Eastern Equine Encephalomyelitis

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Eastern Equine Encephalomyelitis: Etiology

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1) Enveloped RNA virus
2) Family: Togaviridae
3) genus: alphavirus
4) pheasants are sentinels
5) most susceptible animals species: horses, humans, pigs, pheasants, partridges, emus
6) reservoirs: catbirds, song sparrow, yellow warbler

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EEE: Transmission

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1) Vector transmission: a) Culiseta malanura (mosquito)-- bird to bird, b) Coquillettidia perturbans (mosquito)-- bird to mammal, c) Aedes solicitans and aedes vexans
2) Direct bird to bird spread, i.e. pecking in pheasants (viremia)
3) Peak incidence: august to september

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EEE: Pathogenesis and signs in horses

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1) bite of arthropod vector (virus replicates in mosquito salivary gland)--> replication in cells near entry SITE AND THEN IN REGIONAL LYMPH NODes--> PrimarY VIremiA, feVer, DEPResSION, aNoreXia, VIRal eXcretiOn in aErosolS, urInE
2) DissemInation to extraneural areas
3) curtailed or secondary viremia--> brain invasion via passive diffusion through endothelium or by active replication in endothelial cells
3) more clinical signs develop: high fever, paralysis of lips and pharynx, drooping eyelids, incoordination (sitting on hindquarters). Cranial nerve involvement
4) 90% mortality after secondary viremia is seen

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`EEE: Disease Patterns

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2 scenarios: 1) acute infection followed by recovery (1 fever peak); 2) distinct second fever peak (secondary viremia-- can carry virus to brain)

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EEE: Virus Infection in Birds

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1) Pheasants are "sentinels"
2) Culiseta melanura--> causes high viremia w/ signs of paralysis in pheasants
3) Transmitted btwn birds by pecking
4) Mosquitoes--> horses--> humans (dead end)
5) Emus also susceptible

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EEE: Diagnosis

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1) Clinical signs and gross lesions unremarkable
2) Histo-- significant neuronal necrosis (almost pathognomonic), perivascular cuffing
3) IHC
4) VI (dangerious to humans!)
5) RT-PCR w/ formalin-fixed tissues
6) Serology (HI)

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EEE: Prevention

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1) inactivated multivalent vaccine-- administered in spring (before mosquito season, 2 doses at 2 wk interval)
2) surveillance of dz in MI-- birds, horses, and mosquitoes