West Nile Virus Encephalitis

by Lorie A. Valentino, DVM, MS, Diplomate American College of Veterinary Internal Medicine

West Nile Virus (WNV) identified as the causative agent of encephalitis in birds, horses, and humans in New York City and Suffolk County, NY, in 1999.
The virus cycles between bird reservoir hosts and mosquito’s.
Horses, humans, and other mammals are incidental or dead-end hosts. They do not develop a sufficient viremia to infect mosquito’s.
Not all horses that are exposed to WNV develop clinical signs.
By the time WNV appears in a locale, widespread sub clinical exposure has already occurred.
Clinical signs are consistent with myeloencephalitis.
- Ataxia (incoordination) most common, rear limbs worse
- Muscle fasciculations or tremors especially around the nose and lips
- Excessive reaction to touch or sound
- Fever
- Unable to stand
- Seizures or coma

Improvement noticed within days in most cases.
Treatment is primarily supportive therapy.
Euthanasia warranted in severely affected cases.
Approximately 35% died or were euthanized in 1999 and 2000.
Approximately 40% of horses that survived acute illness caused by WNV still exhibit residual changes in gait or behavior.
Relapse of clinical signs occur in approximately 9% of cases, most often within 2 months after the initial illness.
Cases can occur year round in areas with prolonged mosquito season.
Rule out other causes of neurological disease.
Laboratory testing may be performed on serum, whole blood, cerebrospinal fluid or brain tissue.
The IgM capture enzyme-linked immunosorbent assay (ELISA) test is the test of choice for recent exposure in horses.
The IgM response rises rapidly and then declines by 30 days after the onset of clinical signs.
Vaccination does not interfere with diagnostic testing.
Killed virus vaccine became conditionally available in 2001.
Live canary pox vector vaccine now available.
Preventative management practices minimize spread and transmission.