The Changing Face of Mosquito-Borne Diseases

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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."

Although the United States has not had an equine case of VEE since 1971, the risk of its reintroduction remains. The virus exists as multiple subtypes in nature (I-VI), and while subtypes 1AB and 1C are the epidemic strains of the virus, the endemic 1E subtype caused clinical disease and deaths in horses in southwestern Mexico in 1993 and 1996. Recent outbreaks of a horse-virulent strain in Central/ South America serve as a reminder that active surveillance must continue if we are to be prepared to protect our equine population.

It has been nearly 20 years since the last reported equine case of Western equine encephalomyelitis (WEE) occurred in the United States. The fact that the virus is endemic in some of the Western states should not be overlooked; mutations could yield strains with greater potential for epidemic disease.

Active surveillance and reporting must remain at a heightened level to prevent or limit the spread of "exotic" vector-borne diseases that could affect horses in the future. Furthermore, increased education and awareness of the value of vaccination in preventing the major diseases of horses are critical.

CONTACTS: Charles Issel, DVM, PhD, (859) 257-4757,
Peter Timoney, FRCVS, PhD, (859) 257-4757,
Maxwell H. Gluck Equine Research Center
University of Kentucky, Lexington, Kentucky

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd's, London, brokers, and their Kentucky agents.

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Equine Disease Quarterly

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