US horse owners urged to act now to protect at-risk horses from vesicular stomatitis

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Clinical signs of vesicular stomatitis (VS) in infected horses
Clinical signs of vesicular stomatitis (VS) in infected horses: (A) lesions in the ear [photo by Richanne Lomkin]; (B) lesions on the tongue, lips, and nostril [photo by Jason Lombard]; (C) lesions on the tongue [photo by Randy Lewis]; (D) lesions on the lower lip [photo by Piper Norton].
Risk-reduction strategies and biosecurity measures at multiple levels are needed to adequately protect horses from vesicular stomatitis, an equine epidemiologist writes in the latest issue of Equine Disease Quarterly.

Vesicular stomatitis is a viral disease of horses and other livestock caused by the vesicular stomatitis virus (VSV).

It is endemic in southern Mexico and only occasionally moves northward into the United States.

The clinical disease is characterized by blister-like lesions on the muzzle, lips, tongue, ears, udder, sheath, or coronary bands.

“While the lesions usually heal on their own, some horses require supportive care,” says Angela Pelzel-McCluskey, of the Animal and Plant Health Inspection Service, an agency within the US Department of Agriculture.

A 2019 outbreak in the US was the largest in recent history with 1144 premises affected in eight states (Colorado, Kansas, Nebraska, New Mexico, Oklahoma, Texas, Utah, and Wyoming).

Vesicular stomatitis returned this year on April 13, 2020, with equine premises in Arizona, New Mexico, and Texas affected.

“Expansion of the disease to other states is expected this summer,” Pelzel-McCluskey says.

“Taking steps now to prevent VSV transmission on equine premises is imperative to limit the spread.”

The virus is spread two ways: natural transmission by insect vectors, or direct contact with infected animals.

Risk-reduction strategies targeting insect vectors and biosecurity measures to prevent contact with infected animals and contaminated objects (shared water troughs, feed buckets, tack, or equipment) are thought to be the best methods of prevention.

The muzzle of a horse affected by vesicular stomatitis. Photo: Dr Piper Norton

Known insect vectors include black flies (Simulium species), sand flies (Lutzomyia species) and biting midges (Culicoides species), but other insects may also transmit the disease.

“These insect vectors emerge in specific habitats, which helps target the implementation of mitigation strategies. Sand flies prefer dry areas, such as tree holes, rock crevices, and animal burrows.

“Biting midges prefer wet areas, such as wet leaves and mud around ponds or troughs.

“Black flies prefer flowing water, such as irrigation ditches, rivers, or streams. Adult flies move outward from these areas. Some can only fly short distances (sand flies), but others fly longer distances (black flies) or travel on wind currents (black flies, midges).

“Seasonality of the vectors coincides with warmer months, spring through fall, but midges can be more cold-hearty and occasionally transmit VSV in winter.”

Preferred feeding/biting times vary, with sand flies being night feeders, biting midges feeding around sunrise and sunset, and black flies feeding during the day, Pelzel-McCluskey says.

She says vector mitigation strategies should be considered at the neighborhood, premises, barn, and animal level.

“At the neighborhood level, keep animals away from insect emerging sites such as moving water and standing water during the insect seasons.

“Alternatively, time the rotation of animals through pastures to avoid grazing near a stream returning to base flow after reaching peak runoff, a time of black fly emergence.

“If possible, move animals to higher-elevation pastures during the vector season.

“At the premises level, removing manure regularly, maintaining sloped and well-drained footing around water sources, and keeping surrounding vegetation mowed will also reduce insect vectors.

“At the barn level, move animals indoors during vector feeding times or provide access to a run-in shed for pastured horses.

“Installing mesh netting or repellent-treated fabrics on barn openings can further prevent vector entry.”

Minimizing the use of bright lights at night and adding fans blowing down onto horses can also be preventive, she says.

“At the animal level, topical insecticides and repellents reduce exposure to biting insects, but repeated application is necessary and must be combined with insect mitigation at the other levels to be effective.

“Using fly sheets, masks, leg wraps, and ear covers with or without repellent fabric are also animal-level mitigations, but these must cover the areas where VSV lesions occur. Therefore, chemical repellents may need to be used in conjunction with fly sheets and masks to protect the muzzle and lips.

To learn more, go to “Management Strategies for Reducing the Risk of Equines Contracting Vesicular Stomatitis Virus in the Western United States” 
Equine Disease Quarterly is funded by underwriters at Lloyd’s, London.

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