroinvasive compared with other European strains[9]. Exposure to gaseous anesthetics increases WNV neuroinvasion in a concentration-and time-dependent manner[10][11], 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)[12], 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[13].

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[14] or cross-reacted with tick-borne encephalitis virus[15].

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[1].

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[16].

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[17].


  1. 1.0 1.1 Read RW et al (2005) West Nile virus encephalitis in a dog. Vet Pathol 42(2):219-222
  2. Lan D et al (2011) Serological evidence of West Nile virus in dogs and cats in China. Arch Virol 156(5):893-895
  3. Dennett JA et al (2007) Associations between two mosquito populations and West Nile virus in Harris County, Texas, 2003-06. J Am Mosq Control Assoc 23(3):264-275
  4. Hamer GL et al (2008) Culex pipiens (Diptera: Culicidae): a bridge vector of West Nile virus to humans. J Med Entomol 45(1):125-128
  5. Grazzini G et al (2008) West Nile virus in Italy: a further threat to blood safety, a further challenge to the blood system. Blood Transfus 6:235–237
  6. Rossini G et al (2008) First human case of West Nile virus neuroinvasive infection in Italy, September 2008 - case report. Euro Surveill 13
  7. Figuerola J et al (2007) Seroconversion in wild birds and local circulation of West Nile virus, Spain. Emerg Infect Dis 13:1915–1917
  8. Levy JK et al (2011) Prevalence of infectious diseases in cats and dogs rescued following Hurricane Katrina. J Am Vet Med Assoc 238(3):311-317
  9. Beasley DW et al (2002) Mouse neuroinvasive phenotype of West Nile virus strains varies depending upon virus genotype. Virology 296:17–23
  10. Ben-Nathan D (2000) CNS penetration by noninvasive viruses following inhalational anesthetics. Ann N Y Acad Sci 917:944–950
  11. Katz Y (2002) Inhalation anesthetic-induced neuroinvasion by an attenuated strain of West Nile virus in mice. J Med Virol 66:576–580
  12. Yamanaka T et al (2012) No evidence of horizontal infection in horses kept in close contact with dogs experimentally infected with canine influenza A virus (H3N8). Acta Vet Scand 54:25
  13. Buckweitz S et al (2003) Serological, reverse transcriptase-polymerase chain reaction and immunohistochemical detection of West Nile virus in a clinically affected dog. J Vet Diagn Invest 15:324–329
  14. Schaudien D et al (2008) Lack of detectable West Nile virus RNA in brains and kidneys of dogs and cats with immunohistological precipitates using virus-specific antibodies. Vet Microbiol 132(1-2):171-176
  15. Klimes J et al (2001) Prevalence of antibodies to tickborne encephalitis and West Nile flaviviruses and the clinical signs of tickborne encephalitis in dogs in the Czech Republic. Vet Rec 148(1):17-20
  16. Bowen RA et al (2008) Pathogenesis of West Nile virus infection in dogs treated with glucocorticoids. Am J Trop Med Hyg 74(4):670-673
  17. Karaca K et al (2005) Recombinant canarypox vectored West Nile virus (WNV) vaccine protects dogs and cats against a mosquito WNV challenge. Vaccine 23(29):3808-3813