Risk-Based Vaccination Protocols for Horses

Veterinarians administer risk-based vaccines based on a horse’s potential to contract certain diseases. Learn more in this article from The Horse‘s 2024 Preventive Care issue.
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Know when your horse might need one of these important vaccines

how to protect your horse from equine influenza; Massachusetts Horse Owners Urged to Vaccinate
Core vaccines are important at least annually, but the need for risk-based vaccine plans can vary regionally and should be tailored to the individual horse or farm. | The Horse Staff

Vaccination serves as a core component of equine preventive medicine that can help keep your horse and others safe from dangerous and potentially deadly diseases. Ideally, equine veterinarians administer vaccines early in the year to prepare horses’ immune systems for the coming months when they will likely have the most exposure to some potentially infectious pathogens.

Core vaccines are a necessary part of all horses’ yearly wellness plans with their veterinarians. Regardless of your horse’s activity level, housing, or use, the American Veterinary Medical Association (AVMA) and American Association of Equine Practitioners (AAEP) recommend vaccinating horses annually against diseases that have the potential to affect public (horse and human) health, must be vaccinated against by law, or can cause severe disease or death. These include rabies, tetanus, West Nile virus, and Eastern and Western equine encephalomyelitis (EEE and WEE).

“Core (equine) vaccines are those vaccines that are recommended for all horses, at least annually,” says Elizabeth Davis, DVM, PhD, Dipl. ACVIM, professor and associate dean of clinical programs at Kansas State University’s College of Veterinary Medicine, in Manhattan. “These are diseases that we should recognize that a horse could contract while standing in a pasture, and they are typically diseases that if infected, will be likely to result in serious if not life-threatening and possibly zoonotic (contagious to humans, in the case of rabies) disease.”

Veterinarians administer risk-based vaccines, however, based on a horse’s potential to contract certain diseases, after the owner and veterinarian perform a risk-benefit analysis. Risk-based vaccine plans can vary regionally and should be tailored to the individual horse or farm. When managing a group of horses, combining meticulous biosecurity practices with a strict vaccination protocol can help reduce disease risk.

Risk-Based Vaccines for Horses

Most often, horse owners and their veterinarians determine what risk-based vaccines, if any, a horse could benefit from based on his location, workload, frequency of travel, and resident herd status (i.e., how much the horses leave the farm and commingle with others). The AAEP lists the available risk-based vaccines as:

  • Anthrax Horses can contract this serious, septicemic (caused by the spread of bacteria and their toxins in the bloodstream) disease through inhalation, contamination of a wound, ingestion, or mechanical transmission by blood-sucking insects. Horses in geographic locations with alkaline soil are at the highest risk because this environment supports Bacillus anthracis (the causative bacterium) spore survival. Anthrax carries a high mortality rate and can be transmitted between animals of different species.
  • Equine influenza is the most common respiratory disease in horses and is highly contagious. The virus can spread rapidly through aerosolized droplets distributed when infected horses cough and clear their airways. The most common clinical signs include coughing and abnormal respiratory sounds but also fever, edema, and enlarged lymph nodes.
  • Botulism This is the most potent known biological toxin, produced by the bacterium Clostridium botulinum. It causes neurologic clinical signs beginning with weakness and quickly progressing to paralysis and often death. The bacterium can be found in decaying animal carcasses or plant material, and horses contract it by inadvertently ingesting it while grazing or eating hay. 
  • Leptospirosis Infected animals, including wildlife, spread the causative bacteria, Leptospira, through urine and other bodily fluids. Horses can be exposed via open wounds or the mucous membranes, most commonly via water or soil. Clinical signs can include uveitis (ocular inflammation), placentitis or abortion in pregnant mares, or acute renal failure.
  • Potomac horse fever While this infection most commonly occurs in horses in the eastern United States residing near the Potomac River, cases have been reported worldwide. Horses are most likely to be infected with the causative bacterium, Neorickettsia risticii, between late spring and early fall. N. risticii has a complex life cycle, first infecting parasites of freshwater snails, which the snails release when the water is warm, then infecting larval stages of aquatic insects such as caddisflies, mayflies, damselflies, dragonflies, and stoneflies. These aquatic flies thrive abundantly during specific times of the year and can serve as a reservoir for potential infection during the summer and fall months; typically, horses ingest the flies after they’re attracted to lights in the barn and fall into horses’ feed buckets and hay, or they can ingest them in water. Clinical signs vary but can include diarrhea, fever, mild/moderate colic, and acute laminitis.
  • Snakebite In areas where venomous snakes such as rattlesnakes and copperheads are abundant, horses have a higher risk of getting bitten, which can lead to death if not treated immediately. Clinical signs can vary based on the type of snake but typically include pain and swelling at the site of the bite. After horses have recovered from snakebite, veterinarians typically monitor them for heart failure or kidney damage, which can occur in the weeks following the bite.
  • Strangles, caused by the bacterium Streptococcus equi subspecies equi, most commonly infects young horses. Some infected horses might become carriers for an extended time, meaning they can shed S. equi for months or years. This disease is highly transmissible through both direct (i.e., nose to nose contact between horses) and indirect (i.e., a horse drinking from a water bucket that an infected horse used) contact between horses. Infected horses typically exhibit clinical signs such as fever and nasal discharge or pus draining from ruptured lymph nodes around the throat.
  • Rotavirus This viral infection is spread through the fecal-oral route and is a common cause of illness and death in foals, though it can be largely prevented by vaccinating the dam during pregnancy. Infected foals typically have diarrhea, are lethargic, and will not eat. Although a vaccine for foals exists, there is no published research showing evidence that the vaccine can provide them significant protection.
  • Equine viral arteritis While typically not life-threatening to adult horses, equine arteritis virus is spread through respiratory secretions in close quarters (i.e., horses stabled near one another), fomites (brushes, humans, etc.), and breeding. Equine viral arteritis can cause abortion in mares, death in young foals, and stallions typically become lifelong carriers. Clinical signs of equine viral arteritis can vary greatly from fever and depression to localized swelling of the limbs (especially hind limbs), scrotum or mammary glands, and underside of the abdomen.
  • Equine herpesvirus-1 and -4 are most common in horses that commingle with horses from other farms. Both are spread by direct and indirect contact and in many cases establish latent (hidden) infection in horses that then become asymptomatic carriers. Clinical signs of EHV-1 include respiratory disease, abortion, and neurologic defects, while EHV-4 typically causes respiratory disease.
  • Venezuelan equine encephalomyelitis Horses living in Southern Texas, California, Louisiana, Mississippi, Alabama, and the west coast of Florida have the highest risk of contracting this disease, which is most often seen in Central and South America. This virus is also typically transmitted through mosquitoes. Venezuelan equine encephalomyelitis can cause fever, depression, paralysis, gait abnormalities, and seizures, but the prognosis depends upon the subtype of the virus.

Assessing Your Horse’s Risk

“It is important to make a risk-based decision to understand what is best for the individual horse and the population in which it lives,” says Noah Cohen, VMD, MPH, PhD, Dipl. ACVIM, professor of equine internal medicine, Patsy Link professor of equine research, and associate department head for research and graduate studies in the Department of Clinical Sciences at Texas A&M University’s School of Veterinary Medicine and Biomedical Sciences, in College Station. Not all risk-based vaccines are necessary for every horse, and some might be harmful to certain groups of horses, he adds.

2 horses looking out of stall windows in barn aisle
Horses that live close to those that travel frequently are at an increased risk of contracting disease even if they don’t leave the farm. It is important to vaccinate them as well. | Getty images

Practitioners might recommend vaccinating horses based on the resident farm population’s risk. An older horse that stays on the farm could have a decreased risk of developing some diseases, but his veterinarian might recommend vaccinating him because young show horses also live on the property.

“Vaccinating the older horse might strengthen herd immunity even if it has little impact for the individual horse,” says Cohen. “Risk-based vaccination should be considered case-by-case (where the case might be a horse, a herd, or both) and there is subjectivity in this decision-making process. For example, a veterinarian might recommend that horses be vaccinated for strangles at a farm with a history of this disease, but not for other horses living in settings where the disease is considered low risk. Another veterinarian might say vaccinating all horses for strangles would reduce the burden of disease for all horses

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Written by:

Haylie Kerstetter, Digital Editor, holds a degree in equine studies with a concentration in communications and a minor in social media marketing. She is a Pennsylvania native and, as a horse owner herself, has a passion for helping owners provide the best care for their horses. When she is not writing or in the barn, she is spending time with her dog, Clementine.

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