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Vaccination is the process of administering weakened or
dead pathogens to a healthy person or animal, with the
intent of conferring immunity against a targeted form of a
related disease agent.
The term was coined by Edward Jenner (1796) and adapted by
Louis Pasteur for his pioneering work in vaccination.
Vaccination (Latin: vacca—cow) is so named because
the first vaccine was derived from a virus affecting cows:
the cowpox virus, a relatively benign virus that provides a
degree of immunity to smallpox, a contagious and deadly
The vaccinations administered by your veterinarian to your
horse place a protective barrier between the animal and a
whole list of problems: tetanus, encephalomyelitis(sleeping
sickness), influenza, rhinopneumonitis,rabies, strangles,
and Potomac Horse Fever, to name the most common.
Vaccinations are a vital part of proper equine management.
If incorporated into a program that includes regular
deworming, an ample supply of clean water, a good nutrition
program, and a safe environment, you and your horse will be
all set to enjoy many happy, healthy, productive years
injection (left). Vaccination into the upper lip of a horse
A good immunization program is essential to responsible
horse ownership, but just as in humans, vaccination does
not guarantee 100% protection. In some situations,
immunization may decrease the severity of disease but not
prevent it completely. This is due to many complicated
scientific reasons, such as differences in the virulence or
severity of some diseases (such as influenza).
Vaccination involves the injection (with a sterile syringe
and needle) of bacteria or viruses that are inactivated or
modified to avoid causing actual disease in the horse. Two
or more doses are usually needed to initiate an adequate
immune response. Once the immunization procedure is
completed, the protective antibodies in the blood stand
guard against the invasion of specific diseases. Over time,
however, these antibodies gradually decline.
Therefore, a booster shot is needed at regular intervals.
Protection against some diseases such as tetanus and rabies
can be accomplished by boostering once a year. Others
require more frequent intervals to provide adequate
The specific immunizations needed by a particular horse or
horses depend upon several factors: environment, age, use,
exposure risk, value, geographic location, and general
Your local equine veterinary practitioner can help you
determine the vaccination program best suited to your
horse’s individual needs. The following diseases are those
most often vaccinated against. Again, your local
practitioner will know what is best for your horse.
The most common diseases to vaccinate against are equine
influenza and tetanus
EQUINE INFLUENZA (FLU)
This is one of the most common respiratory diseases in the
horse and is one of the most contagious virus groups in the
horse. The virus can be transmitted by aerosol from horse
to horse over distances as far as 30 yards, for example, by
snorting or coughing. Signs to watch for are similar to
those in a human with a cold, i.e., dry cough, nasal
discharge, fever, depression, and loss of appetite. With
proper care, most horses recover in about 10 days. Some,
however, may show symptoms for weeks, especially if put
back to work too soon. Influenza is not only expensive to
treat, but results in a lot of “down time” and indirect
financial loss, not to mention discomfort for your horse.
Unfortunately, influenza viruses constantly change in an
effort to bypass the horse’s immune defense. Therefore,
duration of protection is short-lived and revaccination is
recommended every two to four months. Not all horses need
influenza vaccination. However, animals that travel or are
exposed to other horses should be regularly immunized
against influenza. Follow your veterinarian’s advice as to
whether your horse needs influenza vaccine.
Sometimes called “lockjaw,” tetanus is caused by
toxin-producing bacteria present in the intestinal tract of
many animals and found in abundance in the soil where
horses live. Its spores can exist for years. The spores
enter the body through wounds, lacerations, or the
umbilicus of newborn foals. Therefore, although not
contagious from horse to horse, tetanus poses a constant
threat to horses and humans alike.
Symptoms include muscle stiffness and rigidity, flared
nostrils, hypersensitivity, and the legs stiffly held in a
sawhorse stance. As the disease progresses, muscles in the
jaw and face stiffen, preventing the animal from eating or
drinking. More than 80 percent of affected horses die.
All horses should be immunized annually against tetanus.
Additional boosters for mares and foals may be recommended
by your veterinarian. Available vaccines are inexpensive,
safe, and provide good protection.
Strangles is a disease of the upper
respiratory system and the lymph nodes of the head. It
is caused by the organism Streptococcus equi. It
rapidly spreads from horse to horse through coughing,
or by the horse eating or drinking infective droplets.
The discharge may remain infective for over a month.
Within 3–8 days of becoming infected, the horse will show a
fever (39.5–40.5°C). The throat and larynx become so
extremely inflamed that swallowing food or water becomes
very painful or impossible. The lymph nodes of the head
become swollen and very painful, and may eventually burst
and discharge a thick creamy yellow pus.
Occasionally the infection spreads to other parts of the
body, when it is then known as ‘bastard strangles’. This
form is difficult and sometimes impossible to cure.
Treatment involves isolation, antibiotics and supportive
nursing. Disinfection of saddlery, grooming equipment, food
boxes etc. must be done to stop transmission of the disease
to other horses.
Two distinct viruses, equine herpesvirus type 1 (EHV-1) and
equine herpesvirus type 4 (EJ-IV-4), cause two different
diseases, both of which are known as rhinopneu-monitis.
Both cause respiratory tract problems, and EHV-1 may also
cause abortion, foal death, and paralysis. Infected horses
may be feverish and lethargic, and may lose appetite and
experience nasal discharge and a cough. Young horses suffer
most from respiratory tract infections and may develop
pneumonia secondary to EHV-1.
Rhinopneumonitis is spread by aerosol and by direct contact
with secretions, utensils, or drinking water. Virus may be
present but inapparent in carrier animals.
All pregnant mares must be immunized. Foals, weanlings,
yearlings, and young horses under stress also should be
vaccinated. Immune protection is short. Therefore, pregnant
mares are vaccinated at least during the 5th, 7th, and 9th
months of gestation, and youngsters at high risk need a
booster at least every three months. Many veterinarians
recommend vaccinations at two-month intervals year-round.
More commonly known as sleeping sickness,” this disease is
caused by the Western Equine Encephalomyelitis (WEE) virus
or the Eastern version (EEE). WEE has been noted throughout
North America, while EEE appears only in the east and
southeast. VEE, the Venezuelan variety, has not been seen
in the United States for many years. However, a recent
outbreak of VEE occurred in Mexico.
Sleeping sickness is most often transmitted by mosquitoes,
after the insects have acquired the virus from birds and
rodents. Humans also are susceptible when bitten by an
infected mosquito, but direct horse-to- horse or
horse-to-human transmission is very rare.
Symptoms vary widely, but all result from the degeneration
of the brain. Early signs include fever, depression, and
appetite loss. Later, a horse might stagger when it walks,
and paralysis develops in later stages. About 50 percent of
horses infected with WEE die, and the death rate is 70 to
90 percent of animals infected with EEE or VEE.
All horses need an EEE and WEE vaccine at least annually.
Pregnant mares and foals may require additional
vaccinations. The best time to vaccinate is spring, before
the mosquitoes become active. In the South and West, some
veterinarians choose to add a booster shot in the fall to
ensure extra protection all year-round.
OTHER DISEASE THREATS
Several other diseases are common, although the need for
vaccination against them is a highly individual one. Rely
on your veterinarian to guide you.
Other diseases include:
• Rabies. A frightening
disease which is more common in some areas than others.
Horses are infected infrequently, but death always occurs.
Rabies can be transmitted from horses to humans.
• Botulism. Known as
“shaker foal syndrome” in young horses, this disease can be
serious. Botulism in adult horses, “forage poisoning,” also
can be fatal. Vaccines are not available for all types of
botulism, but pregnant mares can be vaccinated in endemic
• Equine viral arteritis
(EVA). A complicated disease which can result in
some breeding restrictions and export problems. Follow your
• Potomac Horse Fever.
This is a seasonal disease seen generally in the summer
months. It had been reported in 33-plus states as of summer
1998. The disease is characterized by high fever, severe
diarrhea, malaise, depression, anorexia and very often a
severe founder that can effect all four feet. It has a high
mortality rate. There is now an annual vaccine for the
prevention of this disease. It is best to give one in early
spring. Initial vaccination is followed by a booster in
three to four weeks and annual re-vaccination thereafter.
Contact your veterinarian.
nutschell For primary immunization, an initial
vaccination is required, followed by a repeat dose in 3-4
weeks. The following is a handy reference guide for
scheduling your horse’s immunizations:
• Influenza. Most
horses. Foals at 3-6 months, then every 3 months. Traveling
horses every 3 months. Brood mares biannually, plus booster
4-6 weeks pre-foaling.
• Tetanus. All horses.
Foals at 2-4 months. Annually thereafter. Brood mares at
4-6 weeks before foaling.
All horses. Foals at 2-4 months. Annually in spring
thereafter. Brood mares at 4-6 weeks before foaling.
Foals at 2-4 months and younger horses in training. Repeat
at 2- to 3-month intervals. All brood mares at least during
5th, 7th, and 9th months of gestation.
• Rabies. Foals at 2-4
months. Annually thereafter.
• Strangles. Foals at 8-12 weeks.
Biannually for high-risk horses. Brood mares biannually
with one dose 4-6 weeks pre-foaling.
• Potomac Horse Fever.
Foals at 2-4 months. Biannually for older horses. Brood
mares biannually with one dose at 4-6 weeks pre-foaling.
Many combination vaccinations are available. Please check
with your local equine practitioner.
Appropriate vaccinations are the best and most
cost-effective weapon you have against common infectious
diseases of the horse. A program designed with the help and
advice of your local veterinarian will keep your horses —
and you — happy and healthy for many years to come.
• Does my horse need rest after vaccination?
No. This was a recommendation for older vaccines, and is no
longer stipulated with more modern vaccines. Nevertheless,
talk to your vet, as he or she will be able to offer
tailor-made advice for your particular horse and the work
it is doing.
• I have heard vaccines can cause side effects: is this
Serious side effects following vaccinations are extremely
rare. Although all veterinary vaccines undergo thorough,
independent evaluation of their safety, efficacy and
quality, occasional reactions may occur in individual
cases. This may be an injection site reaction, allergic
reaction or general malaise.