ARBOVIRAL ENCEPHALITIDES

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ARBOVIRAL ENCEPHALITIDES

• Western Equine Encephalitis (WEE)
• Eastern Equine Encephalitis (EEE)
• Venezuelan Equine Encephalitis (VEE)
• St. Louis Encephalitis (SLE)
• LaCrosse Encephalitis (LAC)
• Powassan Encephalitis (POW)
• Tick-borne Encephalitis (POW-like)
• California group (LAC, Jamestown Canyon, Snowshoe
  Hare, California Encephalitis)

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ARBOVIRAL ENCEPHALITIDES

• Arboviruses are maintained through biological
  transmission between susceptible hosts by blood
  feeding arthropods
• Those causing human encephalitis come from three
  families
• Togaviridae (Genus Alphavirus) (WEE, EEE, VEE)
• Flaviviridae (SLE, POW)
• Bunyaviradae (LAC)

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ARBOVIRAL ENCEPHALITIDES

• Arthropod-borne zoonoses of US
• Mosquito-borne
• WEE, EEE, VEE
• SLE
• LAC
• Tick-borne
• POW
• POW-like (newly isolated)

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ARBOVIRAL ENCEPHALITIDES

• Mosquito vectors
• EEE
• Culisetta melanura
• Coquilletidia perturbans
• Aedes sollicitans
• Aedes vexans
• Culex nigripalpus

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ARBOVIRAL ENCEPHALITIDES

• Mosquito vectors
• WEE
• Culex tarsalis
• Aedes melaminon (CA)
• Aedes dorsalis (UT, NM)
• Aedes campestris (NM)
• VEE
• Culex spp.

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ARBOVIRAL ENCEPHALITIDES

• Mosquito vectors
• SLE
• Culex nigripalpus (FL)
• Culex pipiens pipiens (Midwest US)
• Culex pipiens quinquefasciatus (Midwest US)
• Culex tarsalis (Western US)
• LAC
• Aedes triseriatus (daytime biting treehole
  mosquito)

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ARBOVIRAL ENCEPHALITIDES

• Tick vectors
• POW
• Ixodes marxi
• Ixodes cookei
• Dermacentor andersoni
• POW-like
• Ixodes scapularis

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ARBOVIRAL ENCEPHALITIDES

• Reservoir
• EEE, WEE
• wild Passerine birds
• LAC
• amplifier hosts - chipmunks, squirrels
• transovarial passage
• horses, humans - dead end hosts since they do
  not produce significant viremia, therefore do not
  contribute to the transmission cycle

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ARBOVIRAL ENCEPHALITIDES

• Reservoir
• VEE
• endemic (enzootic) cycle
• wild rodents, passerine birds
• epidemic (epizootic) cycle
• horse to horse by equinophilic mosquitoes

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ARBOVIRAL ENCEPHALITIDES

• Clinical features (humans)
• majority of infections are asymptomatic
• mild cases non-specific flu-like symptoms with
  insidious onset of fever, headache, malaise
• severe infections acute onset of high fever, CNS
  signs, (encephalitis, stupor, disorientation,
  seizures, coma
• permanent neurologic sequelae may occur depending
  on age and viral agent most severe in infants
  with JE. EEE, WEE

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ARBOVIRAL ENCEPHALITIDES

• Clinical features (humans)
• VEE - usually less severe with fever, not
  encephalitis
• SLE - most common mosquito-transmitted pathogen
  in humans in the US lt 1 are clinically
  apparent milder in children than adults, but
  children have higher rate of encephalitis
• LAC - first discovered in LaCrosse, WI in 1963
  75 cases per year most cases in children under
  16 years of age low (lt1) case fatality ratio

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ARBOVIRAL ENCEPHALITIDES

• Clinical features (horses)
• EEE, WEE
• incubation pd 1-3 weeks
• fever usually only manifestation prior to
  neurologic symptoms, at which time fever and
  viremia disappear
• restlessness, unsteady gait, lack of
  coordination, disoriented, circling
• final paralytic phase falling, lower lip
  paralysis, difficulty swallowing

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ARBOVIRAL ENCEPHALITIDES

• Clinical features (horses)
• EEE, WEE
• death as soon as 1-2 days after neurologic
  symptoms
• if recover, neurologic sequelae, particularly
  abnormal reflexes
• fatality rate between 20-30, my be as high as 50

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ARBOVIRAL ENCEPHALITIDES

• Clinical features (horses)
• EEE vs WEE
• generally shorter course than WEE and more fatal
• biphasic febrile course during initial infection
• fatality rate gt75 with encephalitis serious
  brain damage if recover
• outbreaks in pheasants in eastern US do
  occur--fever, depression, diarrhea, ataxia,
  tremors, paralysis

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ARBOVIRAL ENCEPHALITIDES

• Clinical features (horses)
• VEE
• benign febrile disease fever 1-2 days,
  anorexia, depression (mild leukopenia, low
  viremia)
• encephalomyelitis
• sudden onset of high fever, profound depression,
  pronounced anorexia and weight loss, grinding of
  teeth, diarrhea or constipation

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ARBOVIRAL ENCEPHALITIDES

• Clinical features (horses)
• VEE
• encephalomyelitis (contd)
• more severe--stupor difficulty with balance and
  supporting head aggression, convulsions,
  circling, blind very high fatality rate (gt80)
• leukopenia and high-titer viremia

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ARBOVIRAL ENCEPHALITIDES

• Diagnosis (humans)
• Serology
• detection of IgM and IgG in acute phase serum or
  CSF now done with with ELISA
• IgM more specific early, while IgG more reactive
  with later infection
• inclusion of monoclonal antibodies in assays
• Ddx
• tick-borne encephalitis encephalitic and
  nonparalytic polio, mumps, meningoencephalitis
  meningitis

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ARBOVIRAL ENCEPHALITIDES

• Epidemiology
• occurrence most cases from June through
  September
• distribution
• EEE - Atlantic and Gulf Coast, midwest
• WEE - western US
• VEE - Latin America
• SLE - midwest and southeast
• LAC - upper midwest
• POW - upper midwest and NW

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ARBOVIRAL ENCEPHALITIDES

• Prevention and control
• mosquito surveillance and virus isolation
• sentinel programs with domestic fowl and
  passerine birds, especially sparrows, during
  transmission season (SLE, WEE)
• done on a regular basis throughout the season
• infection in birds strongly correlates with human
  infection