Equine Viral Encephalitides

Question 1

What 4 viruses most common cause encephalopathy in horses? What are the 2 most distinguishing features?

Answer 1

1. West Nile Virus (WNV) - Flaviviridae
2. Eastern Equine Encephalitis Virus (EEEV) - Togaviridae
3. Western Equine Encephalitis Virus (WEEV) - Togaviridae
4. Venezuelan Equine Encephalitis Virus (VEEV) - Togaviridae

mental depression and possible fever

Question 2

What initiates infection of WNV in horses? What does its enzootic cycle rely on?

Answer 2

bite of WNV-infected mosquito –> low magnitude viremia, short duration

transmission of the virus between birds and infected mosquitoes

Question 3

What clinical signs are seen in WNV-infected horses?

Answer 3

• depressed mental state
• ataxia
• weakness
• muscle fasciculations
• fever
• recumbency
• vaccinated horses demonstrate reduced (if any) clinical manifestations

Question 4

What are signs of WNV on CSF evaluation? How is infection confirmed?

Answer 4

elevated protein and mononuclear pleocytosis

antigen (IgM) capture ELISA, virus isolation, and plaque reduction neutralization

Question 5

What treatment is recommended for horses with WNV? What is prognosis like?

Answer 5

supportive –> anti-inflammatories and fluid therapy

variable - many horses recover, mortality rate of 33%

Question 6

How do EEEV, WEEV, and VEEV persist in the environment?

Answer 6

infected, but asymptomatic wild animals, such as birds and small mammals

mosquitoes act as vectors and an important route of infection spread

Question 7

What clinical signs are seen in cases of EEEV, WEEV, and VEEV infection?

Answer 7

• somnolence (sleep sickness)
• hyperesthesia
• fever, anorexia
• depression
• proprioceptive deficits
• recumbency
• cerebral/cranial nerve signs - head pressing, propulsive walking, circling, head tilt

Question 8

What is seen on CSF examination in cases of EEEV, WEEV, and VEEV infection? What is definitive diagnosis based on?

Answer 8

elevated protein and cell counts

serology or necropsy

Question 9

What treatment is recommended for horses with EEEV, WEEV, and VEEV? How does prognosis differ with each virus?

Answer 9

supportive –> anti-inflammatories and fluid therapy

• EEE = high mortality (75-100%)
• WEE/VEE = lower mortality (40-80%)
• residual neurologic defects may present in horses that recover