Vaccinations and Your Horse

How do they work?

Vaccines contain inactivated disease causing agents (most commonly viruses, but also bacteria and toxins), or specific parts of them, called antigens. When given, the horse’s immune system responds, creating proteins called antibodies. These antibodies are protective in the event the horse later encounters the actual virus, bacteria, or toxin. The number of vaccinations required to produce a satisfactory antibody response may be different for each vaccine, so follow your veterinarian’s protocol for any initial vaccination series.  If your horse continues to require protection every year, booster these vaccinations as indicated by the vaccine manufacturer and/or your veterinarian. While no vaccine should be considered 100% protective, many are virtually so. Even vaccines that are not designed to “prevent disease” can be very useful in that they lessen the severity of disease, should your horse be infected.

How do I know which vaccines my horse needs?

There is no simple answer to this question, as it depends on each horse’s individual circumstances. Your veterinarian will ask you a few questions about your horse’s lifestyle that will help determine which diseases he/she is at risk for, and which vaccines may be warranted. That said, most horses should receive at least the “core” vaccinations recommended by AAEP- the American Association of Equine Practitioners, which are described below.

Rabies- Rabies is not a common disease in horses. Rabies is a viral disease spread through saliva, usually by being bitten by an infected wild


animal. If an unvaccinated horse is bitten by a rabid animal such as a bat or raccoon, they will rapidly show neurologic signs and die shortly thereafter. Horses often have very different symptoms than other rabid animals, and unfortunately have the potential to expose many people before they are diagnosed. The rabies vaccine is inexpensive, very effective, generally safe, and ultimately safeguards human health, so we feel that every horse should get this vaccine annually. Because rabies is a reportable disease, this vaccination can only be given by a licensed veterinarian. Horses in our practice are vaccinated against rabies in the fall.

Tetanus- Tetanus is caused by a toxin produced by a bacteria called Clostridium tetani. This bacteria is present in soil, so all horses are at risk of exposure in their environment. When the bacteria enters the horse’s body, usually through a wound, the bacterial toxin attacks the horse’s nerves where they insert on muscles. As a result, horses with tetanus show muscle spasms and stiffness that gets worse and can lead to paralysis and death. It is difficult and expensive to treat tetanus once signs develop. There are several different vaccine types, the most common being tetanus toxoid, which contains the inactivated bacterial toxin. This is available as a single part vaccine and is also commonly included in combination vaccine products. Tetanus toxoid is readily available, inexpensive, and effective. There is also a tetanus antitoxin, which your veterinarian may choose to give if your horse is injured and has never had (or is overdue for) their tetanus toxoid vaccination. We recommend the tetanus toxoid vaccine be given once a year. If your horse has a penetrating injury, and their last tetanus vaccine was more than 6 months ago, we will often give a booster vaccination. Horses in our practice receive the tetanus toxoid vaccine in combination with EEE and WEE in the spring.

Eastern and Western Encephalitis (EEE/WEE)- EEE and WEE are viruses that cause progressive neurologic signs in horses and are spread by mosquitos. EEE is most common in states along the east coast, and in the southeastern US, but has also been reported in Ohio, Michigan, and Wisconsin. WEE is most common west of the Mississippi river, but has also been reported in the Midwest. The clinical signs for both EEE and WEE include depression, circling, wandering, head pressing, paralysis, seizures, and death. Unvaccinated horses have mortality rates between 20-90%, depending on the specific strain of virus. There is no treatment other than supportive care. Reported cases rise with peak mosquito season, in late summer to early fall. Horses living in or traveling through affected areas should be vaccinated, at least once a year. For most horses in our practice, we recommend once a year vaccination in the spring, prior to the start of mosquito season. If your horse travels to warmer regions during the winter (where mosquitos are active year-round), we recommend vaccinating every 6 months.

West Nile Virus (WNV)- West Nile Virus is another disease that is transmitted by mosquitos. It has now been reported in all 48 contiguous states since being first identified in the US in 1999. Signs of the virus in horses include fever, refusing feed, colic, lameness, tremors, facial paralysis, hyper-excitability, weakness, narcolepsy, and ataxia (incoordination). It is estimated that 1/3 of horses infected with WNV die from the disease. There are no specific treatments available, only supportive care. The vaccine is not 100% effective in preventing the disease in all horses, but previously vaccinated horses who become infected have much milder symptoms and generally recover fully. This vaccine is available as a single product or as a combination vaccine, most commonly combined with EEE, WEE, and tetanus toxoid. We recommend this vaccine be given in the spring, prior to the start of mosquito season. Like for the EEE & WEE vaccine, if your horse heads south for the winter, we recommend boosters every 6 months.

Additionally, we may recommend that your horse receive additional “risk-based” vaccinations. Our recommendations are personalized to your horse, farm or boarding environment, and how much/where your horse travels. Risk based vaccines commonly used in our practice are detailed below. 


Equine Influenza Virus (“Flu”, EIV)- Much like the influenza virus in people, the equine influenza virus is highly contagious and causes upper respiratory infection, that is usually mild. Infected horses can quickly spread the virus to an entire barn by coughing and sneezing (droplets can be spread up to 30 yards!) Signs of infection include, depression, not wanting to eat, nasal discharge, coughing, sneezing, and exercise intolerance. Luckily, the virus is generally not life threatening to healthy adult horses, but can be expensive to treat and takes time away from riding. We recommend that horses be vaccinated for influenza once to twice a year, unless your horse lives in a “closed herd” situation. This vaccine is often given in combination with the EHV vaccine.

Equine Herpes Virus (“Rhino”, EHV)- Similar to influenza, equine herpes virus also is a highly contagious upper respiratory virus. Unlike influenza, rhino can also cause more serious signs depending on the type or strain involved.  EHV-1 and EHV-4 can both cause upper respiratory signs (fever, depression, loss of appetite, nasal discharge, cough) but EHV-1 can also cause abortions in pregnant mares, and specific strains have been associated with severe neurologic signs (weakness, incoordination, paralysis, and even death). Horses spread this virus by coughing, sneezing, or sharing equipment (tack, water buckets etc). Pregnant mares should be given a specific vaccine against EHV in months 5, 7, and 9 of pregnancy to prevent abortions. Horses who travel frequently or are in contact with many other horses should receive an EHV vaccine once to twice yearly, usually in combination with EIV.

Potomac Horse Fever (PHF)- Potomac Horse Fever is the clinical syndrome caused by a bacteria, Neorickettsia risticii. The exact mode of transmission is unclear, but aquatic insects are thought to play a role. The disease has been reported throughout the US, but is most common in the Northeast and Midwest during spring, summer, and fall. Farms located near creeks, rivers, and ponds report a higher incidence of disease, which tends to persist year after year. Infected horses are thankfully not contagious to their barn-mates. Signs of infection initially include lethargy and loss of appetite, followed by fever, loose manure, and in about a third of cases, laminitis. Severe cases may become dehydrated and/or septic and require intensive care.  The fatality rate in unvaccinated horses can be up to 30%, and is most often the result of severe laminitis, requiring humane euthanasia. While the PHF vaccine is not regarded as being highly effective at preventing all cases, vaccinated horses who become infected are much less likely to develop laminitis or severe diarrhea, and are much less likely to succumb to the disease. We do see a fair number of PHF cases in the late summer and fall, and may recommend this vaccine in the spring, especially if your farm had a case previously.

Streptococcus equi ssp equi (“Strangles”)- A dreaded term in most barns, strangles is the horse version of strep throat. The most common clinical signs are thick white to yellow nasal discharge, fever, loss of appetite, depression, and swollen lymph nodes. Rarely, the bacteria can travel throughout the body, causing abscesses in other regions (“bastard strangles”), or can cause the horse’s body to produce immune complexes that damage blood vessels (pupura hemorrhagica). These syndromes are rare, but can occasionally be fatal. The disease spreads when horses come into contact with the infectious nasal discharge or abscess material. This exposure can be directly from an infected horse or indirect from bacteria on objects around the barn (bedding, buckets, tack, etc). Frustratingly, about 10% horses recover clinically but go on to become “carriers” meaning that while they show no signs, they did not fully clear the infection and continue to shed bacteria and can infect other horses. These carriers can make clearing up a strangles outbreak a long and tedious process. The best prevention is a strict isolation protocol for new horses and good hygiene, and occasionally vaccination. The vaccinations available for strangles are not 100% effective at preventing the disease, but can decrease the overall incidence and severity, especially when considered at the herd level. In high-risk populations, we may recommend using the modified live vaccine that is administered up the horse’s nose, once a year.

Can’t I just buy my vaccines at the feed store and give them myself?

fant horses

Technically, yes. Many (but not all) vaccinations are available through agricultural supply stores and on various websites. There are several reasons why you should consider having a veterinarian vaccinate your horse, rather than doing it yourself which we’ll highlight briefly. First, we have experience and special training that are crucial in deciding which vaccines are appropriate for which horses and why. We stay current on the best and newest vaccine products as well as relevant disease outbreaks in order to give your horse customized protection, without giving unnecessary vaccines. Second, vaccines must be stored and handled properly in order to be effective. Equine veterinarians receive their vaccine products directly from the manufacturer or supplier, and store them under refrigeration in the clinic or in our vehicle until they’re given. We strategically order our vaccines so that we do not have product that sits around until it expires. We also have a much deeper understanding of anatomy and vaccine labeling, and will ensure each vaccine is being given in the manner that the manufacturer describes (intramuscular vs subcutaneous vs intranasal). As such, manufacturers’ guarantees will only apply to vaccines that are administered by a veterinarian. Additionally, if you need these vaccinations in order to travel or attend an event, a veterinarian must certify their administration on your health papers. We cannot certify vaccines we didn’t give. Third, while vaccine reactions are rare, veterinarians are equipped to deal with them right away. If your horse has a severe vaccine reaction to a vaccine given by a veterinarian, in many cases, the vaccine manufacturer will pay for any treatment needed. What if your horse has a severe reaction to a vaccine you administered? The manufacturer won’t cover any treatment. The same is true if your horse is insured; you are only covered if a veterinarian gave the injection (this actually applies to all injectable medications!). Lastly and most importantly, these vaccination appointments are an important part of your horse’s overall health plan. It’s a great time for a complete physical examination, to do an oral exam and dentistry procedures, to discuss your horse’s diet and to pick your vet’s brain about any number of things! We love seeing healthy horses at wellness visits, and it gives us a good idea about what is normal for your horse. If you ever do need us in an emergency situation or when your horse is ill, it’s invaluable for us to have that baseline to compare with. 


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