The dreaded tick, the ectoparasite (external parasites) that we deal with from spring to the first frost, can expose us to many diseases.  Ticks are hematophagic, or blood feeders, western black legged tickand feed on the blood of mostly mammals and birds. Because of this they are also a vector, or carrier of diseases.  Most of us are familiar with the most common of these, Lyme Disease.

In horses we are most concerned about another tick-borne disease which can be found throughout the year, Anaplasmosis, formerly known as Ehrlichia.  Anaplasmosis is caused by the bacterium Anaplasma phagoytophilum.  Anaplasmosis is spread by the Ixodes tick (deer tick & Western black-legged tick).  Twenty percent of Ixodes ticks found in areas with horses are found to be infected with Anaplasma phagocytophilum.

Clinical signs for Anaplasmosis do not appear right away in the horse.  It can take up to three weeks after exposure from the tick bite to start showing clinical signs.  The most common indicator is a fever of 102-106°F.  Besides the fever other clinical signs are depression, loss of appetite, limb swelling, and occasionally muscle stiffness.  The severity of symptoms is dependent on the age of your horse and duration of the infection.  If you suspect that your horse has Anaplasmosis, call your local veterinarian.

morulaThere are a several things that your veterinarian will do to confirm that your horse has Anaplasmosis.  The first thing is to collect a blood sample to run a CBC (complete blood count).  In the CBC, the veterinarian will be looking for a low number of red blood cells, low platelets, and low number of white blood cells.   Secondly, they will create a blood smear with the sample, and look for an inclusion body called Morula that will be present in Neutrophils, a type of white blood cell.   However, the presence of the inclusion body usually will not occur until three days after the onset of the fever.  Due to this delay, even with a negative blood smear many veterinarians will start treatment for Anaplasmosis if the horse is exhibiting clinical signs.  Your veterinarian might also advise performing a Chemistry profile, specifically looking at kidney values for your horse.  Due to many horses affected by Anaplasmosis being dehydrated as well as the drug of choice to treat Anaplasmosis being metabolized by the kidneys, the veterinarian will want to make sure that the kidneys are functioning properly.

The course of treatment for Anaplasmosis is most commonly Oxytetracycline and Banamine to treat the fever.  Oxytetracycline is generally given for five days with horses usually responding well.   There is currently not a vaccine for Anaplasmosis.  Checking daily for ticks is the best preventative measure against your horses contracting Anaplasmosis. Anoka Equine also advises using a topical product called UltraBoss which we have found to be an effective method of tick prevention on horses.  Please call us at 763-441-3797 for more information.

Difference from Potomac Horse Fever

Many of the clinical signs for Anaplasmosis are similar to that of Potomac Horse Fever.  This may cause confusion as to which your horse may have.  Like Anaplasmosis, horses with Potomac Horse Fever show clinical signs of fever, depression and loss of appetite.  However, horses with Potomac Horse Fever can develop laminitis, colic and severe diarrhea.  Potomac Horse Fever is caused by horses being infected by the bacterium Neorickettsia risticii.  Horses are exposed to this bacterium by ingesting infected insects like Mayflies and Caddisflies.

Unlike Anaplasmosis, there is a vaccine available for Potomac Horse Fever commonly given in the fall.  If you are in an area that has had Potomac Horse Fever in the past or have low-lying areas of water (swamps or stagnant water) that attract Mayflies or aquatic insects, you should consider adding the Potomac Horse Fever vaccine to your fall vet visit.

If you would like more information on Potomac Horse Fever click here.

By: Katie Jones, CVT (revised 5/16 by PHM)

If your horse is experiencing any of the above clinical signs, please be sure to call your veterinarian and schedule an appointment.