West Nile Virus Encephalitis

In August 1999, West Nile virus (WNV) was identified as the cause of encephalitis in birds, horses, and humans in New York. Since then, WNV activity has spread north and south along the East Coast, and west to several states following bird

Share
Favorite
Close

No account yet? Register

ADVERTISEMENT

In August 1999, West Nile virus (WNV) was identified as the cause of encephalitis in birds, horses, and humans in New York. Since then, WNV activity has spread north and south along the East Coast, and west to several states following bird migration patterns. Prior to its introduction in New York, WNV was reported to cause outbreaks in other parts of the world. The virus isolated in the United States is suspected to have come from Israel, but how it was introduced into America is unknown.

West Nile virus cycles between birds (“reservoir hosts”) and mosquitoes. Birds might become ill and die of myeloencephalitis (crows are particularly susceptible), or be unaffected and develop a lifelong immunity. A large number of dead crows is generally the first sign of viral activity in a given area. Horses and people contract WNV encephalitis through the bite of infected mosquitoes and are considered “dead-end hosts” because there is no evidence that they develop sufficient viral amplification (viral reproduction in levels significant in their body) to complete the cycle. Horses and people are therefore not contagious. Other species such as cattle, camels, dogs, and cats develop an immune response without becoming ill except for rare cases reported in cats and dogs.

Serologic surveys show that most horses exposed to the virus fail to become ill, but the ones which do suffer from neurological disease. The severity and nature of clinical signs depend on the area of the nervous system affected by the virus and the extent of the lesions. The most common sign is ataxia–with rear limb involvement more frequently observed–and trembling, particularly around the nose and lips. Some horses become hypersensitive to touch or sound, or conversely might be lethargic and unresponsive. A transient (temporary) fever might precede these signs.

The onset of disease is generally acute, and in most cases, improvement is noticed within days. Euthanasia is warranted in the most severe cases due to paralysis, or severe signs such as seizures or coma. In 1999 and 2000, approximately 36-38% of the horses with WNV neurological disease either died or were euthanized. In 2001, not all outcomes of WNV cases were reported to the United States Department of Agriculture, making numbers hard to report; however, a similar percentage of cases recovered completely over several months.

During the 1999 and 2000 outbreaks, equine cases were observed between mid-August and late October, but in 2001, cases were confirmed from June to December and beyond in Florida, suggesting that cases might be observed year-round in areas with a prolonged vector (mosquito, in this case) season. No significant age, gender, or breed predisposition was detected among affected horses, and the outcome was not influenced by any of these categories.

Currently the diagnosis of WNV is based on a blood test and on clinical presentation in horses located in an area with viral activity. After exposure to the virus, horses develop an immune response characterized by a rapid increase in IgM (a type of antibody) that lasts less than two months, and a delayed increase in IgG (another antibody type) that might persist for several months. Vaccination does not interfere with the ability to diagnose acute cases, because no IgM levels are detectable in vaccinated horses.

Since WNV is transmitted by mosquitoes, measures to avoid exposure of horses to these vectors should be implemented. A killed vaccine has been developed to help prevent WNV encephalitis in horses. Currently, the vaccine is under conditional license since data regarding its efficacy are not available yet. Current recommendations for first-time vaccination are two injections three to six weeks apart followed by boosters every four to six months depending on the length of the vector season. These recommendations might be reviewed when more research results become available

Create a free account with TheHorse.com to view this content.

TheHorse.com is home to thousands of free articles about horse health care. In order to access some of our exclusive free content, you must be signed into TheHorse.com.

Start your free account today!

Already have an account?
and continue reading.

Share

Written by:

Genevieve L. Fontaine, DVM, MS, Dipl. ACVIM, is an assistant professor of equine extension at the University of Florida in Gainesville, Fla.

Related Articles

Stay on top of the most recent Horse Health news with

FREE weekly newsletters from TheHorse.com

Sponsored Content

Weekly Poll

sponsored by:

Where do you primarily feed your horse?
282 votes · 282 answers

Readers’ Most Popular

Sign In

Don’t have an account? Register for a FREE account here.

Need to update your account?

You need to be logged in to fill out this form

Create a free account with TheHorse.com!