Vesicular Stomatitis

The Organism

Vesicular stomatitis is caused by vesicular stomatitis virus, a vesiculovirus (VSV). Two serotypes, or strains distinguished by the immune response they provoke, cause disease in the US: New Jersey and Indiana [1]. The 2025–2026 outbreak is the New Jersey serotype [2].

Heat, ultraviolet light, and common disinfectants, including 1% bleach and 70% alcohol, kill the virus [1].

Where It Lives

Vesicular stomatitis is endemic in Mexico, Central America, and northern South America. In the US, it appears as periodic outbreaks, typically beginning in the Southwest and moving north. The two most recent outbreaks began in 2023 and 2025 and affected Arizona, New Mexico, Colorado, Texas, California, and Nevada [2, 3, 4].

Outbreaks usually occur in warm months and follow waterways, where the virus-carrying insects breed. Pastured animals are more likely to be infected than stabled ones, and animals kept within about a quarter mile of running water are at higher risk [1]. Frost normally decreases transmission, but the 2025–2026 outbreak began on October 31, 2025, and continued through the winter into the following fall [2, 3, 4].

The virus has survived US winters before. In the 2004–2006 epidemic, which caused 751 outbreaks across nine southwestern states, genetic and spatial analysis was consistent with the virus overwintering in limited areas of Colorado and Wyoming. How it survives the winter remains unknown [5].

How It Spreads

Biting insects carry the virus. Black flies are considered especially important in the western US; sand flies and Culicoides midges are also biological vectors, and several other flies and mosquitoes may carry it mechanically [1]. APHIS assessed infected insects arriving on the wind or along waterways as the likely source of the 2025 Arizona index cases, with no animal movement involved [3].

Once in a herd, the virus also spreads by contact. It is shed in saliva and blister fluid and enters through broken skin or mucous membranes. Saliva on mangers, buckets, hay, and milking equipment stays infectious for three to four days [1]. Shared water troughs, feed bins, bits, and grooming tools can carry it between animals.

The virus is not shed in feces, urine, or milk [1].

Infection Pattern

Signs usually appear three to seven days after infection [1]. Excessive drooling is often the first sign. Blisters form on the mouth, tongue, lips, and muzzle; on the coronary band where skin meets hoof; on the teats and udder; and on the sheath, then break within a day or two, leaving raw, painful sores. Animals refuse to eat or drink and may go lame [1].

Horses are usually the first and most visibly affected species, which is how US outbreaks are detected. Some slough the mouth lining; others show only crusting scabs. Laminitis can lead to hoof loss [1]. In the 2025–2026 outbreak, 46 of 49 affected premises had only horses showing signs [2].

Cattle lose weight and milk, and teat lesions can lead to mastitis [1].

Pigs often show foot lesions first and can lose claws. Pigs infected with the New Jersey serotype have higher mortality [1].

Typically 10% to 20% of a herd shows signs, though up to 100% may be infected. Recovery usually takes two to three weeks. Deaths are very rare in cattle and horses, but animals with mastitis or laminitis may be culled [1].

What to Do If You Suspect Vesicular Stomatitis

Call your veterinarian, and do not move the animal or any others off the premises. Treat every blister in susceptible livestock, particularly cattle, pigs, camelids, sheep, and goats, as possible foot-and-mouth disease until the laboratory rules it out, and the veterinarian must notify the state animal health official and USDA immediately [1, 6].

Separate animals with lesions, give each its own feed and water container, and wear gloves when examining mouths or sores; saliva and blister fluid carry the virus [1]. Milk cows with lesions last, and clean and disinfect troughs, buckets, bits, and milking equipment [1].

When possible, bring animals into stalls or pens away from running water, and apply insect repellent, including to the inner surface of the ear, where black flies feed [1].

In a county already confirmed in the current outbreak, state officials may classify a horse with typical signs as a presumptive case without testing [2].

Diagnostic Differential

Vesicular stomatitis cannot be distinguished from foot-and-mouth disease, swine vesicular disease, vesicular exanthema of swine, or Senecavirus A by appearance alone. Only laboratory testing separates them [1, 4]. The US has been free of foot-and-mouth disease since 1929, so the US reports and investigates every blistering disease in a susceptible species.

Horses are not susceptible to foot-and-mouth disease, so a vesicular lesion in a horse narrows the list. It does not remove the reporting obligation.

Other causes of mouth and foot lesions include bluetongue, BVD, malignant catarrhal fever, contagious ecthyma in sheep and goats, photosensitization, chemical or plant irritation, and injury from coarse feed or awns.

Diagnosis

Samples include blister fluid, swabs of ruptured blisters, and loose skin tags from the lesion; sedation is recommended for collection [1]. Samples go through the state animal health official to the USDA National Veterinary Services Laboratories (NVSL) in Ames, Iowa [2].

RT-PCR, virus isolation, and antigen-capture ELISA can be used to detect VSV. Additional characterization, including virus neutralization testing or molecular sequencing, can be used to determine the serotype. NVSL confirmed the 2025 Arizona index cases by sequencing viral genomes from oral lesion swabs [1, 3].

For antibody testing, ELISA and virus neutralization are preferred. Antibodies appear 5 to 8 days after infection, and a fourfold rise between paired samples is diagnostic [1].

Risk to Humans

People can be infected through contact with lesions or saliva, needlestick injuries, laboratory aerosols, and possibly insect bites. Symptoms begin 1 to 6 days after exposure as a flu-like illness: fever, muscle aches, headache, malaise, swollen lymph nodes, and red eyes. Some people develop blisters on the mouth, lips, or hands; many do not [1].

Many infections cause no illness. Among 54 laboratory workers and animal handlers with antibodies, 31 reported a mild illness, and in the 1982–1983 US epidemic, 17 of 133 exposed people showed evidence of infection. People usually recover in four to seven days, and no deaths from laboratory-acquired infection have been reported [1].

Wear gloves and protective clothing when examining or treating animals with lesions [1].

Treatment and Control

There is no specific treatment. Clean lesions with a mild antiseptic, provide soft feed and clean water, treat secondary bacterial infection, and watch teats closely to prevent mastitis [1]. No vaccine is available in the US [1].

The state animal health official quarantines affected and presumptive premises until at least 14 days after lesions appear in the last affected animal [2].

Prevention during an outbreak [1]:

• Keep animals in stables or pens, especially at dusk and dawn, rather than on pasture.
• Keep animals at least a quarter mile from running water where possible.
• Control insects by applying repellent to the inner ear, legs, and belly.
• Do not share feed, water buckets, bits, or grooming tools between groups.
• Check event and interstate movement requirements before hauling; states add entry requirements during outbreaks.

What Vesicular Stomatitis Costs Producers

The main cost is regulatory. A confirmed or presumptive case quarantines the whole premises for at least 14 days after the last animal’s lesions appear, with no animals moving on or off [2].

During outbreaks, other states and events add entry requirements for animals from affected counties, which can keep horses home from shows, rodeos, and sales.

The clinical cost falls mostly on dairies and on horses with severe lesions: mastitis, weight loss, extended veterinary supportive care, and animals culled after mastitis or laminitis [1]. Deaths are rare [1].

Where to Report a Case in the Southwest

Vesicular stomatitis is a notifiable disease on the national reporting list and is handled as an emergency incident because testing is needed to distinguish it from foot-and-mouth disease [6]. Report on suspicion; do not wait for confirmation.

Federal: The veterinarian notifies the state animal health official and the APHIS Area Veterinarian in Charge, who open a foreign animal disease investigation [1, 6].

Documented Incidence in the Southwest

The current outbreak began in Arizona. On October 31, 2025, the National Veterinary Services Laboratories confirmed the New Jersey serotype in two horses on separate premises in Cochise County, and APHIS assessed infected insects as the likely source [3]. By September 18, 2026, the outbreak had reached 49 premises in Arizona, Colorado and New Mexico, 32 confirmed and 17 presumptive. Only three premises, all in Colorado, remained under quarantine [2].

Arizona

15 affected premises in six counties: Cochise, Gila, Maricopa, Pinal, Santa Cruz and Yavapai. One was the Salt River wild horse group in the Tonto National Forest [2]. In the 2020 outbreak, Arizona had 19 affected premises [11].

New Mexico

Four counties had 16 affected premises: Rio Arriba (3), Sandoval (2), Santa Fe (2), and Valencia (9) [2]. In 2020, New Mexico was the first state affected, with the outbreak starting in Doña Ana County, and had 16 affected premises [11].

Texas

No affected premises in the 2025–2026 outbreak [2]. Texas had 10 affected premises in 2020 [11] and was one of three states, with California and Nevada, in the 2023–2024 outbreak of 319 premises [3].

References

1. Spickler AR. Vesicular stomatitis. Center for Food Security and Public Health, Iowa State University, January 2016. https://www.cfsph.iastate.edu/Factsheets/pdfs/vesicular_stomatitis.pdf↑ a b c d e f g h i j k l m n o p q r s t u v w x y z { | }
2. Vesicular stomatitis situation report, 18 September 2026. USDA APHIS Veterinary Services. https://www.aphis.usda.gov/sites/default/files/vsv-sitrep-9-18-26.pdf↑ a b c d e f g h i j k l
3. Vesicular stomatitis situation report, 31 October 2025. USDA APHIS Veterinary Services. https://www.aphis.usda.gov/sites/default/files/vsv-sitrep-10-31-25.pdf↑ a b c d e f
4. WOAH Terrestrial Manual, Chapter 3.1.22: Vesicular stomatitis. World Organization for Animal Health. https://www.woah.org/fileadmin/Home/eng/Health_standards/tahm/3.01.22_VESICULAR_STOMATITIS.pdf↑ a b c
5. Perez AM, Pauszek SJ, Jimenez D, Kelley WN, Whedbee Z, Rodriguez LL. Spatial and phylogenetic analysis of vesicular stomatitis virus overwintering in the United States. Preventive Veterinary Medicine 2010;93(4):258–264. https://repositoriosdigitales.mincyt.gob.ar/vufind/Record/CONICETDig_d48a43ae4cc50d7e4aa5fc18e0e61782↑
6. Voluntary 2026 National Animal Health Reporting System reportable disease list. USDA APHIS, 2026. https://www.aphis.usda.gov/sites/default/files/nlrad-nahrs-disease-list.pdf↑ a b c
7. Arizona Administrative Code § R3-2-402: Mandatory disease reporting by veterinarians and veterinary laboratories. https://www.law.cornell.edu/regulations/arizona/Ariz-Admin-Code-SS-R3-2-402↑
8. 21.30.4.9 NMAC: New Mexico reportable disease list. New Mexico Livestock Board. https://www.srca.nm.gov/parts/title21/21.030.0004.pdf↑
9. 4 TAC § 45.3: Reportable and actionable disease list. Texas Animal Health Commission. https://www.law.cornell.edu/regulations/texas/4-Tex-Admin-Code-SS-45-3↑
10. Reportable diseases. Texas Animal Health Commission. https://www.tahc.texas.gov/regs/reportable.html↑
11. Vesicular stomatitis situation report, 13 November 2020. USDA APHIS Veterinary Services. https://www.aphis.usda.gov/sites/default/files/sitrep-11-13-20.pdf↑ a b c