Anaplasmosis in Horses (Equine Granulocytic Ehrlichosis): Symptoms, Diagnosis & Treatment

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]

• Diarrhea
• Abortion (any stage of gestation)

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. phygocytophilum 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]

• Equine herpesvirus-1
• West Nile virus
• Rabies

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 helps 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]

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.