Equine Granulocytic Anaplasmosis

Equine Granulocytic Anaplasmosis (EGA) is an infectious, noncontagious, seasonal disease in horses caused by the tick-transmitted bacterium Anaplasma phagocytophilum. This pathogen infects neutrophils and occasionally eosinophils, leading to a range of clinical signs from mild fever to severe systemic illness.

Etiology and Transmission:

Causative Agent: Anaplasma phagocytophilum, a rickettsial bacterium.
Vectors: Transmitted primarily by ticks of the Ixodes species, including I. pacificus (western black-legged tick), I. scapularis, I. ricinus, and I. persulcatus.
Hosts: Infects a wide range of hosts, including horses, burros, dogs, llamas, rodents, and humans (where the disease is known as human granulocytic anaplasmosis).
Seasonality: In regions like California, cases occur predominantly in late fall, winter, and spring, correlating with tick activity.

Pathophysiology:

After transmission through a tick bite, A. phagocytophilum enters neutrophils and forms intracytoplasmic inclusion bodies called morulae. The presence of the bacterium within neutrophils leads to systemic inflammation and clinical signs associated with EGA.

Clinical Presentation:

Incubation Period: Approximately 1–3 weeks post-infection.
Clinical Signs:
- Foals (<1 year): May exhibit only fever.
- Yearlings (1–3 years): Fever, depression, mild limb edema, and ataxia.
- Adults and Geriatric Horses: Fever (103°–104°F; can reach up to 107°–108°F), partial anorexia, depression, reluctance to move, limb edema, petechiation, and icterus. Fever typically peaks around day 5 and can persist for 6–12 days.
Hematologic Findings: Leukocytopenia or pancytopenia with moderate to severe thrombocytopenia.
Lesions: Gross petechiation, ecchymoses, and edema in the subcutis and fascia, predominantly affecting the legs.

Diagnosis:

Clinical Evaluation: Consider EGA in horses from endemic areas presenting with acute fever and associated clinical signs, especially during peak tick activity seasons.
Laboratory Testing:
- Blood Smear: Giemsa or Wright-Leishman stained smears may reveal morulae within neutrophils.
- PCR Assay: Detection of A. phagocytophilum DNA in unclotted blood or buffy coat samples.
- Serology: Indirect fluorescent antibody tests may be negative early in the disease; a four-fold rise in antibody titers over 2–4 weeks can retrospectively confirm the diagnosis.

Treatment:

Antibiotic Therapy: Oxytetracycline is highly effective against A. phagocytophilum. Administer intravenously at 6.6 mg/kg once daily for 5–7 days. Dilution in saline can reduce gastrointestinal side effects.
Supportive Care: Ensure adequate hydration and monitor for secondary infections or complications.

Prognosis:

With prompt and appropriate treatment, the prognosis for EGA is generally favorable. Most horses respond rapidly to oxytetracycline therapy. However, if left untreated, the disease can lead to more severe complications, especially in older horses.

Prevention:

Tick Control: Implement measures to reduce tick exposure, such as the use of acaricides, environmental management to decrease tick habitats, and regular inspection and removal of ticks from horses.
Monitoring: Be vigilant for clinical signs of EGA, especially during seasons of high tick activity, and in horses residing in or traveling to endemic areas.

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