West Nile Virus

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West Nile Virus: Etiology

Back 1

1) enveloped, + strand RNA
2) flaviviridae, genus flavivirus
3) highly pathogenic for horses and birds

Front 2

WNV: Transmission

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1) mosquitoes to spread it among birds
2) Different mosquitoes to spread virus from birds to mammals

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WNV: Susceptible species

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1) mosquitoes: replicates in intestine and salivary glands; small percentage of mosquitoes in an anedmic area actually carry the virus
2) Birds: fatal in crows, blue jays, ravens; high level of viremia for 1-4 days
3) Horses
4) Humans: <1% get severe clinical signs; if severe clinical signs develop after an incubation period of 3-15 days, fatality rate is 3-15%
5) recentaly, dogs and squirrels
6) Serological evidence of infection in other species

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WNV 2007

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MT and CA had highest number of equine WNV

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Clinical signs

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1) Birds: corvids most susceptile, acute clinical signs, birds usually found dead and submitted for testing
2) Horses: mostly seen in older horses, acute onset ataxia, weakness, paresis of hind legs, paraparesis, progressing to tetraplegia and recumbency, low-grade fever or no fever, marked hypmetria, muscle fasiculations (common in horses). Less common signs: anisiocoria, slow PLR, seizure, head tremors, lip twitching, tongue paralysis, hypersensitivity to ligh/sound, somnolence, falling to knees
3) Humans: subclinical, fever, meningoencephalitis

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Lesions

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1) Birds: heart muscle, kidney, brain
2) Horses: Brian-- nonsuppurative encephalomyelitis, perivascular cuffs, glial nodules and focal gliosis in the grey matter; Spinal cord-- non-suppurative inflam, possible neuronal degeneration, neuronophagia, gliosis

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Diagnosis

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1) clinical signs
2) gross and histo lesions
3) virus isolation-- need BSL-3 lab
4) IHC-- don't use in horses
5) IgM capture ELISA
6) RT-PCR-- fresh (BSL-3) or formalin fixed tissue (not as good)
7) IgG ELISA (not used much)
8) Plaque reduction neutralization test (not used)

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WNV diagnosis in dead horses

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1) submission of heads or whole bodies to DCPAH for necropsy
2) MDA will arrange for transportation if necessary

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Other testing of live equine suspects

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1) IgG ELISA: screening test for previous exposure (vacc or natural)--NOT USED
2) VN: determines Ab titer. Most common for horses

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Control

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1) mosquito control (reduce breeding sites)
2) limit exposure ot mosquitoes
3) supportive therapy
4) prevention of self-inflicted injuries in horses

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Prevention

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1) inactivated vaccine for horses: Primary series-- two doses IM, 3-6 wks apart; yearly booster
2) vector vaccine by Merial. canarypox vector
3)intervet-- vector vacc w/ flavi vector (1 yr immunity w/ 1 dose)