Equine Health Care and Disease Prevention:
Developing a Vaccination and Deworming Program
Amanda M. House, DVM, DACVIM
University of Florida College of Veterinary Medicine
Gainesville, FL 32610
Routine vaccination is a critical component of developing a health maintenance
program for horses. It is important to emphasize that there is no standard vaccination
program that is suitable for every horse, and that individual programs should be
developed with your veterinarian. Vaccinations help to prime the immune system to
respond quickly when a horse is exposed to an infectious agent. Although vaccinations
cannot guarantee disease prevention in all circumstances, they help minimize the risk of
infection and aid in the prevention of certain diseases. Vaccination is not a substitute for
other good management practices, and should be used in conjunction with proper
nutrition, deworming, pasture management and minimizing stress and overcrowding for
optimal results in each horse and herd. Ideally, all horses in a group should receive
vaccinations and be on the same schedule when possible.
The vaccination program appropriate for an individual horse or herd needs to take
into account things such as age, sex, geographic location, use of the horse, pregnancy
status and risk for developing the disease. Currently, there are vaccines available for
Tetanus, Encephalomyelitis, West Nile Virus, Rabies, Rhinopneumonitis (Equine Herpes
Virus), Influenza, Strangles, Potomac Horse Fever, Botulism, Equine Viral Arteritis,
Anthrax, and Rotavirus. The vaccines are administered by an intramuscular or intranasal
route depending on the disease. Influenza and Strangles have both intramuscular and
intranasal vaccines available. Adverse reactions to vaccination are not common but are a
potential risk of vaccination. Signs of an adverse reaction may include muscle soreness,
swelling, fever, anorexia, and lethargy. If the signs are severe or are not self-limiting,
your veterinarian should be informed and may want to initiate additional therapy.
When considering a vaccination program for your horse, it is important to
remember that each horse's immune system will respond a little differently to
vaccination. Not every horse will be protected exactly the same or for the same amount
of time following vaccination. A primary series of the vaccines with booster doses will
be required for an appropriate immune response before exposure to the disease. It will
take 1-2 weeks after a completed vaccine series for your horse to be protected against the
disease. After the initial vaccine series, most horses will require annual or semi-annual
An overview of basic vaccination guidelines and the diseases follows. This is
only a guideline and a specific plan for your horse should be developed in conjunction
with your veterinarian. Generally, vaccination is recommended for all horses for
Tetanus, Encephalomyelitis, and West Nile virus. Rabies is an uncommon occurrence,
but is fatal in all cases; therefore, vaccination is advised. Vaccination for influenza and
Equine Herpes Virus (EHV) is recommended in most cases, especially in performance
horses and horses exposed to a transient or young equine population. Broodmares and
weanlings have specific vaccination recommendations tailored for their needs, and will
not be covered in detail here. The information presented herein is a program targeted for
the adult pleasure or performance horse.
Tetanus is caused by Clostridium tetani, an anaerobic, spore-forming bacterium.
The bacteria are present in the intestine and manure of horses, as well as in the soil.
Clostridium tetani produces spores that can survive for years in the environment. The
spores can gain access to the horse through wounds or lacerations, or the umbilicus in
foals. The clinical signs of tetanus are a result of toxin production, and include muscle
stiffness and rigidity, third eyelid prolapse, stiff legs and "sawhorse" stance, nostril flare,
and lockjaw. Severe cases are unable to eat, and may be down and unable to get up;
many progress to death or require euthanasia. All horses should be revaccinated annually
for tetanus, and boostered immediately if they sustain a wound or undergo surgery greater
than 6 months after their previous vaccine. Tetanus antitoxin can be administered to
horses that sustain a wound (increasing their risk of disease) and that have not previously
been vaccinated with tetanus toxoid. Tetanus antitoxin rarely can cause fatal liver
disease, and should be discussed with your veterinarian prior to administration.
The equine encephalomyelitis viruses (eastern equine encephalomyelitis (EEE),
western equine encephalomyelitis (WEE), and Venezuela equine encephalomyelitis
(VEE)) are transmitted by mosquitoes to horses and humans from wild birds or rodents.
Horses and humans are considered dead-end hosts for the disease, and cannot pass it to
others. Infection with these viruses can cause fever and neurological symptoms such as
depression, difficulty walking/staggering gait, changes in mentation/behavior, and
seizures. VEE is a reportable foreign animal disease, and has not been seen in the U.S.
for many years. The death rate is 70-90% for horses with EEE or VEE, and about 50%
for horses with WEE. All horses should be vaccinated for EEE/WEE in the spring, prior
to mosquito season, and potentially again in the fall in warm climates such as Florida
(every 6 months).
West Nile Virus is another virus transmitted by mosquitoes that can cause
neurological symptoms such as muscle tremors, loss of coordination, hypersensitivity to
being touched, and recumbency. The death rate for infected horses is about 33%. Three
vaccines are available for use, and horses should be vaccinated annually. The newer
recombinant and modified live chimera vaccines underwent more intensive challenge
studies than the original killed vaccine, and reportedly help protect against disease for a
Fortunately, Rabies is an uncommon disease in the horse. However, in any areas
where Rabies is endemic in the wildlife population, horses can be exposed through a bite
from an infected animal. Rabies results in progressive neurological disease and is fatal in
all cases. It can be transmitted from infected horses to humans. Vaccination is
recommended followed by a yearly booster.
Equine influenza is one of the most common infectious respiratory diseases in the
horse. The virus is highly contagious and can be transmitted through the air from horse
to horse as a result of coughing. The most common signs of infection are fever, cough,
nasal discharge, and reduced appetite. Young horses (<5 years) and horses exposed to
large numbers of other horses through showing or transport seem to be most susceptible
to infection. Most horses recover from infection in about 10-14 days, and treatment
consists of supportive care. Vaccination is available in intramuscular (killed virus, and
canary pox vectored vaccine) and intranasal (modified live virus) formulations. Discuss
the best option for your horse with your veterinarian. Vaccination is recommended every
six months, and is done more frequently (every 3-4 months) in some horse populations.
Equine herpesvirus type 1 (EHV-1) and equine herpesvirus type 4 (EHV-4) can
both cause respiratory infections (Rhinopneumonitis) in horses, generally affecting the
upper respiratory tract and causing fever, nasal discharge, and sometimes cough.
However, EHV-1 can also result in neurological disease, abortion, and foal death. EHV-
1 and EHV-4 are transmitted through the air or by direct contact with secretions from the
nose, on equipment, or in drinking water. It is likely that initial infection with EHV-1
and EHV-4 first occurs in foals, but clinically apparent infections are seen as they get
older and are exposed to new horses. Vaccination is recommended for prevention of
abortion in pregnant mares (with a killed vaccine product licensed for prevention of
abortion) and for reduction in signs and spread of respiratory disease in foals, weanlings,
yearlings, young performance, and show horses that have a higher risk of exposure.
Vaccination does not prevent the neurological form of EHV-1. Adult horses determined
to be at risk for infection are usually vaccinated every 6 months.
Additionally, vaccines for Strangles (Streptococcus equi infection) and Potomac
Horse Fever (PHF) are available and their use should be discussed with your veterinarian.
Potomac Horse Fever is not typically seen in Florida, but vaccination may be considered
in horses that are traveling to the northeast and mid-atlantic regions. The vaccine for
PHF is not completely protective, and additional study on this vaccination and disease is
warranted. Vaccinations for Botulism, Rotavirus, and Equine Viral Arteritis are used
more commonly in breeding populations. The included chart is from the American
Association of Equine Practitioners (AAEP), and was updated in 2005. The AAEP is in
the process of re-updating their guidelines for vaccination of the horse, and is an
excellent resource for owners at www.aaep.org.
Establishing a deworming program for equine parasites has become a somewhat
controversial topic of discussion. Due to the emergence of resistant parasites (worms that
are not killed by traditional dewormers), some of the emphasis is shifting to prevention
and control, rather than just routine rotational treatment with anthelmintics. The
American Association of Equine Practitioners (AAEP) recommends establishing a
program with your veterinarian that works best for your horse and/or herd, in conjunction
with these suggestions for environmental management:
1. Clean and dispose of manure in the pasture at least twice weekly.
2. Mow and harrow pastures regularly to break up manure and expose parasite
eggs to the sun.
3. If possible, rotate pastures by allowing other livestock to graze them.
4. Group horses in a pasture by age to reduce exposure to certain parasites, and
maximize the deworming program geared to that group.
5. Prevent overgrazing and reduce fecal contamination by keeping the number of
horses per acre to a minimum.
6. Feed horses in a feeder for hay and grain rather than on the ground.
7. Remove bot eggs from the hair routinely to prevent ingestion.
Individual horses can be monitored for parasites with a fecal examination and egg
count. In Florida, the peak worm season is fall, winter, and spring. Treatment should be
focused around these times. The effectiveness of different dewormers can be measured
using a fecal egg count reduction test, which involves performing a fecal egg count
before and after deworming your horse. Although it is critical to target the large and
small strongyles in adults, as well as roundworms in foals, rotation of dewormers should
not be done as often as every 4-8 weeks, because this may promote resistant worms.
Ideally, a dewormer can be used for several treatments prior to rotation to a new drug. It
is important to remember that treatment with ivermectin is done at 2 month intervals,
while moxidectin is done at 3 month intervals. Equine tapeworms are difficult to identify
in fecal examinations, and deworming for tapeworms is recommended biannually or
annually with a product containing praziquantel (Zimectrin Gold, Combocare, Equimax),
or double dose pyrantel pamoate or tartrate. A blood test has been developed that
identifies antibodies to tapeworms in horses. This test is only available at one lab in the
United States at the University of Tennessee's College of Veterinary Medicine.
However, there are likely horses with tapeworms that this test will not identify, and it is
more practical to be sure horses are dewormed annually for tapeworms. Consult your
veterinarian for a deworming strategy that works best for your horse and/or herd.