Until 1999, West Nile (WN) virus was unknown in the Western Hemisphere, much less the USA. In August of that year, the virus was identified in New York City and caused the deaths of seven persons from viral encephalitis. 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, member of the Japanese encephalitis virus serocomplex, family Flaviviridae, known for many years to be a highly adaptable virus, as evidenced by its extensive distribution throughout the Old World. As such, it was no surprise that WN virus spread rapidly from its presumed point of introduction in New York. It initially extended southwards along the eastern seaboard, while also migrating westwards into the hinterland of the USA. With the exception of Alaska and Hawaii, WN virus had reached the remaining 48 states by 2004. The virus was later confirmed in Alaska in 2018.

Kentucky reported its first cases of WN virus in birds and horses in 2001. Devastating losses were recorded in crows and other members of the corvid family, 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 of the state. 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. With the exception of 2006 and 2013, the same years also saw corresponding minor surges in human cases (CDC, ArboNET). Cases of WN virus infection have been confirmed in horses every year since 2001 and, with the exception of 2020, also in humans.

The seasonality of equine WN infections, based on the weekly onset of illness, was consistent with other arboviral encephalomyelitides. Apropos 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 approximately mid-September, and virus activity ceased by mid-November.

A breakdown of the 20-year total of WN virus infections in Kentucky revealed that the most common horse breeds/categories included American Quarter Horses, Tennessee Walking Horses, Thoroughbreds and pleasure horses, of which American Quarter Horse and Tennessee Walking Horse comprised almost 50% of the overall number. Females outnumbered males 57% to 43%. Ages of confirmed WN cases ranged from 3 months to 39 years, with a median of 8 years. Of the 765 total cases, 541 (71%) survived and 224 (29%) died or were euthanized.

The first equine vaccines against WN virus-related disease received conditional licensure from the USDA in 2001. Even though vaccination is effective in protecting against disease, the great majority (86%) of WN 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 WN virus is one of five core equine vaccines strongly recommended by the American Association of Equine Practitioners. Of the 656 cases of WN infection without prior vaccination history, approximately 30% died or were euthanized. Greater efforts are therefore needed to encourage horse owners to take advantage of vaccination if continued losses from WN virus infection are to be reduced or even eliminated.

This review confirms that WN 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.

Source: July 2021 Equine Disease Quarterly. Peter J. Timoney, MVB, PhD, FRCVS, is a professor and the Frederick Van Lennep Chair in Equine Veterinary Science at the Gluck Equine Research Center. Rusty Ford is an equine operations consultant with the Kentucky Office of State Veterinarian.