Tick-Borne Diseases by Dr. Scott Wilson

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.