What you need to know to protect your equine companions
Spring is coming, and along with the excitement of shedding horses (finally!) it is time to start
thinking about vaccinations for our equine friends. There are many different vaccines available
for several diseases, and this can be confusing to horse owners. When considering what
vaccines to administer, we need to evaluate each horse as an individual, as their requirements
will differ depending on their age, geographical location and potential exposure to diseases.
The best person to assist in this decision making process is your veterinarian, as he or she can
formulate a plan based on your horse’s personal needs.
In this area (Mid-Atlantic), I prefer to administer yearly vaccinations in two “bunches”, one in
the spring and one in the fall. In this way, we can provide optimum protection as close to the
possible threat of disease as possible, and keep the immune system on its toes without over-
stimulating it. Any vaccinations for mosquito or other insect-borne diseases (West Nile Virus,
Potomac Horse Fever, Eastern and Western Encephalomyelitis) should be given in the spring.
Vaccination for botulism is best done in the fall, as the disease is more common in the wet
months of the year. Rabies vaccination can be done any time of year, as can strangles and
rhino/flu. There are no established protective titer ranges for horse diseases at this time, so
although we are able to draw blood and provide a number, we are unable to determine exactly
what that means for protection of the horse’s health.
If you are planning to begin administering a vaccine for a disease you have not previously
vaccinated for, either in a mature horse or a foal, the horse will need to complete what is called
a “primary course” of vaccination, in order to get the immune system primed to recognize the
diseases as they are presented to the body. This course is generally two administrations,
usually a month apart. The exceptions are botulism, which requires a course of three shots to
be protective, and rabies, which we usually booster a year after the initial dose. Some West
Nile Virus vaccines are also a one-dose primary course, but this varies by the drug company and
product. Vaccines for diseases in pregnant mares such as rotavirus and the abortion-causing
strain of EHV require 3 shots a month apart at a specific time during gestation. Pregnant mares
also require their routine vaccines to be boostered one month prior to foaling to ensure the
foal receives adequate antibodies in the colostrum (first milk). You should consult with your
veterinarian regarding the proper timing of vaccination in pregnant mares and foals on an
individual basis.
So what diseases should we be concerned about? Many clients are surprised to learn that their
unvaccinated horse is at risk for many preventable diseases even though it may never leave the
farm. Although vaccines are not fool-proof for preventing diseases, they definitely reduce the
chance of contracting diseases and lessen the symptoms in affected individuals. Let’s go
through the most common equine diseases covered by vaccines one at a time:
Rabies
Rabies is a viral disease spread by the body fluids of an infected individual either into mucous
membranes such as in the eyes or mouth, or through open wounds or bites. It is obviously also
a zoonotic disease, meaning that it can be transmitted to humans by other animals. While
there is no legal requirement for rabies vaccination in horses since they are not considered
“pets”, and it is not especially common in horses, it is not a vaccine I would skip, as it is fatal.
Any horse that lives outside is at risk of contracting rabies, even if stall-bound. The most
common vectors of rabies to horses are skunks, raccoons, foxes and bats. The disease causes
varying degrees of neurologic signs including inappetance, erratic behavior, drooling, and
depression. Exposed animals are subject to quarantine, more so if unvaccinated. Not all
exposed animals will die, but non-vaccinated symptomatic animals should be euthanized to
reduce exposure to other animals and humans. The only definitive diagnostic test is one that
requires euthanizing the horse and submitting the brain for evaluation. Rabies vaccine should
be administered yearly and is a core vaccine recommended in all horses by the AAEP (American
Association of Equine Practitioners).
Tetanus
Horses are exquisitely sensitive to infection with tetanus, which is caused by a clostridial
bacteria found in soil and manure. Tetanus is another core vaccine recommended by the AAEP
for all horses. Horses are usually infected with tetanus via a wound such as a laceration or a
foot abscess, or also via the umbilicus of new foals. The disease is not contagious between
individuals. Tetanus causes neurologic signs including alterations to the facial nerves that result
in difficulty chewing or opening the mouth, improper function of the eyes, excessive reactivity
to light or sound and a stilted gait. Tetanus is fatal in most horses although early cases may be
treatable with tetanus antitoxin administration. Most horses should be vaccinated for tetanus
once a year, and the vaccine is usually combined with other disease vaccines such as eastern
and western encephalomyelitis. Horses with acute wounds should have a tetanus booster if
they have not had one in the last six months.
Eastern and Western Equine Encephalomyelitis
These viral diseases are spread by bites from infected mosquitoes, with wild birds and rodents
acting as natural reservoirs for the disease. Therefore, any horse with access to the outdoors
should be vaccinated against them. They are also zoonotic diseases, being spread to humans via
infected mosquito bites, although horse to horse and horse to human transmission via
mosquito bite is rare. These diseases cause neurologic signs such as difficulty swallowing,
behavioral changes, impaired vision, seizures, circling and recumbency. EEE and WEE have the
potential to be lethal, although many cases are able to recover with supportive care. The vaccine for EEE and
WEE is usually combined with a tetanus vaccine. In this area, most horses are vaccinated once a year,
although horses living in or travelling to warmer areas such as Florida should be boostered appropriate to their
situation. The AAEP recognizes both EEE and WEE as core vaccines for every horse.
West Nile Virus
Another AAEP core vaccine, West Nile is a virus spread by infected mosquitoes via birds and is another
potentially zoonotic disease. Again, any horses with outdoor access should be vaccinated. The virus cannot
be spread directly from horse to horse or horse to human. Indirect transmission between horses and humans
via mosquitoes is unlikely as horses do not retain a large amount of virus in their bloodstream after infection.
This virus causes similar neurologic signs to EEE and WEE as it leads to encephalomyelitis. Although horses
usually recover (although many have lasting deficits) with supportive treatment, approximately 33% of infected
horses succumb to the disease. The booster recommendations for WNV are similar to those for EEE/WEE as
far as mosquito-prone areas are concerned.
Rhinopneumonitis (Rhino/EHV) and Influenza (Flu)
Although not a core vaccine, rhino/flu (usually combined into one vaccine) virus coverage is important for
horses that come in contact with other horses directly, or indirectly by sharing stalls, water buckets, etc. as
these diseases are spread by contact with infective nasal secretions. Horses that live isolated from other
horses (aerosolized particles from a cough can travel over 30 feet!) with caretakers that do not contact other
horses off the farm are at low risk of contracting these viruses and therefore may not require the vaccination.
This vaccine can be given either once a year or twice a year depending on exposure. These two viruses cause
respiratory disease in most cases, but variants of EHV can cause neurologic disease or abortion in pregnant
mares (Pneumabort is one vaccine used to protect from abortive EHV). Young horses are more susceptible to
both viruses. Currently available EHV vaccines have a very low efficacy against the neurologic form of EHV,
unfortunately.
Botulism
Botulism is a bacterial disease caused by ingestion of, or wound contamination by, infective spores of a
clostridial bacteria found normally in soil.
Check out my earlier post on botulism for more detailed info on the disease. Botulism vaccine is not given in all parts of the country, but it is especially important in this area and
other places where wet weather and mud are commonplace. It is not considered a core vaccine by AAEP, but
having seen several cases in this area, it is one vaccine I would not skip. This vaccine is usually administered
once a year and I prefer to give it in the fall since there are more botulism cases during muddy weather.
Stay tuned for part two, where I will continue discussing more risk-based equine vaccinations, including
Potomac Horse Fever, Strangles, Rotavirus, and Pneumabort. Enjoy this beautiful weather in the meantime!