Other Names
- EGA
- Equine granulocytic anaplasmosis
- Tick-borne fever in horses
- Ehrlichiosis in horses
What is Equine Anaplasmosis (EGA)?
Equine Anaplasmosis (formerly known as Equine Granulocytic Ehrlichiosis) is a tick-transmitted bacterial disease caused by Anaplasma phagocytophilum — an obligate intracellular organism that infects and replicates within the neutrophils and eosinophils (granulocytes) of the horse’s immune system. The disease is transmitted through the bite of infected hard ticks, principally Ixodes species, and has been reported in North America, Europe, and Asia.
In the United Kingdom and Europe, the vector is primarily Ixodes ricinus (the sheep tick or castor bean tick), which is widely distributed and active in spring, autumn, and mild winters. The disease is seasonal in distribution, occurring most frequently in the tick-active months. While often self-limiting with appropriate treatment, equine anaplasmosis can cause significant clinical illness, and severely affected horses — particularly those with concurrent illness — may deteriorate rapidly without treatment.
Causes
Anaplasma phagocytophilum is transmitted to horses through the bite of an infected Ixodes tick. The bacterium is maintained in wildlife reservoir hosts — particularly deer, small rodents, and other mammals — and ticks acquire infection by feeding on these animals. Transmission to horses requires the tick to remain attached for a minimum of 24–48 hours, making prompt tick removal an effective preventive strategy.
Risk factors for equine anaplasmosis include:
- Keeping horses in tick-endemic habitats — deciduous woodland margins, heathland, and rough grassland
- Lack of tick control measures
- Spring and autumn seasons when tick activity peaks
- Access to wildlife-dense areas where the reservoir cycle is maintained
Symptoms and Signs
Clinical signs typically appear 1–2 weeks after tick exposure:
- High fever (39–41°C) — often the first sign
- Depression, lethargy, and anorexia
- Limb oedema — particularly marked swelling of the lower limbs, which can be a prominent feature
- Reluctance to move and stiffness
- Ataxia (incoordination) in some cases
- Petechial haemorrhages on mucous membranes in severe cases
- Tachycardia and tachypnoea
- Jaundice in occasional severe cases
Secondary infections — particularly in immunosuppressed horses — are an important complication of anaplasmosis, as the pathogen specifically impairs the neutrophils that form a primary line of defence against bacteria.
Diagnosis
- Clinical signs and history: Fever, limb oedema, and ataxia in a horse with tick exposure history in a known endemic area is highly suggestive.
- Blood smear examination: Examination of a stained peripheral blood smear (Giemsa or Wright-Giemsa stain) may reveal characteristic morulae — dark-staining inclusion bodies within neutrophils. This is a rapid and inexpensive test when positive, but sensitivity is variable depending on stage of infection.
- PCR testing: Real-time PCR on EDTA whole blood is the most sensitive and specific diagnostic test, particularly early in infection before morulae are easily detected.
- Serology: Indirect fluorescent antibody (IFA) testing demonstrates rising antibody titres between acute and convalescent samples; useful for retrospective confirmation.
- Haematology: Thrombocytopaenia, leucopaenia (neutropaenia), and anaemia are common haematological findings that support the diagnosis.
Treatment
- Oxytetracycline: The treatment of choice. Intravenous oxytetracycline (6.6 mg/kg twice daily for 5–7 days) produces rapid clinical improvement, often within 24–48 hours of commencing therapy. Oral doxycycline has also been used but bioavailability in horses is significantly lower than in other species.
- NSAIDs: Phenylbutazone or flunixin for fever control and anti-inflammatory effect.
- Supportive care: IV fluids if the horse is significantly dehydrated; nutritional support; management of limb oedema with gentle exercise and cold therapy.
- Antibiotic prophylaxis for secondary infections: In immunocompromised or severely affected horses, the risk of opportunistic bacterial infection may warrant broad-spectrum antibiotic cover alongside tetracycline treatment.
Response to oxytetracycline is typically dramatic and rapid improvement within 12–48 hours provides clinical confirmation of the diagnosis. For comprehensive guidance on tick-borne diseases, the British Equine Veterinary Association (BEVA) provides clinical resources.
Prevention
- Implement a rigorous tick control programme — apply licensed tick repellents to horses in endemic areas, particularly in spring and autumn.
- Inspect horses daily for ticks, particularly around the head, mane, groin, and under the tail. Remove any ticks found using a tick removal tool with a gentle rotating motion — do not squeeze the tick body.
- Reduce tick habitat where possible — keep pastures mown and fence off woodland margins.
- There is currently no commercially available vaccine for equine anaplasmosis.
Prognosis
The prognosis for equine anaplasmosis with prompt, appropriate tetracycline treatment is generally good to excellent. Most horses recover fully within 5–10 days of commencing therapy. Untreated, the disease may persist for 2–4 weeks before natural recovery, but immunosuppression during this period carries significant risk of secondary infection, which can be life-threatening. Horses with concurrent illness or pre-existing immune compromise carry a more guarded prognosis.
When to Call the Vet
Contact your vet promptly if your horse:
- Develops fever, limb oedema, and depression in spring or autumn
- Is in a tick-endemic area and showing any of the described signs
- Has recently had ticks found attached for an extended period
- Does not respond to initial NSAID therapy as expected
Frequently Asked Questions
Is equine anaplasmosis the same as Lyme disease?
No — they are distinct tick-borne diseases. Lyme disease in horses is caused by Borrelia burgdorferi, also transmitted by Ixodes ticks, but produces a different spectrum of clinical signs (predominantly musculoskeletal, neurological, and ophthalmological). Both diseases can co-occur in the same tick populations and geographic areas.
Can equine anaplasmosis be transmitted from horse to horse?
No — the disease requires tick transmission and is not directly contagious between horses or from horses to humans. However, humans can be infected with Anaplasma phagocytophilum through tick bites independently of contact with affected horses.
How quickly does a horse improve with treatment?
Response to intravenous oxytetracycline is typically rapid — many horses show a notable improvement in attitude and reduction in fever within 24 hours of the first dose. This rapid response is diagnostically useful and is one of the strongest clinical indicators that the diagnosis is correct.
Is equine anaplasmosis common in the UK?
Cases have been reported in the UK and continental Europe, primarily in areas with established Ixodes ricinus tick populations — woodland areas, upland moorland, and heathland environments. The frequency of diagnosed cases may be underrepresented as the condition is not always considered in the differential diagnosis of acute febrile illness.