Bit & Bridle

Equine Piroplasmosis (Babesiosis)

Other Names

  • Equine Babesiosis
  • Equine Theileriosis
  • Equine Biliary Fever
  • Horse Tick Fever
  • Equine Malaria

What is Equine Piroplasmosis

Equine piroplasmosis is a tick-borne protozoal disease caused by two intraerythrocytic (red blood cell-invading) parasites: Babesia caballi and Theileria equi (formerly classified as Babesia equi). Both organisms are transmitted by ticks and cause destruction of red blood cells, resulting in haemolytic anaemia of varying severity. The disease affects horses, donkeys, mules, and zebras worldwide.

Equine piroplasmosis is endemic across large areas of Africa, Asia, Central and South America, Southern and Eastern Europe, and the Middle East. It is a notifiable disease in many countries, including the United Kingdom and the United States, where it is not considered endemic. The UK remains officially free from piroplasmosis, and any suspected case must be reported to the Animal and Plant Health Agency (APHA) under the Equine Disease Notification (EDN) system.

Both B. caballi and T. equi can produce chronic carrier states in horses — animals that have recovered from acute infection but remain persistently infected and capable of infecting feeding ticks and spreading disease. This chronic carrier state makes eradication and international trade regulation particularly challenging.

Causes of Equine Piroplasmosis

Transmission occurs primarily through ticks, though other routes are possible:

  • Tick transmission: Multiple species of hard ticks (Ixodidae) transmit piroplasmosis, including Dermacentor, Rhipicephalus, Hyalomma, and Amblyomma species. B. caballi is transmitted transstadially (between tick life stages), while T. equi is also transmitted transovarially (from an infected adult female tick to her offspring), meaning infection can persist through tick generations without a horse host.
  • Iatrogenic transmission: Transmission via contaminated blood — through shared needles, syringes, blood transfusions, or contaminated surgical instruments — is a significant risk in endemic areas.
  • Vertical (transplacental) transmission: T. equi can be transmitted from an infected dam to her foal in utero, resulting in neonatal piroplasmosis.
  • Predilection areas: Horses in tick-endemic environments — particularly where horses graze in scrub, moorland, or woodland — are at highest risk during peak tick activity seasons.

Symptoms and Signs

Clinical signs vary by infecting organism, parasite burden, and the immune status of the horse. T. equi infections tend to be more severe than B. caballi:

Peracute/acute piroplasmosis:

  • High fever (39–42°C)
  • Severe depression and weakness
  • Tachycardia and tachypnoea
  • Pale or icteric (yellow) mucous membranes due to haemolytic anaemia
  • Dark, red-brown, or coffee-coloured urine (haemoglobinuria)
  • Haemoglobinuria-associated kidney damage
  • Petechiae (pinpoint haemorrhages) on mucous membranes
  • Rapid deterioration and potentially death without treatment

Subacute piroplasmosis:

  • Intermittent fever
  • Mild to moderate anaemia
  • Reduced performance and exercise intolerance
  • Variable appetite, weight loss
  • Mild jaundice

Chronic piroplasmosis (carrier state):

  • No obvious clinical signs in many horses
  • Subclinical anaemia detectable on haematology
  • Vague poor performance or recurrent low-grade illness under stress

Diagnosis

In the UK and other non-endemic countries, laboratory confirmation is essential, as clinical signs overlap with many other febrile illnesses. Notify your official vet immediately if piroplasmosis is suspected — it is a notifiable disease:

  • Blood smear (Giemsa stain): Examination of a thin blood smear allows direct visualisation of intraerythrocytic piroplasms. Sensitivity is higher during acute phases when parasitaemia is at its peak. The pear-shaped paired piroplasms of B. caballi and the “Maltese cross” tetrad form of T. equi are characteristic findings.
  • PCR testing: Polymerase chain reaction is the most sensitive method for detecting piroplasm DNA in blood, capable of identifying both B. caballi and T. equi, including in chronic low-level carriers where blood smear sensitivity is poor.
  • Serology: The competitive enzyme-linked immunosorbent assay (cELISA) is widely used for surveillance and export/import testing, detecting antibodies to both parasites. Positive serology may reflect past or current infection. Paired samples 2–4 weeks apart can confirm active disease.
  • Indirect fluorescent antibody test (IFAT): An alternative serological test for antibody detection.
  • Haematology and biochemistry: Anaemia (often regenerative), thrombocytopenia, elevated bilirubin, haemoglobinuria, elevated urea and creatinine (renal involvement), and elevated liver enzymes may all be present in acute cases.

Treatment

Treatment should be initiated promptly in acute cases and carried out under veterinary supervision. In the UK, all confirmed cases must be reported to the APHA:

  • Imidocarb dipropionate: The drug of choice for both B. caballi and T. equi. Given by intramuscular injection, repeated according to the infecting species and clinical response. B. caballi infections can be cleared with treatment, but T. equi carriers may not be fully sterilised even after repeated courses. Adverse effects include sweating, salivation, colic-like signs, and in overdose, cholinesterase inhibition — atropine may be needed to manage these reactions.
  • Buparvaquone: Licensed for treatment of theileriosis in some countries; may be used in T. equi infections with specialist veterinary guidance.
  • Supportive care: Intravenous fluids to maintain hydration and support kidney function; blood transfusion for severe anaemia; NSAIDs for fever and systemic inflammation; nutritional support.
  • Rest: Affected horses should be rested for a minimum of 2–3 weeks after clinical recovery.

Prevention

In the UK and other non-endemic countries, prevention focuses on preventing importation and managing tick exposure:

  • Test horses for piroplasmosis before importation or export — many countries require negative piroplasmosis serology as a condition of entry.
  • Implement rigorous tick control — regular tick checks (particularly after exercise in tick-infested habitats), use of licensed acaricidal products, and avoidance of tick-endemic grazing where possible.
  • Do not share needles, syringes, or blood-contaminated equipment between horses.
  • Quarantine and test new arrivals, especially those from endemic regions.
  • Report any suspected cases immediately to your vet and APHA (in the UK).

Prognosis

Prognosis depends on the infecting organism and the severity of illness at presentation. B. caballi infections treated promptly carry a good prognosis, and horses can be fully cleared of infection with treatment. T. equi infections carry a more guarded prognosis: acute cases can be fatal without treatment, and even successfully treated horses may remain lifelong carriers, limiting their international movement. Horses that have recovered from acute piroplasmosis generally return to normal work, though some may take weeks to months to regain full fitness. In countries where piroplasmosis is notifiable, carrier horses may face significant restrictions on movement and competition.

When to Call the Vet

Contact your vet and official vet (APHA in the UK) immediately if your horse:

  • Develops fever, jaundice, or dark discoloured urine.
  • Has recently arrived from or visited a tick-endemic region.
  • Shows unexplained acute haemolytic anaemia.
  • Has been found to have ticks — particularly if the horse has travelled internationally.

In the UK, suspected piroplasmosis is a notifiable disease — failure to report is a legal offence. For further guidance see the APHA guidance on equine piroplasmosis and BEVA. For related tick-borne conditions see our guide on equine leptospirosis.

Frequently Asked Questions

Is equine piroplasmosis present in the UK?

The UK is considered officially free from equine piroplasmosis. Cases have been reported in horses imported from endemic regions or following travel abroad. Any suspected case must be reported to the Animal and Plant Health Agency (APHA) immediately. Routine surveillance and testing of imported horses are key to maintaining the UK’s disease-free status.

Can a recovered horse still spread piroplasmosis?

Yes. Horses that recover from Theileria equi infection remain lifelong carriers and can infect feeding ticks, which can then transmit the infection to other horses. Babesia caballi carriers may be cleared with repeated imidocarb treatment, but T. equi carriers cannot reliably be sterilised. This has significant implications for international movement, as carrier horses are subject to restrictions in many countries.

Which ticks transmit piroplasmosis in Europe?

In Europe, Dermacentor reticulatus (the ornate dog tick) is the principal vector for B. caballi. Hyalomma and Rhipicephalus species are important vectors in southern Europe and the Mediterranean region. Tick distribution is expanding northward in Europe due to climate change, potentially extending the geographic range of tick-borne diseases. Horse owners in all regions should be vigilant about tick control, particularly after travelling to high-risk areas.

How does equine piroplasmosis affect competition horses?

Competition horses that test positive for piroplasmosis face significant restrictions. Many countries require negative serological testing for piroplasmosis as a condition of entry, meaning carrier horses may be ineligible to compete internationally. In countries where the disease is notifiable, positive horses may be subject to movement restrictions and quarantine measures. Riders and owners should be aware of testing requirements before planning international competition travel, and should include piroplasmosis in routine pre-export health screening.