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Equine Anaplasmosis: A Seasonal Threat

Equine Anaplasmosis, also known as Equine Granulocytic Anaplasmosis or EGA, is a seasonal, non-contagious, tick-transmitted disease. It is caused by a bacterium ( Anaplasma phagocytophilium) that is spread via deer tick bite. This is the same tick species that carries Lyme disease. Possible vectors of this bacteria are deer, rodents, and birds. This disease is not considered contagious between horses, and there is no direct horse-to-horse transmission of equine anaplasmosis.

Equine anaplasmosis cases in Wisconsin are common in spring and fall but keep in mind that “spring” to a deer tick is when the weather initially warms up. That could be February or March. Also, we’ve seen cases of equine anaplasmosis in Wisconsin as late as December. It’s far less common to see equine anaplasmosis during June, July, and August.

There is an anaplasmosis-like infection that presents during mid-summer months which responds to the same treatment protocol as Anaplasmosis. Don’t be alarmed if your horse’s test results are negative for all of the typical infections as long as the horse is responding to treatment.

Here are the symptoms, diagnostics, treatment, prognosis, and prevention measures of equine anaplasmosis:


Symptoms of Equine Anaplasmosis vary depending on the animal’s age. Young horses under 3 years old often have mild symptoms. In some cases, they may only develop a fever. Older horses often display more severe symptoms, including jaundice and swelling of the limbs. Geriatric horses generally display the most severe symptoms.

Another factor that increases the severity of symptoms is the presence of underlying medical conditions, especially ongoing infections.

Common symptoms include:

Sudden high fever (greater than 103) which may spike and subside over several days

Lethargy. Infected horses may become lethargic and show a lack of interest in their surroundings.

Loss of appetite. Infected horses may refuse to eat or exhibit reduced appetite.

Muscle Soreness. Some infected horses may display muscle stiffness and soreness, which can lead to a reluctance to move or lameness.

Edema. Swelling of the limbs, particularly the lower legs can occur.

Jaundice. In severe cases, jaundice (yellowing of the mucous membranes and sclera) may be observed.

Respiratory signs. Coughing and respiratory distress are possible, although far less common.

While we’re discussing symptoms, let’s talk about one of the most important things for you to do when you first notice your horse isn’t acting right, is off their feed, or is laying down more than you feel they should. What is this one thing? Take your horse’s temperature! As soon as you notice that something is “off” with your horse, check his temperature and THEN call the vet! This vital piece of information helps your vet know if your horse is sick or in pain, which informs their planning and scheduling.


Your vet will perform a physical exam of your horse. She will also run diagnostics including a CBC (Complete Blood Count) as well as a blood smear. A blood smear is a blood sample that is smeared on a glass slide and examined under a microscope.


If it’s diagnosed with Equine Anaplasmosis, your horse will need daily intravenous antibiotic injections as well as anti-inflammatories to reduce swelling. Depending on your location, Dr. Suzanne may not be able to administer these injections daily for you. If it is not possible for Dr. Suzanne to see your horse daily for treatment, be prepared to haul your horse to the Dells Equine Clinic for the duration of the treatment. If you don’t have a truck and trailer to haul the horse yourself, begin now to establish a relationship with an equine hauler in the area. We have area horse haulers listed on the Resources page of the DEV website.

In addition to antibiotics, supportive care may be necessary to manage symptoms and aid in recovery. This can include anti-inflammatory medications, fluid therapy, pain management, and rest.


For most horses who contract Equine Anaplasmosis, the prognosis is excellent if treatment is started early. In most cases, we see the symptoms resolving in the first 12 to 24 after beginning treatment.

If left untreated, the prognosis is more guarded. Most animals will recover within two weeks, but rare cases of death have been recorded.


Preventing equine anaplasmosis primarily involves tick control and management. There is no vaccine to prevent this disease. Immunity is acquired after infection and normally lasts around two years following an active infection. Here are some preventive measures you can take:

Tick control. Implement tick control measures, such as regular grooming and the use of tick repellents, to reduce the risk of tick infestations.

Tick removal. If you find ticks on your horse, promptly remove them using fine-tipped tweezers or a tick removal tool. Please be careful not to leave the mouthparts embedded in your horse’s skin.

Eliminate ticks from the area. Reduce your horse’s exposure to ticks, if possible, by eliminating the areas that ticks love to hide: tall grass, shrubs, under canopy trees and bushes, and leaf piles.

Keep the vectors at bay. Fence off pastures from white-tailed deer if possible. Minimize rodent populations near horses.

Equine Anaplasmosis is a tick-transmitted, seasonal illness that can make your horse feel miserable for several days and require daily antibiotic treatment. It’s essential to work closely with your equine veterinarian to diagnose and treat equine anaplasmosis and to develop a tailored management plan for your horse’s specific needs. Early diagnosis and treatment can lead to the most favorable outcomes.

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