Equine Vaccines

An important part of equine preventive health care is vaccination.  Vaccines are a way to help prevent problems before they occur.  A vaccine targets the horse’s immune system presenting it with a non-infectious form of a bacteria or virus; the horse can then create immune cells against that agent, so if they are exposed to the real thing, these cells destroy the pathogen before it has a chance to cause disease.  Not all horses are at equal risk for every disease, so it is important to work with your veterinarian to determine a vaccination schedule unique to your horse’s lifestyle.

Core Vaccinations:  These are defined as core vaccines (highly recommended for every horse) by the American Association of Equine Practitioners because they include those diseases with potential public health significance, which are highly infectious, and/or those posing a risk of severe disease. Each of these diseases has a vaccine with proven safety and a high level of effectiveness.

Tetanus- Tetanus is caused by a bacterium which forms spores that can survive harsh conditions and for many years in the soil.  It sets up infections in puncture wounds, lacerations, or surgical incisions and can also travel up the umbilical cord of foals.

The bacterium releases a potent toxin which muscle twitching and rigid paralysis.  Affected horses are rigid, easily excited, and have difficulty eating.  By the time these signs develop, horses do not respond well to treatment and can die due to effects on muscles that control breathing.

The tetanus vaccine is a toxoid, meaning an inactivated form of the toxin that causes disease.  Previously unvaccinated horse should receive one dose and then a booster four to six weeks later with annual vaccination thereafter.

Important Note: Tetanus anti-toxin is not the same thing as tetanus toxoid.  Anti-toxin is used in horses at high risk of disease, who had not been previously vaccinated.  A rare but fatal liver disease can result from use of tetanus anti-toxin in horses; it should never be used without consulting your veterinarian.

Eastern/Western Equine Encephalomyelitis- These diseases are caused by closely related viruses that survive in the wild bird population and then are transmitted to other animals via mosquitoes.  They get their names from the distribution patterns in the US with most cases of EEE occurring in the Midwest and East, while WEE is seen most commonly in the western part of the country.  Because of movement of horses and spontaneous cases outside the normal range, it is recommended that all American horses are vaccinated for both viruses.  Humans are also susceptible to EEE and WEE, although they cannot get it directly from horses, but rather from the same mosquito population that infects horses.

The EEE virus, the most common form seen in Michigan, travels to the brain after entering the bloodstream of a horse via mosquito bites.  There it sets up an infection that leads to rapid deterioration.  Horses begin acting depressed and disoriented within a week of infection.  Over the next few days, they become progressively weaker eventually leading to death in ninety percent of cases.

An inactivated virus vaccine against both viruses is recommended to be given annually in the spring before the start of mosquito season with a booster four to six weeks later in those being vaccinated for the first time.  In warmer climates, where mosquitoes are active year round, it is recommended to vaccinate horses every six months.

West Nile Virus- This virus has some similarities to EEE/WEE in that is maintained in the wild bird population, transmitted by mosquitoes, and can affect humans as well as horses.

Fortunately, horses are much more likely to recover from this disease.  Many horses show only mild depression and decreased appetite, with the virus never making it to the nervous systems.  Those that do develop neurologic signs (often weakness and changes in gait) often can recover with supportive care.

There are multiple vaccines available against this disease all of which help decrease the risk and severity of disease.  The vaccine should be given annually in the spring with a booster three to six weeks later (timing depending on which vaccine used) to horses not previously vaccinated.

Rabies- This virus is carried in the saliva of an infected animal and spread through bites.  Infected horses are typically bitten by wild animals such as raccoons, skunks, or bats.  Rabies is rare in horses, but one hundred percent fatal.  Rabies is also a significant risk to any humans exposed to an infected animal, so it is recommended that all horses be vaccinated.

There is a wide range of possible signs seen in the rabid horse including weakness, head tilt, difficulty swallowing, paralysis of facial muscles, and aggression.  Despite treatment, all of these cases will die typically within ten days of showing clinical signs.

There are a few killed virus rabies vaccines available for horses and it is recommended that they are vaccinated annually with no additional booster needed in the weeks following the first vaccine.

Horses that die or are euthanized due to neurologic disease should be submitted to a diagnostic lab for post-mortem testing.  That way, if the horse was infected with rabies or another disease potentially transmissible to people, appropriate precautions can be taken by those people exposed to the horse.

Highly Recommended Vaccinations:  While these vaccines are not denoted as core by the AAEP, we still recommend them for all horses, as these viruses can be quite contagious and are capable of causing severe disease.  These include diseases that can spread through the air or on surfaces.  Young horses who have not built up immunity are at particularly high risk as are any horses that travel or have exposure to other horses.

Equine Rhinopneumonitis- Equine Herpesvirus type I and type IV can infect the respiratory system causing depression, fever, nasal discharge, and cough.  It is spread from horse to horse through nasal secretions in the air or shared equipment.  EHV-I can also cause spontaneous abortion in pregnant mares or neurologic disease.  The disease can also be spread through contact with aborted fetuses or placenta.  Horses can carry the virus and have no signs of disease but still transmit it to more susceptible horses (particularly young horses.)

There are modified live viral vaccines available that can help prevent respiratory disease or both respiratory disease and abortions.  No vaccine prevents neurologic disease.  At risk horses should receive an initial vaccine followed by one to two boosters at three to four week intervals and then be vaccinated on a yearly basis.  Horses on breeding farms, performance horses, and animals at facilities with many horses coming and going (such as boarding stables) are considered high risk and should be vaccinated every six months.  Pregnant mares should be vaccinated with a product that protects against abortion in the fifth, seventh, and ninth month of pregnancy.

Equine Influenza- Equine Influenza is caused by a virus which infects the respiratory system leading to coughing and nasal discharge.  It is spread through nasal droplets in the air and is very contagious.  Young horses are more susceptible, but you can see outbreaks in older horses as the virus mutates and immunity decreases.

Several vaccines are available including a form that is administered in the nostril rather than as an injection.  The vaccine should be given yearly with a possible booster four to six weeks after the first dose depending on which vaccine used.  Young horses that come into contact with lots of other horses may need to be vaccinated every six months.

Other Vaccinations:

Strangles- Strangles is a disease caused by a bacterium characterized by large amounts of nasal discharge, abscesses of the lymph nodes under the jaw, and occasionally abscesses in other internal organs.  It is very contagious and is spread by direct contact with discharges or indirect contact via shared tack or equipment.

The vaccine is recommended for horses at high risk of infection, those coming into contact with many other horses, or where there has been a previous outbreak.  One complication of the disease or sometimes the injectable form of the vaccine is a syndrome called purpura hemorrhagica; this is a non-contagious immune response which leads to swelling of the head and limbs, with small areas of bleeding under the skin.

Two forms of the vaccine are available: an injectable form and one administered intranasally (via the nostril).  The injectable form will decrease the severity of disease, but not prevent it completely.  The intranasal vaccine will rarely cause a mild version of the disease, but offers a higher level of protection.  We recommend the intranasal vaccine.          

Potomac Horse Fever- Potomac Horse Fever is a disease caused by a bacterium and characterized by fever, diarrhea, and an increased chance of laminitis.  It is spread through ingestion of certain fly larvae or snails found near water sources.  It is relatively common in Michigan due to the amount of water here.  Cases occur most commonly in the late summer months.

The vaccine does not prevent disease completely, but does decrease the severity.  Horses should be vaccinated in the spring so that immunity is maximized throughout the summer.

Equine Protozoal Myeloencephalitis- EPM is a neurologic disease caused by a protozoal parasite that migrates throughout the spinal cord.  Horses with EPM generally have asymmetric muscle loss with an ataxic (uncoordinated) gait.

There are a few vaccines available for this disease, but it has not been shown how effective they are in preventing the disease.  They do tend to increase antibody levels in the blood, which may actually make the disease more difficult to diagnose if it does occur.  Talk to your veterinarian before deciding to vaccinate your horse for this disease for the costs may outweigh the benefits.

Vaccine reactions:

As most vaccines are injected into the muscle, horses may have some swelling or soreness at the injection site.  They may also develop a slight fever and be a little lethargic following vaccination.  These tend to be self-limiting and resolve without treatment.  Serious reactions are rare, but can occur (including anaphylaxis with severe facial swelling and difficulty breathing.)  Because of this risk, vaccines should always be administered by a veterinarian or under veterinary supervision.

For more information on horse vaccines, including those vaccines specific to breeding mares and young foals, visit the vaccination guidelines website from the American Association of Equine Practitioners: http://www.aaep.org/vaccination_guidelines.htm