Anaplasma Phagocytophilum Equine Granulocytic Anaplasmosis Tick

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-25

Key Takeaways

Equine Granulocytic Anaplasmosis (EGA) is caused by the obligate intracellular bacterium Anaplasma phagocytophilum, which primarily infects granulocytes.
Transmission occurs via the bite of infected Ixodes ticks, with common clinical signs including fever (102-106°F), lethargy, and limb edema.
Diagnostic confirmation relies on a combination of clinical findings, history of tick exposure, and laboratory tests such as PCR for bacterial DNA and serology for antibody detection.
Thrombocytopenia and leukopenia are characteristic hematological findings on a Complete Blood Count (CBC).
The primary treatment involves a 7-14 day course of antibiotics, with doxycycline being the drug of choice; oxytetracycline is an alternative.
Prevention focuses on minimizing tick exposure through environmental management, acaricide application, and diligent daily inspection of horses for ticks.

Immediate Direct-Answer Summary Box Equine Granulocytic Anaplasmosis (EGA) is a tick-borne disease caused by the bacterium Anaplasma phagocytophilum. It manifests with symptoms such as fever, lethargy, and limb edema. Immediate veterinary care, including antibiotics like doxycycline, is essential for effective treatment.

Introduction

Equine Granulocytic Anaplasmosis (EGA) is a significant health concern for horses, particularly in regions where ticks are prevalent. This guide provides a comprehensive overview of EGA, including its causes, symptoms, diagnostic methods, and treatment options. Our goal is to equip horse owners and veterinarians with the knowledge needed to manage and prevent this disease effectively.

What is Equine Granulocytic Anaplasmosis?

EGA is an infectious disease caused by the bacterium Anaplasma phagocytophilum. This bacterium is transmitted to horses through the bite of infected ticks, primarily Ixodes species. Once inside the horse's body, the bacterium infects granulocytes, a type of white blood cell, leading to a range of clinical symptoms.

Symptoms of EGA

The symptoms of EGA can vary in severity but typically include:

Fever: Often the first sign, with temperatures ranging from 102°F to 106°F.
Lethargy: Affected horses may appear depressed and less active.
Limb Edema: Swelling in the legs is common.
Ataxia: In severe cases, horses may exhibit incoordination.
Petechiae: Small red or purple spots on the mucous membranes due to bleeding.

Diagnostic Criteria

Accurate diagnosis of EGA involves a combination of clinical signs and laboratory tests:

Clinical Examination

History: Recent exposure to tick-infested areas.
Physical Examination: Presence of fever, limb edema, and lethargy.

Laboratory Tests

Complete Blood Count (CBC): Thrombocytopenia and leukopenia are common.
PCR Testing: Detects Anaplasma phagocytophilum DNA.
Serology: Identifies antibodies against the bacterium.

graph TD;
    A[Clinical Signs] --> B{History of Tick Exposure?};
    B -->|Yes| C[Physical Examination];
    B -->|No| D[Consider Other Diagnoses];
    C --> E[Fever, Lethargy, Limb Edema];
    E --> F[Laboratory Tests];
    F --> G[CBC, PCR, Serology];
    G --> H[Diagnosis of EGA];

Treatment Options

The cornerstone of EGA treatment is the administration of antibiotics. The most commonly used antibiotic is doxycycline, which is highly effective against Anaplasma phagocytophilum.

Antibiotic Therapy

Doxycycline: Administered orally or intravenously for 7-14 days.
Oxytetracycline: An alternative if doxycycline is unavailable.

Supportive Care

Fluid Therapy: To prevent dehydration.
Anti-inflammatory Drugs: To reduce fever and pain.

Prevention Strategies

Preventing EGA involves minimizing tick exposure and implementing effective tick control measures:

Tick Control

Environmental Management: Regular mowing and removal of leaf litter.
Acaricides: Application of tick repellents on horses.
Regular Inspections: Daily checks for ticks, especially after outdoor activities.

Vaccination

Currently, there is no commercially available vaccine for EGA. Research is ongoing to develop effective vaccines.

Frequently Asked Questions (FAQ)

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Conclusion

Equine Granulocytic Anaplasmosis is a serious but manageable disease. Early recognition of symptoms, prompt veterinary care, and effective tick control measures are crucial for the health and well-being of horses. By staying informed and proactive, horse owners can significantly reduce the risk of EGA in their animals.

References

Pusterla, N., et al. (2003). Equine Granulocytic Anaplasmosis: A Review. Journal of Veterinary Internal Medicine.
Madigan, J. E., & Pusterla, N. (2000). Ehrlichial Diseases. Veterinary Clinics of North America: Equine Practice.
Stuen, S., et al. (2013). Anaplasma phagocytophilum, a widespread multi-host pathogen with highly adaptive strategies. Frontiers in Cellular and Infection Microbiology.
Franzén, P., et al. (2009). Acute clinical, hematologic, serologic, and polymerase chain reaction findings in horses experimentally infected with a European strain of Anaplasma phagocytophilum. Journal of Veterinary Internal Medicine.
Dumler, J. S., et al. (2001). Reorganization of genera in the families Rickettsiaceae and Anaplasmataceae in the order Rickettsiales: unification of some species of Ehrlichia with Anaplasma. International Journal of Systematic and Evolutionary Microbiology.