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 Hampshire, 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 NH by equine practitioners for at least the past four years.
Clinical signs of this disease in horses appear to be quite variable including (but
not limited to) one or more of the following:
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.
Lyme Disease Vaccine
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.
Currently, Dr. Legault is NOT recommending vaccination of horses with any of the
Lyme disease vaccines licensed for use in dogs. When an approved and
effective vaccine against Lyme disease in horses becomes available we will make
attempts to inform all of our clients.