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Equine Vaccinations 101
by Shanna Nelson DVM
Twice a year, you dutifully schedule your horse’s vaccination appointment. The veterinarian comes out, performs a wellness examination, delivers a shot or two, and (barring additional health concerns) deems your horse good to go. You know that vaccinations are important to protect your horse’s health, but do you know exactly what diseases we commonly vaccinate against, and what is included in the yearly schedule? Below is a typical recommended vaccine protocol for the Midwest, based on guidelines set out by the American Association of Equine Practitioners. But first, a list of the diseases we vaccinate against:
Rhinopneumonitis (Equine Herpes Virus 1 & 4)
EHV-1 and EHV-4 are common causes of fever and respiratory disease, especially in young horses. Like all herpes viruses, EHV establishes lifelong latency in infected horses, meaning that animals who have seemingly recovered from the disease can begin showing clinical signs and shedding infective virus particles later in life, especially during times of stress or immunocompromise. Immunity for EHV is short-lived, even following natural infection, so frequent vaccination is needed, especially for horses at high risk of infection. Notably, EHV-1 is a leading cause of pregnancy loss in mares and is also responsible for the life-threatening “neurologic herpes” outbreaks that have been seen throughout the country in recent years. Older horses are more likely to display devastating neurologic symptoms than younger animals.
Equine influenza is a common respiratory infection, again primarily affecting younger animals. While the disease is rarely fatal, it can lay a horse up for some time while recovering, and it also predisposes to secondary bacterial pneumonia, which complicates treatment. Influenza is highly contagious.
Encephalitis Viruses (Eastern Equine Encephalitis and Western Equine Encephalitis)
EEE and WEE are mosquito-borne viruses and are thus most prevalent in the spring and summer. They cause severe neurologic disease which is commonly fatal. Humans can also contract EEE and WEE from mosquitoes, but horses cannot directly transmit the disease to people or other animals.
West Nile Virus
WNV has now been established in the United States for well over a decade. Like EEE and WEE, it is spread by mosquitoes and causes neurologic disease. While some horses can recover from WNV, the treatment is involved, costly, and carries no guarantees.
The bacterium that causes tetanus, Clostridium tetani, lives in the soil and is often introduced via an otherwise innocuous wound. It produces a toxin that causes rigid muscle paralysis which develops over several days; horses are especially sensitive compared to other animals. Once a horse is showing clinical signs, treatment is seldom successful, and death occurs due to respiratory paralysis.
While horses rarely develop the “furious” form of rabies portrayed in pop culture (think Cujo or Old Yeller), they are still susceptible to the virus, as even the most pampered show horse invariably has some exposure to wildlife. Rabies is always fatal, and affected horses can show any number of neurologic signs, most commonly a change in attitude that manifests as lethargy and disinterest, then progresses to inability to eat or drink and eventually paralysis and death. Rabid horses can transmit the virus to people, which is why all neurologic horses should be handled with extreme caution (and gloves) and why all horses that die of unknown neurologic disease should undergo rabies testing.
In the spring, we typically vaccinate against all of the above agents, using two separate injections (one for rabies, and a combo shot for the other diseases). In the fall, we recommend boostering against all but rabies to ensure adequate immunity in a midwestern climate, which can see active mosquitoes throughout most of the year. Foals or adult horses receiving their initial shots will need additional boostering to develop full immunity. Pregnant broodmares should be immunized against rhinopneumonitis at 3, 5, 7, and 9 months of gestation to help prevent herpes-associated abortion. Horses who frequently travel to shows or intermingle with outside horses should be immunized more frequently against rhinopneumonitis and influenza. Horses who have immunocompromise (such as the very old or those affected by Cushing’s disease) may need to have their vaccines spread out over several visits, rather than being administered via the usual combination shot. Animals who have previously experienced significant vaccine reactions (which are uncommon, but do occur) may have to do without certain vaccines, or may require pre-treatment with an anti-inflammatory agent to reduce their reaction risk.
Another commonly administered vaccine is for Streptococcus equi subspecies equi, the causative agent of Strangles (equine distemper). This is an intranasal vaccine, not an injection, which is usually administered once per year. Those who have seen a horse affected by Strangles are unlikely to forget it, as the draining, pus-filled abscesses and swollen lymph nodes are an impressive sight. However, this vaccine is not as effective as some of the others, and in the face of an outbreak, vaccinated horses may only develop a less severe form of the disease than their unvaccinated herdmates. Additionally, horses who already have a high titer against Strep equi can sometimes mount an immune response to the vaccine that results in severe inflammation of the blood vessels and resultant disease. This is why the vaccine is usually administered based on an assessment of a horse’s individual risk factors, weighing the pros and cons.
Potomac Horse Fever
Potomac Horse Fever is another common vaccine for this region. This disease is caused by rickettsial organisms, a type of bacteria, and spreads when a horse accidentally consumes an invertebrate host such as a mayfly. Severe diarrhea subsequently develops, and laminitis occurs secondarily. Antibiotics and intensive supportive care (intravenous fluids, etc.) are needed for treatment, but affected horses are often euthanized due to a poor prognosis. Studies have shown that the vaccine is not highly effective, but may lessen the severity of disease. Vaccination is recommended every 6-12 months.
There are several other vaccines available for horses. These vaccines are less commonly administered, since these diseases are not prevalent in our region (and because for some, the efficacy and safety data is lacking). A short (but not exhaustive) list is below.
This disease is caused by a relative of the tetanus-producing bacterium, Clostridium botulinum. Rather than a rigid paralysis, botulism results in flaccid paralysis. Foals can acquire the disease from their environment, but adult horses are usually infected from contaminated round bales of hay. Treatment is rarely effective and death occurs from respiratory failure.
This is a brand new vaccine, and therefore there is limited information out regarding its safety and efficacy, although initial data has been promising. Leptospirosis is a bacterial infection that can cause kidney or liver disease, pregnancy loss, and uveitis (inflammation of the eyes which may eventually lead to blindness). However, many horses are exposed to the bacterium without ever developing noticeable disease. Transmission occurs through contact with wildlife or contaminated water sources.
A snake venom toxoid product is available for horses in endemic regions at risk of suffering a rattlesnake bite. Frequent vaccination (twice yearly) is recommended to provide adequate protection.
Hopefully this short list helps to clarify exactly what your horse is receiving when he gets his “shots.” At Fox Creek Veterinary Hospital, we are willing to work with you to formulate a customized vaccination schedule for your horse that more accurately reflects his needs and risk factors, and are happy to answer any questions you may have about vaccinations and the diseases they are helping to prevent. Please don’t hesitate to give us a call at 636-458-6569 if you have any questions or concerns about vaccination.