What is Anaplasma (Ehrlichiosis)?
Equine granulocytic ehrlichiosis, is an infectious disease resulting from Anaplasma phagocytophilum bacteria that are transmitted by ticks from the Ixodes family (deer ticks). What had been known as ehrlichia has been renamed anaplasma; anaplasma in horses is not contagious and while it was first seen in northern California, it is now found in many other states as well as Europe, Africa and South America. Typically, the disease will occur in late fall to spring as a result of the state of the ticks where the horse resides. The disease, which can impact horses of all ages, is related to Lyme disease and rocky mountain spotted fever and has been seen more often over the last several years. Typically, the symptoms will show up suddenly and are most often seen in fall, winter and spring.
An infectious disease, anaplasma in horses, also known as equine granulocytic ehrlichiosis, is caused by the Anaplasma phagocytophilum bacteria being transmitted by ticks, leading to symptoms 10-20 days after the horse is infected.
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Symptoms of Anaplasma (Ehrlichiosis) in Horses
The symptoms that are experienced as a result of anaplasma in horses will vary based on your horse’s age as well as the length of the illness. Horses that are under the age of four will typically experience more mild symptoms:
- Horses that are less than a year old may experience no clinical signs or just a fever as a result of the illness
- Horses that are aged one to three years old may experience a fever, depression, mild swelling in the limbs and a lack of muscle coordination
- Horses four years of age and older may not want to move, lose interest in eating, experience depression, have swelling in their limbs, and show petechiation as well as jaundice
Typically, the fever will be at its highest from days 1-3 of infection, usually between 103 and 104 degrees Fahrenheit. The fever will continue at 102-104 degrees for 6-12 days after infection. Symptoms will typically worsen over several days. In rare cases, the horse will experience myocardial vasculitis which can lead to transient ventricular arrhythmias. Vasculitis may be seen, typically on the legs of your horse.
Should your horse have another infection, it can be aggravated as a result of the infection by anaplasma. Once the clinical signs of infection begin, the health of your horse will decline quickly.
Anaplasma will display different symptoms in horses depending upon their age. While horses that are under a year old may experience a fever only or no clinical signs at all upon infection, horses between the ages of one and three years of age will experience additional symptoms. Horses aged four and over will likely experience more symptoms than their younger counterparts.
Causes of Anaplasma (Ehrlichiosis) in Horses
Anaplasma in horses is caused by the bacteria Anaplasma phagocytophilum, which will infect horses through transmission by deer ticks. The white blood cells flood the bacteria from the tick bite which leads to the white blood cells, which fight infection, being destroyed. In the United States, infection has been confirmed in the following states: Connecticut, Illinois, Arkansas, Washington, Pennsylvania, Colorado, Minnesota, New York, Massachusetts, and Florida. Infection has also been found in Sweden, Great Britain, and South America.
Diagnosis of Anaplasma (Ehrlichiosis) in Horses
Upon noticing any of the symptoms noted above in your horse, you will want to schedule an appointment with your veterinarian, who will conduct an examination of your horse and ask you for information on the symptoms you have noticed and when you noticed them. Your veterinarian will consider the location where you live and whether deer ticks are present there, as well as whether any ticks are noticed on your horse. In addition to considering the symptoms that your horse is displaying (particularly fever, depression and lethargy) your veterinarian may consider the following:
- A blood test to look for cytoplasmic inclusion bodies (these may be difficult to see in the first day or two that your horse has a fever, however can increase to 30-40% of the circulating neutrophils as the infection has been present from 3-5 days); blood test results may show a decrease in your horse’s white blood cell count, a decreased platelet count and/or a decreased red blood cell count
- SNAP 4Dx to see if antibodies are present
- A PCR (polymerase chain reaction) test can tell if there is A phagocytophilum present in non-clotted blood
- Increasing antibody titers to A phagocytophilum can be found through an indirect fluorescent antibody test
Often, the symptoms your horse is displaying and his response to treatment are more valuable than the tests; the results often come back after your horse has had symptoms resolved through treatment.
Treatment of Anaplasma (Ehrlichiosis) in Horses
Should your horse be diagnosed with anaplasma, your veterinarian may choose to treat him with oxytetracycline; systemic treatment with tetracycline at 7mg/kg/day intravenously for eight days has been noted to resolve the infection. Should your horse receive treatment early in the course of the infection for a shorter period, he may experience a relapse within a few weeks. If your horse is experiencing swelling and trouble with muscle coordination, short-term treatment with corticosteroids may be helpful. Banamine for the fever your horse is experiencing may be recommended, as well as supportive treatments like intravenous fluids.
Recovery of Anaplasma (Ehrlichiosis) in Horses
Once your horse has recovered from infection with anaplasma he will typically be immune to infection by the bacteria for at least two years. Horses that are treated promptly for the disease have an excellent prognosis, typically with symptoms resolving within 12-24 hours. Once symptoms resolve, it is important that you continue the full course of treatment to make sure that the infection is eliminated.
Your veterinarian may recommend follow up appointments to ensure that your horse is responding to medication and any supportive treatment being provided.