Horse owners urged to vaccinate to reduce toll of West Nile virus

"Greater efforts are needed to encourage horse owners to take advantage of vaccination if continued losses from West Nile virus infection are to be reduced or even eliminated."
Image by April Anderson

Greater efforts are needed to encourage Kentucky’s horse owners to vaccinate against West Nile virus if continued losses from the infection are to be reduced or even eliminated, Professor Peter Timoney says.

Until 1999, West Nile virus was unknown in the Western Hemisphere, much less the United States.

In August of that year, the virus was identified in New York City and caused the deaths of seven persons from viral encephalitis (brain inflammation).

Coincidentally, the virus was also implicated in the death of birds in New York’s Bronx Zoo and crows in the precincts of the zoo.

West Nile virus is a mosquito-transmitted arbovirus from a family known to be highly adaptable, as evidenced by its extensive distribution throughout the Old World.

“As such, it was no surprise that West Nile virus spread rapidly from its presumed point of introduction in New York,” Timoney, with the Maxwell H. Gluck Equine Research Center, writes in the latest issue of Equine Disease Quarterly.

It initially extended southwards along the eastern seaboard, while also migrating westwards into the hinterland of the United States. With the exception of Alaska and Hawaii, it had reached the remaining 48 states by 2004. The virus was later confirmed in Alaska in 2018.

Kentucky reported its first cases in birds and horses in 2001. Devastating losses were recorded in crows and other related birds, and eight cases were diagnosed in horses in counties in the north-central part of the state.

In 2002, the virus dramatically spread throughout much of Kentucky with the exception of the eastern part. To date, 2002 was witness to the greatest number of cases recorded in horses (513), humans (75), and birds in Kentucky. Case numbers in horses (102) and humans (14) declined significantly the following year.

“With the exception of minor surges in horse cases in 2006, 2012, 2013, 2017, and 2018, the annual number of reported cases of infection remained in single digits.”

Timoney noted that cases have been confirmed in horses in Kentucky every year since 2001 and, with the exception of 2020, also in humans.

In terms of the 765 equine cases of infection recorded in Kentucky since 2001, the onset of virus activity ranged from early June to mid-July, depending largely on the level of mosquito activity in any particular year. The peak of infections occurred in about mid-September, and virus activity ceased by mid-November.

A breakdown of the 20-year total of West Nile virus infections in Kentucky revealed that the most common horse breeds/categories included quarter horses, Tennessee walking horses, thoroughbreds, and pleasure horses, of which quarter horses and walking horses comprised almost 50% of the overall number.

Females outnumbered males 57% to 43%. Ages of confirmed cases ranged from three months to 39 years, with a median of eight years.

Of the 765 total cases, 29% died or were euthanized.

The first equine vaccine against West Nile was conditionally licensed in 2001.

“Even though vaccination is effective in protecting against disease, the great majority (86%) of West Nile cases recorded in Kentucky had never been vaccinated or were only partially vaccinated (10%). Only approximately 4% of cases had current vaccination histories.

“While this is not totally surprising, it is disappointing considering that West Nile virus is one of five core equine vaccines strongly recommended by the American Association of Equine Practitioners.

“Of the 656 cases of West Nile infection without prior vaccination history, approximately 30% died or were euthanized,” he said.

“Greater efforts are therefore needed to encourage horse owners to take advantage of vaccination if continued losses from West Nile virus infection are to be reduced or even eliminated.”

Timoney said the evidence shows that the West Nile virus has become endemic in Kentucky and will likely continue to cause horse and human disease.

“Its broad host and vector range, capability for transovarial transmission in mosquitoes, and horizontal transmission in birds and a diversity of mammals has ensured its perpetuation.”

Equine Disease Quarterly is funded by underwriters at Lloyd’s, London.

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