Flaviviruses and Encephalitides

Equine Encephalomyelitis

• Equine Encephalomyelitis is a group of diseases caused by viruses transmitted by mosquitoes, including Eastern Equine Encephalitis (EEE), Western Equine Encephalitis (WEE), Venezuelan Equine Encephalitis (VEE), and West Nile Virus (WNV).

Eastern Equine Encephalitis (EEE)

• Mosquito-borne disease with a mortality rate of 50-90%
• Caused by an alphavirus
• Transmitted by mosquitoes, not horse-horse or horse-human
• Sylvatic cycle between passerine birds and Culiseta melanura

Venezuelan Equine Encephalitis (VEE)

• Mosquito-borne disease with a mortality rate of 50-75%
• Caused by an alphavirus
• Has two life cycles: enzootic and epizootic
• Enzootic cycle involves mosquitoes (subgenus Melanoconion/C. spp)
• Epizootic cycle involves several bridge vectors, with horse infection becoming a predominant feature

West Nile Virus (WNV)

• Mosquito-borne disease with a wide geographic distribution
• Caused by a flavivirus
• Has a mortality rate of 35-45%
• Transmitted by mosquitoes, not horse-horse or horse-human

Clinical Signs

• Neurologic signs occur 9-11 days after infection
• Quiet and depressed
• Ataxia, wandering, impaired vision, reduced reflexes, circling, inability to swallow, drooling, fever
• Paralysis usually followed by death 2-3 days after onset of signs

Diagnosis

• Clinical assessment
• CSF examination with neutrophilic pleocytosis (count > 50%)
• IgM capture ELISA (85-90%); neutralizing antibodies (IgG) are the gold standard
• PCR - midbrain and brain stem

Treatment

• Supportive nursing care
• Management of pain and inflammation
• For WNV, flunixin meglumine 1.1mg/kg, IV, bid to prevent muscle tremors and fasciculations

Prevention

• Equine viral diseases vaccination
• Prevent exposure to other horses attending shows or other equine events

Equine Herpes Virus (EHV)

• Species-specific virus with three subfamilies: alpha, beta, and gamma
• 9 documented strains, but only 5 affect domestic horses
• Only EHV1, EHV3, and EHV4 commonly cause diseases in horses

EHV-1 (α)

• Subtype 1 of EHV1
• Causes rhinopneumonitis (respiratory form), abortion, birth of weak foals, peracute vasculitis, and neurological disorders
• Most important viral cause of abortion in horses
• Can be sudden and deadly
• High temperature, "fulminating" respiratory disease

EHV-3 (γ)

• Venereal disease
• Equine Coital Exanthema
• Rare but important
• Self-limiting
• Characteristic lesions on genitalia of stallions and mares

EHV-4 (α)

• Subtype 2 of EHV1
• Respiratory infection only ("Rhino"); rhinopharyngitis and rhinotracheitis
• Non-fatal, rarely results in abortion
• Severe, widespread outbreaks in young horses; foals → 3 years old in training

EHV-2 and EHV-5 (γ)

• "Ubiquitous" but rarely causes disease
• Found in nearly 100% of healthy foals
• Can cause problems in the following diseases:
  • EHV2 - Herpetic keratoconjunctivitis in young foals; may act as a forerunner that leads to Rhodococcal infection (through immune suppression)
  • EHV5 – equine multinodular pulmonary fibrosis (EMPF); in middle-aged horses

This quiz covers the transmission and life cycle of flaviviruses, including VEE and WNV, and their role in causing encephalitides. It also explores the role of vectors and hosts in the spread of these diseases.