Lyme Disease and Anaplasmosis
If you have been walking in the woods or tall grass, you have probably noticed that the ticks are out in full force. With them comes the emergence of tick-borne diseases. In horses in the Northeast, we mainly see Anaplasmosis ( previously Ehrlichiosis) and Lyme Disease as causing problems.
Formerly known as Ehrlichia equi, Anaplasma phagocytophilum is a rickettsial bacteria in the same family as the organisms causing Typhus and Rocky Mountain Spotted Fever in humans. A. phagocytophilum is not contagious to humans, nor can horses transmit it to each other. The disease is transmitted to the horse by Ixodes sp. ticks, which in the Northeast is Ixodes scapularis, more commonly known as the Deer Tick or Blacklegged Tick. Immature ticks pick up the bacterium from rodents who serve as reservoirs, maintain it as they mature, and then transmit it to the horse they feed off of as adults. It is unknown how long the tick has to be attached before transmission occurs. It takes approximately 2-3 weeks after disease transmission for the horse to develop clinical signs of Anaplasmosis, meaning that by the time signs are noticed the tick is long gone. The most common signs seen by owners are depression/lethargy and a high fever, as high as 104-105° Fahrenheit. Other signs can include limb edema (swelling), petechial hemorrhages (small red/purple spots) on the mucous membranes, icterus (yellow mucous membranes), and poor appetite.
A. phagocytophilum organisms infect neutrophils and eosinophils in the blood, and this is one method of diagnosis. A direct evaluation of a blood smear from the horse can reveal the organisms within these cells, making the diagnosis very straightforward. Another method of diagnosis, usually used on farm and then confirmed with a blood smear, is the use of the canine 4Dx snap test. In dogs this simple test using only a few drops of blood tests for heartworm (Dirofilaria immitis), Lyme (Borrelia burgdorferi), Anaplasmosis (Anaplasma phagocytophilum and platys), and Ehrlichiosis (Ehrlichia canis). Conveniently, this test can be used in our horses for both Lyme and Anaplasma diagnosis. While it is not perfect, it is a good, quick, stall-side diagnostic tool that can be used to direct further testing or treatment. If laboratory bloodwork is submitted, thrombocytopenia (low platelet count) is very commonly seen, which can be low enough to cause spontaneous bleeding and hemorrhages of the mucous membranes. Low red blood cell and white blood cell levels can also be seen, which can be severe enough to cause weakness, and increase susceptibility to other infections, respectively. If questions exist about a diagnosis, an immunofluorescent antibody test (IFA) can be submitted to quantify an antibody response against A. phagocytophilum.
In the Northeast, a horse with an extremely high fever but no clinical signs of viral respiratory disease (another possible cause of such high fevers) is highly suspected of having Anaplasmosis. If confirmed by snap test or blood smear, the treatment is fortunately straightforward. Oxytetracycline intravenously for 3-5 days is the preferred treatment, although oral doxycycline may also be used with somewhat more variable results and timeline, as its absorption from the GI tract is not as predictable. The fevers usually subside very quickly and the horse feels much better within 2-3 days, with no lasting effects. Prolonged treatment is not necessary, and horses gain some protective immunity from the infection, although how long this protection lasts is not known. Anytime a horse has a very high fever, the concern for potential side effects such as laminitis and abortion cannot be ignored, however these are not generally seen with Anaplasmosis. Prevention is difficult; some horses have a natural immunity likely stemming from exposure and disease which was so mild it went unrecognized, but the key lies in tick control. Permethrin-containing tick repellant products are available but ticks may still be able to attach and transmit disease. Environmental management such as keeping grass trimmed short and horses out of the wooded and brushy areas preferred by ticks is helpful. Grooming your horse thoroughly after rides in tall grass or wooded areas may help dislodge ticks which haven’t attached, but it is difficult to comb through every hair of your horse to find all the areas ticks might be hiding. No vaccine is currently available.
Lyme disease is a commonly diagnosed problem in the Northeast, though many lameness or neurologic issues may be unfairly ascribed to it. A very large proportion of horses have been exposed to Borrelia burgdorferi and have antibodies to it, which are picked up in the canine snap test. A positive snap test may be associated with an active infection, but horses with a positive snap test plus clinical signs consistent with Lyme are commonly treated and in many cases, improvement is seen. The disease, like Anaplasmosis, is transmitted by Ixodes scapularis which pick up this spirochete bacterium from rodents as nymphs then transmit it to the horse as adults. Again, it is not known how long the tick must be attached for transmission (research has shown it may be 24-48 hours), and it may be several weeks before signs are noted, and infected horses do not transmit disease to humans or other horses. The signs of Lyme disease are varied and vague in horses, including shifting limb lameness (“Today the right front, yesterday the left hind, what’s going on?!?!”), mild fevers, stiffness, sensitivity to touch, muscle soreness, attitude changes, swelling in multiple joints, decreased appetite, and a host of things which fall into the ‘he’s just not quite right’ category.
If a snap test is positive, confirmation can be obtained by Western Blot and ELISA testing (usually performed concurrently). Likewise, if the snap test is negative but the disease is still highly suspected, these tests may be submitted to make sure the snap result was not a ‘false negative’. In horses with neurologic disease in which Lyme is suspected, cerebrospinal fluid may be submitted as well, to determine whether organisms have affected the central nervous system. Lyme frequently causes inflammation in the synovial lining of multiple joints, which accounts for many of the signs seen. A full lameness and neurologic evaluation may need to be performed along with testing to help rule in or rule out the disease. Laboratory bloodwork (CBC, Chemistry panel) does not tend to show anything specific.
If a horse is suspected or confirmed to have Lyme disease, like with Anaplasmosis, tetracyclines are the drugs of choice. Most horses are put on oral doxycycline as this is easy to do at home, but the course of treatment can be many weeks to months. Another way to treat is to give intravenous oxytetracycline in a hospital setting twice daily for 3 weeks. For these horses, kidney values must be monitored as the prolonged treatment can cause kidney damage in some cases. If elevations are seen, the treatment can be reduced to once a day; if elevations continue or are severe, the treatment must be stopped. The kidney damage, as long as it is caught quickly, is not permanent, but prolonged treatment with no knowledge of kidney function could result in permanent damage. Horses undergoing intravenous treatment may also continue treatment with oral doxycycline once they go home. Once treated, horses can be tested in 3-6 months as the antibodies take a long time to disappear from the blood stream, and a false positive result may be obtained. And again, a positive test in a horse who has been treated does not necessarily indicate another active infection, it could be exposure with an immune response that protects the horse from disease. There is no approved equine Lyme vaccine. However, in horses that test negative on ELISA and western blot, a canine Lyme vaccine may be used to attempt to prevent infection. This should be discussed with your veterinarian and the decision made based on an individual horse by horse basis.
If you have any questions about Anaplasmosis or Lyme disease please contact your veterinarian or the veterinarians at New England Equine Medical and Surgical Center.
Susan Barnett, DVM Jacqueline Bartol, DVM, DACVIM