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Anaplasmosis in horses, also known as Equine Granulocytic Anaplasmosis (EGA), is a tick-borne disease caused by the bacterium Anaplasma phagocytophilum.
This condition typically affects horses in areas where the tick vectors are present, including various parts of the United States, Europe, Africa, and South America. [1]
Horses affected by anaplasmosis experience symptoms that can include fever, depression, swelling of the limbs, uncoordinated gait, and yellowing of the eyes and gums. [1][2][3][4] A variety of blood tests are available for diagnosing EGA. [1][2][5]
Most horses have a favorable prognosis without the need for treatment. However, in more severe cases, the condition is highly responsive to antibiotic therapy and supportive care. [1]
Equine Granulocytic Anaplasmosis
Equine granulocytic anaplasmosis (EGA), formerly known as equine granulocytic ehrlichiosis, is caused by infection with Anaplasma phagocytophilum. This bacterium can infect horses, other domestic animals, and humans. [5]
A. phagocytophilum is part of the Rickettsia group of gram-negative bacteria that are obligate intracellular organisms. [1] This means that these bacteria can only grow and multiply within cells of a host. They require arthropod vectors, such as ticks and mites, for transmission between animals. [6]
The bacterium A. phagocytophilum , was previously classified as Ehrlichia equi. This earlier nomenclature is the source of the alternative name “Equine Granulocytic Ehrlichiosis.”
The term “granulocytic” in this context refers to the type of white blood cells, namely granulocytes, that are primarily affected by the infection. [1][2] Granulocytes, a key component of the immune system, play a crucial role in fighting infections.
Prevalence
EGA was first identified in the United States in 1969 and has since been reported in various countries worldwide. [2][7] The disease is most commonly reported in tick-infested regions, including Canada, the United States, South America, Europe, Asia, and North Africa. However, it has the highest prevalence in the northwestern United States. [1][8]
The actual prevalence of anaplasmosis is challenging to determine as many infected horses are asymptomatic with no outward signs of infection. [2][5]
EGA is a seasonal disease, with the highest incidence in the late fall, winter, and spring. [4][8] This pattern aligns with the activity of infected ticks, which are the primary vectors of the disease.
Climate change poses a risk of increased anaplasmosis exposure in horses. Climate change significantly influences the behavior and distribution of tick populations, as well as extending the duration of tick-active seasons. [5]
Clinical Signs
In the initial stages after infection, horses with EGA commonly exhibit clinical signs such as: [1][2][3]
- Lethargy
- Fever
- Decreased appetite
- Increased heart rate
- Increased breathing rate
Younger horses, typically those under 4 years old, tend to show less severe clinical signs. Foals less than a year old often present with only fever, whereas older horses are more likely to experience a more severe progression of the disease. [1][8][9]
Severe Clinical Signs
As the infection progresses and spreads throughout the body, symptoms tend to worsen. This can lead to the emergence of additional clinical signs, including: [1][4]
- Yellow eyes and gums
- Signs of dehydration, such as sunken eyes and dry gums
- Swelling of the limbs and skin around the abdomen
- Red spots on the gums
- Unwillingness to move
- Stumbling, uncoordinated gait
Although horses with anaplasmosis may appear unwilling to move, the disease is not associated with laminitis. [1]
Rarely, EGA can also be associated with symptoms like: [1][9]
What's your top priority with your horse's health?
What's your top priority with your horse's health?
Secondary Complications
While complications arising from anaplasmosis are uncommon, they have been reported in horses.
Rhabdomyolysis
Severe infection with A. phagocytophilum can lead to breakdown of muscle tissue, also called rhabdomyolysis. This results in firm, painful muscles, typically along the horse’s back and upper hindlimbs. [1][10]
Abnormal muscle tissue breakdown can be identified on bloodwork. In horses with rhabdomyolysis, blood tests reveal elevated levels of muscle enzymes, specifically creatine kinase and aspartate aminotransferase. [1]
Neurologic Disease
Occasionally, anaplasmosis can lead to severe neurologic disease in horses. Neurological symptoms can include: [1][3][9][11][12]
- Head tilt
- Difficulty eating or swallowing
- Recumbency
- Seizures
- Collapse
The mechanism by which A. phagocytophilum causes neurological issues is poorly understood, but is believed to involve a combination of inflammation of the nerves and damage to blood vessels. [4][5]
Pleural and Pericardial Effusion
In rare cases, infection by A. phagocytophilum can lead to fluid build-up around the lungs, known as pleural effusion. It can also cause fluid accumulation around the heart, referred to as pericardial effusion. [1][13]
Symptoms of these complications are similar to those of anaplasmosis and include fever, weakness, and swelling of the limbs. [1]
Breathing Difficulties
EGA can also lead to respiratory difficulties in horses, primarily due to fluid accumulation in the upper airway and impaired functionality of the pharyngeal muscles situated near the windpipe. [1][14]
This complication can occur if the nerves that control the muscles surrounding the upper airway are affected by infection or inflammation. [5][15]
Pathogenesis
Horses commonly become infected with EGA after being bitten by ticks carrying the A. phagocytophilum bacteria. After penetrating the skin, the bacteria travels through the bloodstream, where they target and infect granulocytes. [4][6][8]
Granulocytes are a category of white blood cells that include neutrophils, eosinophils, and basophils. These cells contain granules in their cytoplasm and play a role in the body’s immune response. [2][18][19] Although A. phagocytophilum can infect any granulocyte, it primarily targets neutrophils.
The pathogen also infects endothelial cells that line the blood vessels, facilitating further spread of the bacterium within the bloodstream. [5]
After a horse is exposed to the bacteria, the incubation period for EGA is typically 6 – 12 days before symptoms become apparent. [1]
The mechanisms by which horses naturally clear the infection on their own are still not fully understood. [5]
Transmission
Anaplasmosis is a vector-borne disease, primarily transmitted by infected ticks. The main tick species responsible for transmitting this disease are Ixodes scapularis (Deer tick) and Ixodes pacificus Western Black Legged tick. [1]
The disease can also be transmitted through contaminated needles or syringes carrying blood from infected horses, as well as blood transfusions. [1][4]
Rarely, anaplasmosis can be transmitted from mares to their fetuses through transplacental infection. Occasionally, foals can be born already infected with anaplasmosis, especially if the infection occurs late in gestation. [1][16]
A. phagocytophilum is not found freely in the environment. Instead, small mammals, birds, and lizards are considered reservoir hosts for the organism. Ticks acquire the bacteria from a reservoir host and transmit it to horses, or they can transmit it directly from one horse to another. [1][5][17]
Diagnosis
Diagnosis of anaplasmosis in horses is based on clinical findings and blood tests. Veterinarians typically suspect EGA in horses presenting with high fever and related symptoms, especially in regions known for high tick activity.
However, diagnostic tests are required to rule out other conditions and confirm the diagnosis. [4]
Bloodwork
Bloodwork in cases of suspected EGA can reveal a decrease in white blood cells, platelets, and red blood cells (anemia), along with elevated protein levels and liver enzymes.
However, these bloodwork findings are not specific to anaplasmosis. [1][2] [3][14]
Microscopic Blood Analysis
Microscopic examination of blood is sometimes used to diagnose EGA. Under a microscope, the bacterium A. phagocytophilum can be detected within granulocytes, including neutrophils.
One of the key features of anaplasmosis infection is the presence of inclusion bodies or morulae within these cells. These morulae are usually detectable in neutrophils around 3 to 5 days after symptom onset. [1]
The presence of morulae in neutrophils is a key diagnostic indicator of EGA, especially in the absence of other diagnostic tools like PCR (Polymerase Chain Reaction). [1][2][5]
Diagnostic Blood Tests
Three types of blood tests are commonly used to detect A. phagocytophilum infection in horses. These tests help veterinarians determine if a horse is positive for EGA by looking for different disease markers in the blood.
- PCR test: The polymerase chain reaction test detects genetic material of the bacteria in the horse’s blood, showing if an infection is currently present. The PCR test is a highly effective diagnostic tool, capable of identifying the infection from as early as 3 days to as late as 129 days post-infection. However, false negatives can occur in horses that have already undergone treatment. [1][2][4]
- Serology test: This test identifies antibodies in the blood, indicating the horse’s exposure to A. phagocytophilum bacteria within the past two years. The test requires two blood samples taken 2 to 4 weeks apart. While early infection stages may yield a negative result, a horse with active infection will exhibit increased antibody levels against the bacterium. [1][2]
- ELISA or SNAP test: These tests detect specific proteins or antibodies related to EGA in the horse’s blood, providing a quick indication of whether the horse is infected.[5][20]
Post-Mortem Examination
Horses that succumb to anaplasmosis infection may undergo post-mortem examination. Typical findings in these cases include: [1][7]
- Swelling under the skin
- Red spots or streaks under the skin
Upon microscopic examination of tissues, inflammation of the blood vessels, or vasculitis, is often detectable. This condition predominantly affects certain tissues, including the reproductive organs (such as the testes and ovaries), lungs, kidneys, brain, heart, and limbs. [7][14]
To confirm a diagnosis of EGA, tissues such as bone marrow and spleen that can harbor high numbers of the bacteria may be used for a PCR test. [9]
Differential Diagnoses
Other diseases that produce symptoms similar to EGA must be ruled out during diagnosis. Alternative diagnoses to consider include: [4]
- Viral encephalitis
- Liver disease
- Equine infectious anemia
- Purpura hemorrhagica
- Viral arteritis
Horses with neurologic disease and fever are commonly screened for other diseases such as: [12][21]
Treatment
For many horses, EGA resolves on its own without the need for specific treatment. In such cases, the disease typically resolves in 2 to 3 weeks. [1][8]
However, in cases where the disease presents more severely, the primary approach to treatment includes antibiotics and supportive care.
Antibiotics
Tetracycline-based antibiotics are effective for treating anaplasmosis in horses. Symptoms typically start to improve within 24 to 48 hours after starting treatment. [1]
Common antibiotic medications used to treat EGA include: [1]
- Oxytetracycline – 7 mg/kg intravenously once daily (diluted and administered slowly)
- Doxycycline – 10 mg/kg orally twice daily
- Minocycline – 4 mg/kg orally twice daily
- Tetracycline – 10 mg/kg intravenously twice daily
Bloodwork to assess hydration and kidney function is recommended prior to treatment in severe cases, or if prolonged antibiotics are needed. [1]
If symptoms do not improve within 24 to 48 hours after beginning treatment, veterinarians may suspect a secondary bacterial infection or another diagnosis. [14]
Supportive Care
In severe cases of anaplasmosis, supportive care is administered to manage symptoms and prevent complications. Examples of supportive care include: [1]
- Intravenous fluids
- Electrolytes
- Wrapping swollen limbs
- Stall rest
Horses with severe neurological signs may require additional support including pain medications or steroid injections (20 mg/kg of dexamethasone). [4][8]
Prognosis
Most horses fully recover from Equine Granulocytic Anaplasmosis within 2 – 3 weeks. However, in some cases infection can recur even after treatment. [1]
Horses that are not treated may suffer from weight loss and experience prolonged swelling of the limbs. [1]
Rarely, horses can die from A. phagocytophilum infection. When deaths occur, they are often due to secondary infections or complications. [1][8]
Prevention
The most effective way to prevent anaplasmosis in your horse is to reduce tick exposure. This can be accomplished by using tick repellants or controlling the tick population in the environment. [1]
Tick Repellants
There is a variety of sprays and tick repellents available to protect horses from tick bites and the diseases they carry. These products often contain one or more of the following active ingredients: [1]
- Cypermethrin
- Permethrin
- Pyrethrins
- Piperonyl butoxide
Each of these chemicals has different levels of efficacy and durations of action, making it important to choose a product that suits the specific needs of the environment and the horse. [1]
Reducing Tick Population
There are several steps you can take to minimize the presence of ticks on your horse property. These arthropods often populate in loose, organic materials such as tall grasses and wooded debris. Implementing the following measures can help reduce tick populations: [1]
- Removing piles of leaves and wood
- Mowing long grasses
- Controlling wildlife populations, such as deer
Regularly checking your horse for ticks is also important for preventing tick-borne diseases. Frequent checks help minimize their time of attachment in case one happens to bite your horse. The risk of a tick transmitting infectious agents increases the longer it remains attached to the horse’s skin. [5][22]
Vaccines
There is no vaccine available to protect horses against A. phagocytophilum infection. [8] However, horses that have successfully recovered from the infection are considered immune to new infections for at least two years. [8]
Other Considerations
Anaplasmosis is not contagious among horses, meaning isolation of affected horses is not required for preventing the spread of the disease. [1]
Additionally, the bacterium does not survive in the environment outside of its hosts, including ticks and horses. Therefore, environmental decontamination is not necessary if horses on your property become infected with EGA. [1]
While EGA cannot be transmitted directly from horses to humans, humans can become infected with A. phagocytophilum through bites from infected ticks. This makes tick control and prevention measures important for both horse and human health in areas where anaplasmosis is prevalent. [1][4]
Frequently Asked Questions
Here are some frequently asked questions about anaplasmosis in horses:
Anaplasmosis in horses is a tick-borne bacterial disease caused by Anaplasma phagocytophilum. It is also called Equine Granulocytic Anaplasmosis, or EGA. The infection affects white blood cells involved in the immune response and is most often seen in areas where infected ticks are active. Many horses recover well, but fever, swelling, and worsening illness still require veterinary attention.
Signs of equine anaplasmosis commonly include fever, depression, low appetite, limb swelling, and an uncoordinated gait. Some horses also develop yellowing of the eyes or gums, red spots on the gums, dehydration, or reluctance to move. Severe cases can involve neurologic signs such as head tilt, trouble swallowing, seizures, collapse, or recumbency.
Tick season can explain a sudden fever in a horse when the horse lives in or has traveled through an area where infected ticks are common. Anaplasmosis often causes fever early, sometimes along with low energy, reduced appetite, swelling, or stiffness. Other diseases can look similar, so a veterinarian may recommend blood tests to confirm the cause.
Horses get anaplasmosis most often from the bite of an infected tick, especially Ixodes species such as deer ticks and western black-legged ticks. Less common routes include contaminated needles, blood transfusions, and rare transmission from a mare to a fetus. The bacteria do not live freely in the environment, so exposure usually depends on infected ticks or blood contact.
Anaplasmosis is not considered contagious between horses through normal contact. A positive horse usually does not need stall isolation to protect barnmates, since the bacteria are spread mainly by ticks rather than casual contact. Prevention should focus on tick control, tick checks, and avoiding shared needles or syringes that could transfer infected blood.
Anaplasmosis can make a horse look lame when fever, limb swelling, stiffness, or muscle pain affects how the horse moves. The disease is not associated with laminitis, so the abnormal movement is not usually from the hoof disease itself. A veterinary exam helps separate anaplasmosis from true lameness, injury, or other causes of colic-like discomfort.
Equine anaplasmosis is diagnosed using clinical signs, tick exposure history, and blood testing. Bloodwork may show low white blood cells, low platelets, anemia, or changes in liver enzymes, but these findings are not specific. Veterinarians may use microscopic blood analysis, PCR testing, serology, or ELISA and SNAP tests to look for current infection or recent exposure.
Anaplasmosis treatment in horses often includes tetracycline-based antibiotics when signs are moderate or severe. Many horses improve within 24 to 48 hours after treatment begins. Supportive care may also include intravenous fluids, electrolytes, limb wrapping, and stall rest. Mild cases can resolve without specific treatment, but a veterinarian should guide care because complications can occur.
Most horses recover from anaplasmosis within 2 to 3 weeks, especially with prompt diagnosis and care when signs are severe. Prognosis is generally favorable, and antibiotic therapy is usually effective when needed. Rare deaths can occur, often due to complications or secondary infections. Some horses may have lingering weight loss or limb swelling if the disease is not treated.
Anaplasmosis prevention for horses depends on reducing tick exposure. Tick repellents, regular tick checks, mowing tall grass, removing leaf and wood piles, and managing wildlife pressure can all lower risk. Checking horses after turnout is especially useful because the chance of disease transmission increases the longer an infected tick stays attached to the skin.
No equine vaccine is available to protect horses against Anaplasma phagocytophilum infection. Tick prevention remains the main way to reduce risk. Horses that recover from infection are considered immune to new infections for at least two years, but that does not replace tick control. Regular monitoring is still important in regions where infected ticks are common.
Humans do not catch anaplasmosis directly from horses, but people can become infected after being bitten by infected ticks. A horse with anaplasmosis may indicate that infected ticks are present in the area. Tick checks, protective clothing, and careful removal of attached ticks help protect both horses and people on properties where the disease occurs.
Summary
- Equine granulocytic anaplasmosis (EGA) is a bacterial disease that is transmitted to horses by ticks carrying the bacterium Anaplasma phagocytophilum.
- EGA is commonly a self-limiting disease in horses, characterized by clinical signs such as fever, depression, ataxia, and jaundice.
- While many horses recover from the disease without treatment, antibiotics and supportive care are effective when necessary.
- No vaccines are available to protect against anaplasmosis in horses. The best preventative strategy is to reduce tick exposure and the tick population on the property.
References
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- Siska W.D. et al., Clinicopathologic Characterization of Six Cases of Equine Granulocytic Anaplasmosis In a Nonendemic Area (2008-2011). Journal of Equine Veterinary Science. 2013.
- Janet E. Foley, Equine Granulocytic Anaplasmosis - Generalized Conditions. Merck Veterinary Manual. Accessed Dec. 12, 2023.
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