Bit & Bridle

Equine Anaplasmosis

Other Names

  • Equine Granulocytic Anaplasmosis (EGA)
  • Equine Granulocytic Ehrlichiosis (EGE) — former name
  • Tick-borne fever in horses

What is Equine Anaplasmosis?

Equine Anaplasmosis is a tick-borne infectious disease caused by the intracellular bacterium Anaplasma phagocytophilum. The organism infects white blood cells — specifically neutrophils and eosinophils — impairing the immune system’s ability to fight infection and causing a range of clinical signs from mild fever and depression to severe neurological complications.

The disease has a distinct geographical distribution, largely determined by the range of the tick vectors. In the UK, the primary vector is Ixodes ricinus (the sheep tick or castor bean tick), which is widespread across moorland, heathland, woodland edges, and rough grazing in many parts of the country. Cases are most frequently reported in the spring and autumn — peak tick activity periods.

Equine anaplasmosis is the same organism that causes human granulocytic anaplasmosis (HGA) in people and can also affect dogs, llamas, and small ruminants, making it a disease of broader veterinary and public health relevance. For other tick-related or infectious conditions, see our overview of Equine Nasal Discharge.

Causes

Anaplasma phagocytophilum is a gram-negative, obligate intracellular bacterium transmitted primarily through the bite of infected Ixodes ticks. In the UK, Ixodes ricinus is the principal vector. In North America, Ixodes scapularis (deer tick) and Ixodes pacificus (Western black-legged tick) are the primary vectors. The tick must typically be attached and feeding for at least 24–48 hours for transmission to occur.

Wild deer are considered the primary reservoir host in the UK, maintaining the infection cycle in tick populations. Horses grazing in areas with high deer and tick populations are at greatest risk. Transmission does not occur horse-to-horse through normal contact; ticks are the required vector.

Symptoms and Signs

Clinical signs typically appear within one to two weeks of infection (the incubation period following tick bite):

  • Fever — often the first sign; rectal temperature may reach 39–41°C (102–106°F)
  • Depression and lethargy — affected horses appear dull, disinterested in food, and unwilling to move
  • Inappetence — reduced or absent interest in hay and feed
  • Ventral oedema — swelling of the legs (particularly the lower limbs), ventral abdomen, and prepuce is a characteristic feature
  • Ataxia (incoordination) — wobbliness, particularly in the hindlimbs; may be severe enough to cause the horse to stumble or fall
  • Reluctance to move or exercise intolerance
  • Petechial haemorrhages — small pinpoint haemorrhages visible on the mucous membranes due to thrombocytopenia (low platelet count)
  • Jaundice (icterus) — yellow discolouration of the mucous membranes in some cases

The combination of fever, ventral oedema, and ataxia in a horse with access to tick-infested grazing is highly suggestive of equine anaplasmosis and should prompt immediate veterinary assessment.

Diagnosis

Diagnosis can be made using a combination of clinical findings, blood tests, and specific laboratory tests:

  • Haematology — blood count characteristically shows thrombocytopenia (low platelets), neutropenia (low neutrophils), and lymphocyte abnormalities. Anaemia may be present in some cases.
  • Morulae identification — examination of a blood smear may reveal characteristic inclusion bodies (morulae — clusters of bacteria) within neutrophils. This is pathognomonic when found but sensitivity is variable.
  • PCR testing — polymerase chain reaction testing of whole blood for A. phagocytophilum DNA is the most sensitive and specific diagnostic test. Available through specialist veterinary laboratories.
  • Serology (IFA test) — antibodies to A. phagocytophilum can be detected in blood; a rising titre between acute and convalescent samples (two to three weeks apart) confirms recent infection, but serology is not useful for diagnosing acute disease as antibodies take time to develop
  • Biochemistry — elevated liver enzymes (particularly GGT and AST) are commonly seen; bilirubinuria may be present on urinalysis

Treatment

Equine anaplasmosis responds well to appropriate antibiotic therapy when initiated promptly:

  • Oxytetracycline — the treatment of choice. IV administration at appropriate doses typically results in rapid clinical improvement, often within 12–24 hours of beginning treatment. A course of five to seven days is usually recommended; longer courses may be required in severe cases.
  • Doxycycline — oral tetracycline, used in some cases as a follow-on treatment after initial IV oxytetracycline therapy
  • Supportive care — NSAIDs (phenylbutazone or flunixin) to reduce fever and inflammation; ensuring adequate hydration; hand-walking to reduce limb oedema
  • Rest — affected horses should not be exercised until clinical signs have fully resolved and blood parameters have normalised

Horses with severe ataxia must be managed carefully to prevent injury from falls. Padded stabling and restricted movement may be necessary during the acute phase.

Prevention

Prevention relies on reducing tick exposure and detecting ticks before transmission occurs:

  • Daily tick inspection — check horses thoroughly after turnout in tick-prone areas, paying particular attention to the mane, tail base, ears, belly, groin, and between the legs
  • Prompt tick removal — remove any attached ticks as soon as possible using a tick hook or fine-tipped tweezers. Grasp the tick as close to the skin as possible and pull with a steady, gentle upward motion — do not twist or squeeze the tick’s body, which may inject more bacteria.
  • Permethrin-based repellents — applied to the horse’s coat before turnout, particularly effective against ticks and some other biting insects. Reapply as per product instructions.
  • Pasture management — keeping pasture well-grazed and reducing areas of long grass, bracken, and woodland edge vegetation reduces tick habitat near stabled horses
  • Avoid endemic areas at peak times — if possible, reduce turnout on high-risk land in spring and autumn when tick activity peaks
  • There is currently no licensed vaccine against equine anaplasmosis in the UK

For guidance on tick control in horses, the British Equine Veterinary Association (BEVA) provides resources and clinical guidance for equine practitioners and owners.

Prognosis

The prognosis for equine anaplasmosis is generally good with prompt diagnosis and appropriate antibiotic treatment. Most horses recover fully within a week of commencing oxytetracycline therapy. Delayed treatment or failure to diagnose the condition can result in more severe disease, prolonged recovery, and rarely, secondary complications. Co-infections with other tick-borne pathogens (such as Borrelia burgdorferi, the agent of Lyme disease) may complicate the clinical picture and require broader diagnostic investigation.

When to Call the Vet

  • Your horse is febrile, depressed, and lethargic — particularly if recently grazing on tick-prone land
  • You notice ventral leg or belly swelling alongside reduced appetite
  • The horse appears uncoordinated or wobbly, especially in the hindlimbs
  • You find a tick attached to your horse and are in an area where tick-borne disease is known to occur
  • Symptoms are not improving within 24–48 hours of apparent onset — prompt antibiotic therapy is time-sensitive

Frequently Asked Questions

Is equine anaplasmosis the same as Lyme disease?

No — they are distinct conditions caused by different organisms, though they share the same tick vector (Ixodes species) in the UK. Equine anaplasmosis is caused by Anaplasma phagocytophilum, which infects white blood cells. Lyme disease is caused by Borrelia burgdorferi, a spirochaetal bacterium. A horse can theoretically be co-infected with both if bitten by a tick carrying multiple pathogens.

How quickly does anaplasmosis respond to treatment?

Response to oxytetracycline is typically rapid and gratifying — many horses show a noticeable improvement in attitude and temperature within 12–24 hours. This rapid response to tetracyclines is itself considered supportive of the diagnosis. Most horses are well within three to five days of treatment, though a full course should always be completed.

Can humans get anaplasmosis from horses?

Humans cannot catch anaplasmosis directly from horses — transmission requires a tick bite. However, ticks in the same environment that have fed on an infected horse reservoir host can bite and infect people. Human granulocytic anaplasmosis (HGA) causes similar signs to the equine disease. If you are working in a tick-prone environment, use tick repellents, check yourself for ticks after being outdoors, and seek medical attention for any unexplained febrile illness.

Which parts of the UK have the highest tick risk for horses?

Scotland, Wales, South West England, and the New Forest are particularly high-risk due to their terrain and deer populations. The Lake District, North Yorkshire Moors, and Dartmoor are also recognised areas of elevated tick activity. However, ticks are now found more widely across the UK than historically, and risk should be assessed locally rather than assumed absent. See our horse terminology guide for more on tick identification and safe removal.