Equis ISSN 2398-2977


Synonym(s): Equine piroplasmosis, equine biliary fever

Contributor(s): Alex Morrow, Carla Sommardahl


  • An acute, subacute or chronic tick-borne disease of equidae.
  • Cause: intra-erythrocytic protozoa Theileria equi (originally named Babesia equi) and Babesia caballi Babesia spp.
  • Signs: fever, anemia, jaundice, bilirubinuria and in later stages hemoglobinuria.
  • Occurrence can be seasonal, related to tick activity.
  • Susceptible adult animals imported into endemic areas at greatest risk.
  • Diagnosis: history, signs, hematology, serology.
  • Treatment: imidocarb dipropionate, diminazene aceturate, amicarbalide or parvaquone.
  • Sterilization of infection may be difficult.
  • Prognosis: guarded.
Print off the Owner factsheet on Babesiosis (pirioplasmosis) Babesiosis (pirioplasmosis) to give to your clients.



  • B. equi was reclassified asT. equibecause the organism more closely resembles members of the genusTheileriawith its exoerythrocytic (lymphocytic) stages within the vertebrate host with the development of microschizonts and macroschizonts.
  • Two species of Theileria involved:
    • T. equi (a 'small' Babesia).
    • Babesia caballi (a 'large' Babesia).
  • B. caballi is less pathogenic, infections often being clinically inapparent.
  • Principal vectors of both species are Dermacentor (Anocentor), Hyalomma and Rhipicaephalus spp and possibly Haemaphysais and Boophilus spp of ticks.

Predisposing factors


  • Age and immune status of the horse.
  • Virulence of the Babesia spp - T. equi causes more severe/acute disease than B. caballi.
  • Stress including strenuous exercise - premunity is easily broken by stress leading to chronic infection.
  • Concurrent infections.
  • Administration of steroids.


  • Infestation with infected ticks.
  • At least 14 species of ticks of the genera Dermacentor, Hyalomma and Rhipicephalus are responsible for transmission


  • Babesiosis is a tick-transmitted protozoal infection in which parasitized erythrocytes rupture and the inflammatory response results in damage to vascular endothelium.
  • The varied clinical manifestations relate to the occurrence of hemolytic disease.
  • Ticks acquire the piroplasms when they ingest blood from an infected horse.
  • Babesia reproduce sexually and asexually in the tick and depending on the species of Babesia, can be passed transovarially and transtadially.
  • Sporozoites, the infective stage, form in the salivary gland and are transmitted during feeding.
  • T. equi has an exoerythrocytic cycle in lymphoid mononuclear cells.
  • Parasitized erythrocytes are ruptured by the multiplying parasites or are removed by phagocytosis.
  • The released parasites invade other erythrocytes.
  • Intravascular and extravascular haemolysis and erythrocyte phagocytosis result in icterus, bilirubinuria, and hemoglobinuria while the inflammatory response contributes to endothelial damage.
  • The hemolytic crises seen in acute infections results in a regenerative anemia and hypoxia.
  • Cerebral signs, although rare, can occur inB. caballiinfections - probably as a result of sludging of parasitized erythrocytes in small blood vessels.


  • Incubation period after tick inoculation of T. equi varies from 12-19 days and for B. caballi from 10-30 days.


  • T. equ iis transmitted, transtadially only, by 9 tick species.
  • B. caballi is transmitted both transtadially, by 4 tick species and transovarially by 9 tick species.
  • Both T. equi and B. caballi can be transmitted mechanically - but little evidence for the role of this in the field.
  • Infection has been transmitted by contaminated injection needles.
  • Babesiosis affects horses, mules, donkeys and other equines including Burchells Zebras, which can act as carriers.
  • Recovered carrier animals remain a source of infection to the tick population. T. equi infected animals are probably carriers for life while horses may remain carriers of B. caballi for up to 4 years.
  • Occurrence of the disease may be seasonal, relating to periods of increased tick activity.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • ​Tamzali Y (2013) Equine piroplasmosis: an updated review. Equine Vet Educ 25 (11), 590-598 VetMedResource.
  • Mehlhorn H & Schein E (1998) Redescription of Babesia equi Lavern 1901 as Theileria equi. Parasitol Res 84 (6), 467-475 PubMed.
  • Bruning A (1996) Equine piroplasmosis an update on diagnosis, treatment and prevention. Br Vet J 152, 139-151 (review of diagnosis, treatment and control) PubMed.
  • de Waal D T (1992) Equine piroplasmosis: a review. Br Vet J 148 (1), 6-14 PubMed.