Fly Control Key to Protecting Horses Against Vesicular Stomatitis Virus

Weather extremes and changing patterns are more than inconvenient. Their impact can play a role in the spread of disease, as horse owners in Southern California realized this summer.

Each year, vesicular stomatitis virus (VSV) circulates between livestock and insects that can transmit the disease. Because those insects are impacted by climate — especially wet weather — their migration can carry the disease into different regions.

The year's first confirmed U.S. case of VSV was detected in San Diego County, California, on May 17, 2023. As of August 8, 2023, 177 VSV-affected premises have been identified in three states — California, Nevada, and Texas.

California has recorded 174 affected premises in eight counties (Los Angeles, Orange, Riverside, San Bernardino, San Diego, San Luis Obispo, Santa Barbara, and Ventura). In Nevada, White Pine County had one confirmed positive, while the Texas counties of Maverick and Shackelford had two confirmed positive.

"This is unprecedented for California. Our last VSV outbreak was in the early 1980s, and that was from animal movement, not insect vectors," notes Emily Nietrzeba, DVM, MPH, equine specialist for the animal health branch of the California Department of Food and Agriculture (CDFA).

"This year's unusual winter weather pattern created the unfortunate opportunity for this disease incursion," says Nietrzeba, citing the extraordinary amount of rain the state has received.

What is VSV?

VSV is a highly contagious disease that primarily affects horses, donkeys, mules and cattle. Swine, sheep, goats, llamas and alpacas are also susceptible.

The disease is often transmitted through black flies, sand flies and biting midges that carry the disease. Flies that transmit a disease are called "vectors." Often a concern in various states, VSV usually circulates in southern Mexico before crossing the border into the U.S.

Signs of VSV Infection

Although it isn't typically life-threatening, and most animals fully recover, VSV causes painful blisters (vesicular lesions), which is where the disease gets its name.

Hairless regions of the animal's body are generally where the insects bite. Lesions develop on the nose, lips, tongue, inside the mouth and ears, udder, sheath, and coronary bands.

These fluid-filled blisters contain the virus, and when they break open, VSV is easily spread by animal-to-animal contact, shared feed and water sources, and contaminated surfaces. After the lesions burst, secondary bacterial infections sometimes develop in the raw, ulcerated skin.

The incubation period is two to eight days after exposure. Once lesions form, animals can be contagious for up to a week.

Clinical signs of VSV infection include:

• Excessive salivation
• Drooling/frothing
• Lesions (blisters)
• Crusting/ulceration
• Reluctance to eat
• Fever
• Lameness (if lesions develop around coronary bands)

What to Do if You See Signs of VSV

Contact your veterinarian immediately if you notice any clinical signs. Positive confirmation of VSV is made through blood tests or swabs from lesions or blister fluid.

"VSV is a reportable disease at both the state and federal level. It's considered an emergency condition because it is clinically indistinguishable from Foot and Mouth Disease (FMD) in livestock species. You cannot confirm or rule out what it is until you actually test for it," says Nietrzeba.

FMD is a highly contagious and deadly viral disease that impacts food animals like cattle, swine, and sheep.

"Horses can't get FMD, but one of the reasons VSV causes quarantine and restrictions on animal movement is because it mimics FMD and is also highly contagious," explains Nietrzeba.

What to Do if Your Horse Contracts VSV

Isolate positive horses as far as possible from others on the premises to prevent animal-to-animal contact and sharing of feed and water sources.

"We obviously can't quarantine the flies, but animals with any active VSV lesions are quarantined to reduce the risk of disease transmission," notes Nietrzeba. "

Because humans can spread the disease to other horses through contact, only handle those with VSV after caring for healthy horses on the premises.

Monitor your horse's comfort level throughout the disease process. Mouth lesions are painful and can make eating difficult. You may need to provide soft feed, such as dampened grain or soaked hay cubes, to maintain nourishment. To encourage horses to eat, veterinarians often recommend anti-inflammatory medication.

Your veterinarian may prescribe antibiotics if secondary bacterial infection occurs after lesions burst.

VSV typically runs its course in about two weeks. Most horses recover without incident once they get through the painful stage of the lesions.

"Even though the animals create antibodies to the virus once they're infected, they can get infected again once they clear the virus," adds Nietrzeba, noting that there is no VSV vaccine.

Protect Against VSV with Fly Control

Combatting the spread of disease through fly control is essential for battling VSV and other insect-borne health concerns. Nietrzeba urges horse owners to be meticulous about fly control.

Physical deterrents include fly masks, sheets, and boots. On-horse sprays offer additional protection against multiple pests. When applying products, pay close attention to areas most at risk of fly bites. Don't forget the coronary bands!

Take a close look at your horse's environment and remove anything that attracts flies. Manure, standing water, spilled feed, damp hay, or old bedding draw flies and can put your horse at risk of being bitten.

"Frequent manure removal is really critical," says Nietrzeba. "Putting manure right behind the barn is not helpful. You want it moved as far away from the animals as possible."

Monitoring the Situation

As of August 8, 2023, 105 VSV-affected cases had completed quarantine with no new clinical cases, and 72 premises remained under quarantine.

Horse owners should continue to closely monitor their animals, particularly in areas where the insects associated with VSV are present.

Stay vigilant and take precautions, especially when traveling to equine events.