Mitigation Strategies to Prevent VSV Transmission to Equids

The 2019 outbreak of VSV was the largest in recent history with 1,144 premises affected in eight states: Colorado, Kansas, Nebraska, New Mexico, Oklahoma, Texas, Utah, and Wyoming.
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Mitigation Strategies to Prevent VSV Transmission to Equids
The clinical disease is characterized by vesicular (blister-like) lesions on the muzzle, lips, tongue, ears, udder, sheath, or coronary bands. | Photo: Courtesy Dr. Ashley Salinas
Vesicular stomatitis (VS) is a viral disease of horses and other livestock caused by vesicular stomatitis virus (VSV), which is endemic in southern Mexico and only occasionally moves northward into the United States. The clinical disease is characterized by vesicular (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. A 2019 outbreak of VSV was the largest in recent history with 1,144 premises affected in eight states (Colorado, Kansas, Nebraska, New Mexico, Oklahoma, Texas, Utah, and Wyoming). VS 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. Taking steps now to prevent VSV transmission on equine premises is imperative to limit the spread.

VSV is spread two ways: natural transmission by insect vectors or direct contact with infected animals. Mitigation strategies targeting insect vectors and implementation of biosecurity to prevent contact with infected animals and contaminated fomites (shared water troughs, feed buckets, tack, or equipment) are thought to be the best methods of disease prevention. Known competent insect vectors include black flies (Simulium spp.), sand flies (Lutzomyia spp.), and biting midges (Culicoides spp.), but other insects may also transmit the disease.

These insect vectors emerge in specific habitats, which helps target 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-hardy 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.

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

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