Infectious Diseases and Prevention in 2006
By Doug Thal, DVM
In the world of infectious diseases, the one thing that can be counted on is that things constantly
change. Diseases wax, wane, and evolve. The battle to control these agents is ever-changing as we
learn more about the diseases, and produce new vaccines and control methods. Vaccine type,
regulation and availability changes frequently as well.
This article is a brief summary of my perspective on the important current points of infectious disease
prevention.
Some basics
Every horse owner should understand a few basic points about the biology of infectious disease and
vaccines. This basic understanding makes it easier to make sound decisions regarding the health of
your horses. Infectious disease is disease caused mostly by bacterial and viral organisms. These
organisms (agents) are transmitted in many different ways (specific to the agent) from direct contact
between horses to transmission by insects.
Whether or not a horse becomes infected depends on a balance of several factors including:

The animal’s susceptibility to the disease: A combination of genetics, health, general immunity
and immunity to the specific disease in question (altered by vaccination).

The characteristics of the organism: How potent and contagious the organism is, how high a
dose of the organism is presented, and how this dose comes into contact with the horse.
Prevention of infectious disease involves more than just vaccination. General health is critical to a
functioning immune system and disease prevention. Proper management is vital in reducing exposure
to infectious disease.
The status of some important equine diseases
West Nile Virus has been a severe problem for the horse industry since the first cases in the U.S. on
the east coast in 1999. It spread rapidly westward over the next few years, killing large numbers of
birds, horses and some people. The peak in our area occurred in 2003. There were hundreds of cases
in New Mexico that year and our practice saw many cases. Since then, West Nile infection has become
less common in our area, but it is now considered endemic (established permanently) throughout the
U.S.
The disease is transmitted by mosquito. All horses in our area should be vaccinated for West Nile Virus
at least once annually. This is generally done in the spring, before mosquito season.
Symptoms of Eastern and Western Encephalomyelitis are similar to those caused by West Nile
Virus. These diseases have been overshadowed by West Nile Virus over the past few years, causing
some horsemen to forget about vaccinating for these important diseases. Like West Nile, these
diseases cycle dependent on immunity in bird populations and many other factors. They will always be
present in the environment and should be vaccinated for routinely. The vaccine is effective, inexpensive
and strongly recommended. It is now often combined with West Nile and tetanus. Our general
recommendation is annual vaccination, timed before mosquito season
Tetanus is a paralytic disease caused by a toxin produced by the bacteria Clostridium tetani. This
organism is common in the environment on many surfaces and becomes a problem when it is
introduced into wounds. The vaccine is highly effective, safe, and inexpensive, and should be given to
every horse each year. It is often combined with encephalitis and West Nile vaccines.
“Strangles” (distemper) is caused by an abscess forming bacteria, Streptococcus equi. The classic
sign of strangles is severe abscessation of the lymph nodes of the throat and under the jaw. The name
strangles comes from these large swellings, which in their most severe form can cause difficulty in
swallowing and breathing. The disease is very common and highly contagious to at-risk horses. It is
transmitted by contact with secretions containing the organism. It can easily be transmitted by direct
contact between horses. It is also carried on the hands of handlers and on shared tack and equipment
and through shared water.
Vaccination has proven difficult for this disease. There is a modified live intranasal strangles vaccine
available. The intramuscular vaccines have not been very effective. The problem with all of the
vaccines has been a high vaccine reaction rate and limited effectiveness. The modified live intranasal
vaccine causes strangles in a small percentage of vaccinated horses. It is dangerous because it can be
inoculated into and cause abscess in the tissues at other vaccination sites. Our current
recommendation is to use the vaccine whenever the risk of strangles infection outweighs the risk of
side effects from the vaccine. We strongly recommend management to reduce strangles exposure.
Influenza. There has been lots of talk recently about human and bird influenza. Influenza is common in
horses as well. As with all flu viruses, the virus is constantly changing, making it difficult to make a
vaccine which contains the most current version of the virus. Influenza causes high fever, cough and
nasal discharge in horses. It is highly contagious and can be spread by coughing horses from a
distance. Our current recommendation is to vaccinate for this disease using one of several recently
developed intranasal vaccines. These have been shown to produce better immunity. Typically these
are given at least twice annually to at-risk horses.
Equine Herpes Virus (rhino) are several viruses which can cause respiratory, neurologic signs, or
abortion in horses. There have been recent outbreaks of the more rare neurologic form at eastern race
tracks.
One unique characteristic of this virus is that it can lay dormant in infected horses and then can causes
signs of disease when immunity decreases because of stress or other factors. Our recommendation is
frequent vaccination using specific, newer generation vaccines, based on the specific situation.
Rabies is a rare but fatal disease in horses that is transmitted by the bite of an infected animal (bats
and skunks in our area). Our current recommendation for our region is to vaccinate horses for it. This is
more for peace of mind than any other reason. The vaccine is relatively safe, effective and inexpensive.
An example of a common disease that there is currently no vaccine available for is Pigeon Breast
(dryland distemper, pigeon fever). Control measures for this disease are therefore limited to
management to minimize contamination of the environment by infected horses.
There are many other less common infectious diseases that affect horses in the United States and that
are beyond the scope of this article. An example is Potomac Horse Fever, a rare disease in our area.
This disease can cause life-threatening diarrhea in horses. It is difficult disease to diagnose definitively.
The vaccine is of questionable effectiveness. For this reason we do not recommend routine vaccination
for this disease.
Vaccination, important points

We currently routinely recommend vaccination for West Nile Virus, Encephalitis, tetanus,
influenza, rhino and rabies.

Many other vaccines are available, but we recommend them based only in specific situations.

Pregnant mares and foals have special vaccination requirements.
Vaccines are not all created equal and are constantly being improved. Your equine veterinarian should
be current on vaccine development and can help you select the best vaccine for your needs.
Vaccines have historically been created by preparation of all or part of the infectious agent, either killed,
or inactivated and alive, mixed with a delivery liquid called an adjuvant.
Some new vaccines take advantage of huge advances in our understanding of the immune system and
genetic engineering. These products may contain only a small, key part of the organism or its genetic
code but still promote excellent immunity. They tend to be more effective and safer than older vaccines.
New delivery approaches target key organ systems more directly and so may produce better immunity.
Examples are the newer intranasal vaccines.
There is currently debate in the horse community over how much vaccination is necessary. There is
some suggestion that holistic means are sufficient to prevent infectious disease and that vaccination is
not necessary. I strongly disagree. Proper vaccination is critical to good horse health care. Ask your
veterinarian before you rely on alternative approaches. While we cannot rule out that some “holistic or
natural agents” may be helpful, there is very little known at this time about their true effects. The best
policy for now is not to take a chance. We have available highly effective and safe vaccines which have
been proven to reduce your horse’s chances of succumbing to serious disease. Let your veterinarian
help meld your own philosophies on health care with good science and good sense.
While many vaccines are now available over the counter, I feel strongly that you should have your
veterinarian vaccinate your horse on his or her recommended schedule. Anyone can give a vaccine if
they have learned the simple technique involved. There are several key reasons to continue to involve
your equine veterinarian. Your veterinarian stays educated on new vaccine development and so can
provide the most effective, current and safest vaccines. He or she administers these vaccines correctly
and on a proper schedule, and maintains complete vaccination records.
I strongly believe in vaccinating horses because it gives me chance to talk to my client and examine the
horse. This is my opportunity to get to know horses in health rather than crisis. I enjoy discussing goals
and management of my clients’ horses as it relates to their health care. It is also an opportunity to
educate my clients and to catch health problems early.
© Thal Equine, LLC