Tick-Borne Diseases by Dr. Scott Wilson
Horses
are afflicted by a variety of parasites that can carry diseases, including
ticks. These small spider-like
blood-suckers can be found on horses almost any time of year, causing a variety
of clinical signs. A commonly observed
behavior that might be seen is intense scratching (sometimes until the skin is
raw). Other signs often seen include a
swollen area of skin where the tick has attached, and this area may drain yellow
or white discharge. Ticks usually inhabit
areas of tall grass or brush piles, and wait for animals to walk nearby to
crawl onto. Once on the animal, ticks
will crawl to an area where they are covered by hair or where it is very warm,
and begin to feed. Ticks will usually
feed for several days before releasing and falling off of an animal.
During this feeding time on a
horse, there is a potential for regurgitation of blood from the tick’s stomach
into the horse. This transfusion takes
place approximately 1-2 days after the tick first attaches. In certain ticks, namely Ixodes
species, this regurgitation of blood can cause the horse to be inoculated with
any bacteria that the tick may be carrying.
Commonly encountered bacteria within Ixodes ticks include Borrelia
burgdorferi and Anaplasma phagocytophlium. These are the agents that cause Lyme disease
and equine anaplasmosis, respectively.
Lyme disease is a commonly
misunderstood disease of horses. It has
been documented to be the causative agent in only a few cases, including horses
with neurologic disease, skin lesions, or uveitis. It has also been found in the lining of
inflamed joints and has been suspected to have caused laminitis. A diagnosis of Lyme disease has classically
become associated with a horse that has lethargy, shifting-leg lameness, and
hyperesthesia (overly dramatic response to being touched). Numerous research projects involving B.
burgdorferi have attempted to elicit clinical signs in horses that were
experimentally infected, but these studies have not been able to produce
consistent clinical disease in these test subjects. That being said, we do know that ticks
commonly carry B. burgdorferi and can transmit this bacteria to horses
while the tick is feeding. The most
recently developed diagnostic test comes
from Cornell University and can differentiate
vaccination from acute exposure and from chronic infection. There is no licensed vaccine for horses, but
numerous veterinarians elect to administer a vaccine that was developed for use
in dogs. There are numerous anecdotal
reports of horses testing positive for Lyme disease and having a favorable
response to antibiotic treatment; that makes this disease a difficult one to
argue against. Lyme disease has been
shown to regularly affect people, dogs, cats, and other common species, but it
still remains a bit of a mystery in horses.
Some more common differential diagnoses for horses suspected of Lyme
disease include arthritis, muscle disorders, neurologic diseases (such as Equine
Protozoal Myeloencephalitis), or gastric ulceration.
Equine anaplasmosis, caused by Anaplasma
phagocytophilum, is an emerging disease that has gained a lot of attention
in the past half-decade. This disease is
also caused by a bacteria transmitted by ticks, and its prevalence is
increasing rapidly. The first case in Virginia was reported by
the Virginia Tech Equine Field Services team in 2009, and the number of cases
per year is on the rise. Horses
afflicted with anaplasmosis consistently have a high fever, lethargy, and have
a very poor appetite. They may have
yellow-tinged mucous membranes, and also may have limb swelling or even mild
neurologic signs. Diagnosis can be
confirmed by sampling the blood and observing a particular life-stage of the
bacteria within some white blood cells under a microscope. Other available tests include PCR (Polymerase
chain reaction) that detects the bacteria’s DNA within the blood, and checking
the horse’s antibody titer against the bacteria.
Treatment for both of these diseases is similar: daily
administration of a tetracycline antibiotic.
A horse infected with B. burgdorferi needs to be treated for at
least 3-4 weeks, as this bacteria can be difficult to clear from the horse’s
system. Clinically signs usually improve
relatively quickly, but it is important to finish the antibiotic regimen so
that persistent infection does not ensue.
A recheck test can be performed a
few weeks to months after initiation of treatment to assess the effectiveness
of the antibiotic therapy. A horse
infected with A. phagocytophlium should be treated with a non-steroidal
anti-inflammatory drug (NSAID), such as Banamine, in addition to a
tetracycline, for a few days to help stimulate appetite and to decrease
fevers. Antibiotic therapy to treat anaplasmosis
is much shorter than that for Lyme disease, with regimens lasting approximately
10 days. Clinical signs associated with
anaplasmosis typically abate within 1-2 days with appropriate treatment
There are three choices for a tetracycline antibiotic
in the horse: Oxytetracycline, minocycline, and doxycycline. Oxytetracycline is an injectable drug that
has been shown to reach therapeutic levels in the blood when given once daily
by intravenous injection. Minocycline is
an oral antibiotic that has recently been discovered to have excellent efficacy
against B. burgdorferi, and is thus preferred over doxycyline, which is
poorly absorbed as an oral tablet.
Minocycline and doxycycline must be given twice daily by mouth. Oxytetracycline, if given too quickly
intravascularly, can cause some horses to faint, as it binds the calcium in the
horse’s blood. Minocycline and
doxycycline, as with any oral antibiotic, have the potential to cause a serious
colitis. The recommended treatment
for a horse with Lyme disease is 2 weeks of intravenous oxytetracycline,
followed by 2 weeks of oral administration of minocycline.
There is evidence that some Ixodes
ticks in this region of the nation carry both B. burgdorferi and A.
phagocytophilum, and thus can infect horses with both of these bacteria in
a single feeding. Not all ticks carry
one or both of these diseases, so simply finding a tick on your horse does not
mean they will get sick. Some simple
management strategies for decreasing ticks on your farm include keeping grass
length short, removing brush or leaf piles, and checking your horse daily for
ticks. There are some products available
for killing ticks once they are attached to the horse, like Frontline
(Fipronil) spray. This product can be
sprayed in the usual places that ticks like to attach, such as the mane,
between the front legs, between the hind legs, and around and underneath the
tail. If your horse presents with any of
the signs listed above, please contact Old Waterloo Equine Clinic and schedule
a visit.
