Risk: In the
United States, Lyme disease is mostly localized to states in the
mid-Atlantic, and upper north-central regions, and to several
counties in northwestern California.
In 1999, 16,273 cases of Lyme disease
were reported to the Centers for Disease Control and
Ninety-two percent of these were from the states of Connecticut, Rhode
Island, New York,
New Jersey, Maryland,
Massachusetts, and Wisconsin.
People and dogs are susceptible to infection
with the tick-borne agent
which causes Lyme disease. Can horses also become infected with this
agent and become clinically sick?
Yes! Lyme disease (or "Borreliosis") is caused by a bacterial spirochete
called Borrelia burgdorferi. This agent is transmitted by a species of
tick called Ixodes, commonly called the "deer tick". These ticks are
found throughout New Jersey, as is the disease in people and dogs.
Horses do not appear to be as susceptible to the disease as people and
dogs, however. Lyme disease in horses has been reported in NJ by equine
practitioners over the past ten years. What are the clinical signs of
Lyme disease commonly seen in horses?
Clinical signs of this disease in horses appear to be quite variable
including (but not limited to) one or more of the following:
. eye inflammation
. joint swellings
Multiple organ-systems of the horse may be directly affected by this
spirochete, so that wherever the organism causes the most damage may be
where clinical signs of disease are found (like arthritis when the
bacteria is in a joint and hepatitis when it is found in the liver). In
addition, there may be variations in the severity of disease from horse
to horse following exposure to this spirochete. Some of the variation in
clinical signs appears to be related to the response of the horse's
immune system to the presence of the spirochete. While some horses
appear to tolerate infection with this organism and demonstrate no signs
of illness, other horses may be infected and clinically sick. There's a
lot yet to learn about this disease in horses!
How is Lyme disease diagnosed in horses?
In people, Lyme disease is called the "great imitator". The same may be
said of trying to diagnose this disease in horses. In the horse,
Borrelia burgdorferi may cause clinical signs of disease which resemble
other diseases or the organism may be present and cause no clinical
signs of disease. In addition, this spirochete is extremely difficult to
isolate from a live horse. So, in order to diagnose a horse with Lyme
disease, common causes of the clinical signs of disease must first be
After ruling out other possible causes of disease, veterinarians use
blood tests to determine exposure of a horse to the Borrelia burgdorferi
spirochete. This is done by checking for antibodies in the serum. Two
different types of lab tests are used to measure the level of serum
antibodies to Borrelia burgdorferi: the ELISA and Western blot. Most
horses need to have both blood tests performed in order for a
veterinarian to determine whether the horse has been exposed to Borrelia
burgdorferi or has other antibodies which may interfere with this test.
Once antibodies are identified in the blood of a horse showing signs of
Lyme disease, there is still no guarantee that the clinical signs of
disease are being caused by Borrelia burgdorferi. The presence of
antibodies means that the horse has been exposed to or vaccinated for
this organism. If no other cause of disease is identified, a sick horse
would then be treated with appropriate antibiotics to kill Borrelia
burgdorferi spirochetes. Follow-up blood tests are needed to determine
if the level of antibodies to the spirochete are decreasing. Decreasing
antibody levels and diminishing signs of Lyme disease in a horse
undergoing treatment indicate that the therapy is having a positive
effect against this bacteria.
Is there a Lyme disease vaccine approved for use in horses?
No! Currently, there is no USDA approved Lyme disease vaccine for use in
horses. Some horse owners have elected to vaccinate their horses using
one of three Lyme disease vaccines approved for use in dogs. However,
there is no research or clinical information to support the use of any
dog vaccine for Lyme disease prevention in the horse. In fact, use of
the vaccine will make interpretation of the diagnostic tests more
difficult should this disease be suspected. In addition, researchers
have reported finding the spirochete organism in clinically ill horses
who had previously been given the dog vaccine! Insect control is
probably the best method of preventive medicine for horse owners to
decrease their horses= exposure to ticks. Talk with your veterinarian
and ask for some recommendations on appropriate insecticides to use on
your horse to minimize contact with ticks.