The Changing Face of Mosquito-Borne Diseases

Mosquito-borne virus infections of horses pose a continuous and expanding threat to equine health in the United States.

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Mosquito-borne virus infections of horses pose a continuous and expanding threat to equine health in the United States and internationally. The majority of equine mosquito-borne diseases in the United States today are preventable by immunization; however, the delivery of preventive health care to the equine industry can be improved.

In the last 40 years, two arthropod-borne equine pathogens were introduced into the United States. The virulent 1AB epidemic strain of Venezuelan equine encephalomyelitis (VEE) virus made its entry in 1971 with much warning and sufficient time to prepare. It was eradicated by strict local quarantines, large-scale aerial spraying for vectors, and extensive mandatory immunization of equids. The effective response was accomplished at great expense because VEE was judged an important human, as well as equine, disease.

West Nile virus (WNV), first seen in the United States in 1999, was unanticipated and challenged the U.S. diagnostic infrastructure. Once the virus spread from the initial epicenter in New York, it quickly established itself in multiple mosquito species and a diversity of susceptible vertebrates. This establishment caused unprecedented viremia levels (level of virus in the blood) in some bird species and high mortality rates in numerous bird species. Its transcontinental spread was unparalleled. A safe and effective vaccine to protect horses against the now-endemic WNV was developed rapidly and licensed conditionally in August 2001. Several additional WNV vaccines have since been produced.

The distribution and host range of Eastern equine encephalomyelitis (EEE) virus has not changed significantly over the last 20 years. EEE virus is the arbovirus (disease spread by arthropod vectors) of great concern to horses and humans because of high case-fatality rates (approximately 40% in humans and 90% in horses). Safe and effective EEE vaccines for horses are widely available. Regretfully, the vast majority of cases in horses each year continue to occur in animals that have had no history or an incomplete history of EEE immunization. Although the capability of stimulating durable protective immunity against EEE with a single immunization would be welcome, available vaccines provide an excellent level of protection if administered appropriately. The American Association of Equine Practitioners considers both WNV encephalitis and EEE of such significance that it recommends the vaccines against those diseases be considered "core vaccines

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