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What are the trends in West Nile virus activity and the risk to U.S. horses?

This column, brought to you by Merck Animal Health, features insightful answers from leading minds.

The fact that West Nile virus affects horses in most of the continental United States is fairly common knowledge. What may be surprising is that data point to disease risk beginning in early spring in parts of the country not commonly considered WNV-disease hot spots.

Overall WNV-disease risk in horses is highest between July and October, but local risks are nuanced throughout the year, according to one of the largest studies investigating WNV-risk for horses (Humphries et al 2021). Beginning in May, horses in the North and West face the highest risk of WNV illness. Cases increase in this region through June, when risk begins to ramp up in the South, then spreads homogenously across the country through September when total cases peak. Through fall and winter, most disease occurs in southern coastal states with continued warm, humid weather.

DROUGHT MIGHT NOT DRY UP DISEASE Our growing understanding of how precipitation—or the lack thereof—affects WNV might explain why states with moderate rainfall or even drought can have high annual incidence of WNV-disease in horses. Low levels of rain or drought do appear to suppress disease risk, but moderate and severe drought can increase disease transmission. During drought, mosquitoes, birds and horses cluster at the same water source, allowing for spillover of disease into horses. When rain falls after dry conditions in Midwestern and Northern climates, standing water can exacerbate mosquito activity. Conversely, torrential rains in the South can wash out WNV-carrying mosquitoes, diminishing disease risk to horses.

EARLIER SEASON, EARLIER VACCINATION

What do these trends mean for your equine patients? The timing of WNV vaccination may need to be adjusted and careful documentation of vaccine history is essential for virtually all locations in the U.S. For instance, even in Northern and Midwestern climates, consider administering WNV vaccinations in early spring, well before May and perhaps while snowfall is still possible. In Southern states with year-round risk, horses may need additional WNV boosters as activity increases in the fall.

BOOSTING MARE/FOAL PROTECTION

Multiple boosters a year are especially important for young horses that may not have a long history of repeated vaccinations. In foals, given the unpredictability of WNV, give three injections to account for waning maternal immunity. Administer the first dose at 4 to 6 months of age, followed in 3 to 4 weeks (per label) with a booster. Give another booster at least 8 weeks later. Owners can experience ‘vaccination fatigue’ after a long foaling season, and weanlings are especially at risk for missing that last booster due to changes in management.

Yearling horses must receive their first annual vaccination no later than March to maintain protection obtained as a weanling. Owners are often under the mistaken impression that these horses will be protected for a full year after the third booster, which is often administered in early fall. Thus they may skip another closely timed injection the following spring. Any yearling with questionable vaccine history as a foal should undergo a full series of two injections in the spring. All pregnant mares should be vaccinated in late gestation to achieve high levels of colostral antibody.

Remember WNV is a nationwide disease that affects different areas of the country at various times throughout the year. If interested in a specific location, the CDC provides the most reliable indication of real-time WNV activity because it includes mosquito, sentinel, horse and human activity. Access it at https://www. cdc.gov/westnile/statsmaps/index.html

ABOUT THE AUTHOR

Maureen T. Long, DVM, PhD, DACVIM (large animal), is a professor in the Department of Comparative, Diagnostic & Population Medicine at the University of Florida College of Veterinary Medicine. She has written and edited several hundred research papers and educational publications, such as the reference book, Equine Infectious Diseases.

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For more information on vaccination against WNV, visit the Merck Animal Health Prestige® 5 + WNV vaccine page.