VSV, or vesicular stomatitis virus, is a viral infection in horses that causes painful blisters around the mouth, nose, and hooves. It appears most often during warmer months in the Americas and is a federally reportable disease in the United States, meaning any suspected case must be reported to animal health authorities. While most horses recover within two to three weeks, the disease triggers quarantines and movement restrictions that can significantly disrupt travel, competition, and sale schedules.
How VSV Affects Horses
Symptoms typically appear 2 to 8 days after a horse is exposed to the virus. The first sign owners usually notice is excessive drooling or frothing at the mouth. If you look inside the mouth at that point, you’ll often see raised, whitish blisters forming on the inner surfaces of the lips, gums, tongue, or dental pad.
These blisters can also develop on the nose, ears, sheath, udder, and the coronary band, which is where the skin meets the hoof. When lesions form at the coronary band, lameness is common. Fever often appears at the same time as the first blisters or slightly before. As the blisters swell and rupture, they leave raw, painful tissue behind. Horses become reluctant to eat or drink because of the mouth pain, and weight loss can follow quickly if the animal goes off feed for several days.
How the Virus Spreads
VSV is primarily transmitted by biting insects. The major carriers include black flies, sand flies, mosquitoes (particularly Aedes species), and Culicoides biting midges. These insects can spread the virus either mechanically, by carrying it on their mouthparts from one animal to the next, or biologically, by becoming truly infected and passing the virus when they feed.
The disease is endemic in tropical and subtropical regions of the Americas. During summer months, it pushes northward into temperate zones of the U.S. and occasionally into Canada. Outbreaks typically appear first along the Gulf of Mexico in April or May, then move further north as the insect season progresses. In the first year of an outbreak in a new area, cases tend to cluster near flowing water where black flies breed. In the second year, the disease often spreads into drier areas, suggesting a shift in which insect species are doing most of the transmitting.
Direct contact between animals can also spread the virus. Shared water troughs, feed buckets, and tack are potential routes, especially when an infected horse has open, oozing lesions.
Two Strains, Different Severity
VSV comes in two main serotypes: Indiana and New Jersey. This distinction matters because the New Jersey strain causes more frequent and more severe outbreaks than the Indiana strain. Research published in the Journal of Virology found that the New Jersey strain’s surface protein is structurally more stable under acidic conditions, which helps it infect cells more efficiently. In practical terms, if your area is dealing with a New Jersey strain outbreak, expect a higher number of affected animals and potentially worse clinical signs.
Why VSV Triggers Quarantines
VSV is a reportable disease at both the state and federal level in the United States, and the reason is straightforward: in cattle and pigs, vesicular stomatitis looks identical to foot-and-mouth disease. The blisters, the drooling, the lameness from coronary band lesions are clinically indistinguishable from FMD without laboratory testing. Since foot-and-mouth disease would be catastrophic to the U.S. livestock industry, any animal showing vesicular lesions must be tested to rule it out.
When a case is confirmed, state and federal veterinarians typically place the affected premises under quarantine. Movement of horses on and off the property is restricted until a set period has passed after the last lesion heals, usually 14 to 21 days depending on the state. Horses showing no symptoms on the same property may also be held. For horse owners who compete, trail ride, or board at facilities with multiple owners, this can mean weeks of disrupted plans.
If you suspect VSV, your veterinarian is required to notify the state veterinarian’s office. You’ll need to provide your name and address, directions to the property, how many animals are affected versus how many are on the premises, and a complete clinical history. Prompt reporting speeds up testing and helps contain the outbreak.
Treatment and Recovery
There is no antiviral medication for VSV. Treatment is entirely supportive, focused on keeping the horse comfortable and nourished while the virus runs its course. Soft feeds can encourage eating when mouth lesions make chewing painful. Clean, fresh water should always be available, and some horses do better with slightly warm water that’s less irritating to raw tissue. Keeping feed and water containers clean helps prevent secondary bacterial infections in the open sores.
Flushing the mouth with a mild antiseptic rinse can help lesions heal faster and reduce the risk of bacterial complications. If coronary band lesions develop, keeping the hooves clean and dry is important, and your veterinarian may recommend topical treatments to protect the damaged tissue. Anti-inflammatory medications can help manage pain and fever.
Most horses recover fully within 2 to 3 weeks. Lesions generally heal without scarring unless secondary infection sets in. Horses that lose significant weight from reduced eating typically regain condition once the mouth heals and they return to normal feed intake.
Risk to People
VSV can infect humans, and handling infected horses is a well-documented risk. People who develop symptoms typically experience a high fever that may spike, subside, and spike again. This is followed by flu-like symptoms: severe fatigue, headache, muscle and joint pain, and nausea. Blisters on the lips, mouth, or nose can develop but are rare. The illness is generally self-limiting in healthy adults. Wearing gloves when treating lesions and washing hands thoroughly after handling affected horses reduces your risk significantly.
Preventing Exposure
Because insects are the primary transmission route, insect control is the most effective prevention strategy. Fly sprays, fans in stalls and run-in sheds, and removing standing water near the barn all reduce the number of biting insects your horses encounter. Fine-mesh fly sheets and masks provide an additional physical barrier.
During active outbreaks in your region, avoid sharing equipment between horses. Clean and disinfect water troughs, buckets, and bits regularly. If you’re traveling with horses, check state veterinarian websites for active quarantine zones before hauling. Many states require a recent negative Coggins test and health certificate for interstate travel, and during VSV outbreaks, additional testing or documentation may be required before your horse can cross state lines or enter a show ground.
Isolating new arrivals to your property for at least two weeks gives you time to watch for symptoms before they’re exposed to the rest of the herd. This is good practice year-round but especially important during VSV season.