“My horse was fine yesterday, but today he’s not interested in eating and seems really depressed.”
Written By: Lauren Alderman, DVM, CVA, CVSMT
This is a very common phone call received by the veterinarians at Irongate Equine Clinic in the fall and early winter months. If you’re on the other end of this call, chances are we’ll ask if you’ve taken your horse’s temperature… and we’ll recommend scheduling an exam as soon as possible. We’ll also be suspicious that your horse’s signs could be caused by Anaplasmosis.
What does Anaplasmosis look like?
Image from Zuku Review showing petechiations.
Horses sick with Anaplasmosis will typically have a fever (a normal horse temperature ranges from about 98.5 to 100.5 degrees fahrenheit). They are often depressed and off feed, and may have edema (swelling) of their limbs or along their abdomen. They also may show icterus (yellow-tinged mucous membranes) or petechiations (small red or purple spots on their mucous membranes).
What causes Anaplasmosis?
Image from CDC.gov showing the size of different stages of Ixodes ticks.
This infection is caused by the bacterium Anaplasma phagocytophilum (formerly known as Ehrlichia phagocytophila and Ehrlichia equi). The organism is transmitted to horses through the bite of an infected tick (specifically, Ixodes ticks such as the deer tick and the Western black-legged tick). Once inside the body, Anaplasma infects the horse’s white blood cells, where it can often be seen under a microscope when a blood smear is performed.
How is Anaplasmosis diagnosed?
Your veterinarian will perform a physical examination of your horse. If they suspect Anaplasmosis, they will likely recommend bloodwork such as a complete blood count, chemistry panel, and polymerase chain reaction (PCR) test. They may also perform a stall-side serum amyloid A (SAA) test to screen for infection. If your horse’s bloodwork must be sent out to a laboratory and the results will be delayed by a day or two, your veterinarian may recommend beginning treatment immediately. Horses sick with Anaplasmosis tend to improve rapidly with treatment, so monitoring their response to the first treatment or two can help support a diagnosis while waiting for confirmation from the lab.
Image from Journal of Clinical Pathology of white blood cells containing morulae (arrows) that are consistent with Anaplasmosis.
How is Anaplasmosis treated?
Anaplasmosis is treated with tetracycline antibiotics. Most commonly, these medications are administered intravenously (IV) for several days. Certain cases may require a longer course of IV treatment, or may be followed with a course of oral antibiotics. Your veterinarian may also administer or recommend the use of a nonsteroidal anti-inflammatory drug (NSAID) to control fever in the initial days of the infection.
What else should I know?
Anaplasmosis is not directly contagious between horses, although multiple horses on a property may be affected at the same time if there is a heavy tick population in the area. Horses that have recovered from Anaplasmosis typically show immunity for a year or two following infection. Unfortunately, no vaccine is currently available to prevent Anaplasma infection. Horse owners can help prevent infection by inspecting their horses daily for ticks, and promptly removing any ticks found.
