The Return of Vesicular Stomatitis in Horses

After a more than three-year hiatus, vesicular stomatitis (VS) has returned to the southwestern and western United States. At the time of this writing (early August), more than 500 equine premises had been affected with VS in Colorado, New Mexico, Oklahoma, Texas, and Wyoming. We expect continued spread within those states and potentially others over the summer.

Vesicular stomatitis is a viral disease that primarily affects horses and cattle, but it occasionally affects swine, sheep, goats, llamas, and alpacas. People handling infected animals without proper biosecurity precautions can also contract the virus and experience symptoms of fever, headache, muscle aches, and severe fatigue. The virus is known to spread naturally to livestock via black flies, sand flies, and biting midges, although other insect vectors may also be involved in transmission. Clinical signs in infected horses include vesicles (blisters) on the muzzle, lips, tongue, ears, sheath, udder, or coronary bands. These vesicles rupture after a few days into crusty and oozing lesions from which the virus can further spread by direct contact. The virus can also contaminate surfaces or objects. Shared water sources, buckets and feeders, salt licks, and other objects contaminated with saliva or discharge from the lesions can lead to ongoing disease spread.

Veterinarians are required to report suspected VS to state and federal animal health officials. If they confirm the disease, they’ll place that property under state quarantine for at least 14 days from the onset of lesions in the last affected animal on the premises. This means if new animals develop lesions, the quarantine countdown restarts with each new case until no animals show clinical signs of the disease. Property owners should also practice disease mitigation strategies during the quarantine period, such as isolating lesioned animals away from others and implementing vector control measures to reduce the likelihood of continued spread.

Most VS-lesioned horses heal on their own with just some supportive care, but older horses or those with underlying medical conditions might need more veterinary intervention, especially if they are having difficulty eating or drinking.

In addition to the lesions’ direct impact on horses and the hassle of associated quarantines, interstate and international movement during a VS outbreak can be fraught with challenges. Nearly all U.S. states enact interstate movement restrictions for animals traveling from VS-affected states during an outbreak. These often include requirements such as having a recent certificate of veterinary inspection issued with specific statements attesting to the absence of VS lesions in the horse. Horses being transported internationally from the U.S. might not be allowed to travel at all if coming from a VS-affected state, or the receiving country might require them to test negative for VS first. Always contact the destination state or country for transport requirements related to VS, as these can vary widely.

Shows and events can also impose more stringent entry requirements during a VS outbreak. Organizers might even cancel some events due to concern over potential disease spread or reduced attendance. Event participants must weigh the potential pitfalls of attending. If they are coming from an affected state, they will need more stringent veterinary certification to get to the event. If the event is being held in an affected state, the out-of-state participants might have trouble getting back home after they attend. Additionally, outbreak situations can change in an instant; a county that wasn’t affected today might become an infected county tomorrow. You must decide whether to take the chance of traveling during this time.

Although there are many unanswered questions about how VS outbreaks occur, we know the virus circulates annually in southern Mexico and Central America. We believe certain climatic factors, such as rainfall and temperature, favor VSV vectors, and when these insects move north along waterways they bring the virus into the U.S.

The last VS outbreak in 2015-2016 was the largest in recent history and resulted in 823 infected premises in eight states (Arizona, Colorado, Nebraska, New Mexico, South Dakota, Texas, Utah, and Wyoming). While we don’t yet know how big this year’s outbreak will get, horse owners in historically affected states should be aware of the current situation and implement aggressive vector mitigation and biosecurity strategies now to protect their horses throughout the outbreak. Current situation reports are available on the USDA-APHIS website.