Piroplasms (Babesia caballi and Theileria (Babesia) equi)

Babesia and Theileria are the two genera comprising the protozoan Phyllum Piroplasmorida.  They are tick-transmitted and the various species within the genera infect a wide variety of mammalian species, including people, and birds.  Many species of the parasites are pathogenic in these hosts, and several are zoonotic.  To date in Canada, autochthonous (locally acquired) infections have been reported only in wildlife, for example foxes on Prince Edward Island.  Because of the serious health consequences of the species infecting cattle and horses, bovine babesiosis is classified by the CFIA as an “immediately reportable disease”, and equine piroplasmosis as a “reportable disease”, and every effort is made to exclude the parasites from this country.

The basic life cycles of Babesia and Theileria are generally similar, with sexual reproduction in the tick, and asexual reproduction in the mammalian or avian host.  The parasites are transmitted from the tick, during blood-feeding, as sporozoites in the saliva of the tick.  The sporozoites of Theileria initially enter lymphocytes, where they undergo merogony (asexual reproduction), producing merozoites which rupture the host cell as they leave to enter erythrocytes; sporozoites of Babesia enter erythrocytes immediately and begin merogony.  For both species many of the merozoites in the erythrocytes develop into trophozoites which divide by binary fission, becoming ring (single and round) and cross forms (four as a Maltese cross).  These trophozoites disrupt the erythrocytes when they leave, often to infect more erythrocytes.  These repeated cycles of asexual reproduction can come to destroy more and more erythrocytes.  (This process is somewhat similar to what happens with the enteric coccidian, e.g. Eimeria, although different host cells are involved: erythrocytes versus enterocytes).  It is continuing erythrocyte destruction and sludging in small blood vessels by parasitized cells that are the major pathogenic mechanisms for piroplasms. 

Some of the intra-erythrocytic trophozoites develop into gametocytes, and it is these life cycle stages that are transmitted to the tick vectors during blood-feeding. Following ingestion by the tick, in the lumen of the tick gut these gametocytes form gametes which fuse to form a zygote, which, aided by a piercing organelle (ray body or Strahlenkorper), enters the gut epithelial cells and thence across the haemocoel to the secretory cells of the salivary glands.  Here they form multinucleate trophoblasts which give rise to sporozoites which enter the saliva.  As indicated above, infection of the mammalian and avian hosts occurs when the saliva containing sporozoites is introduced by the feeding tick.  Some piroplasms can also invade the ovary of the tick vectors and be transmitted to their offspring.

Babesia caballi and Theileria equi are the two piroplasms infecting horses and donkeys in many parts of the world, particularly the tropics or sub-tropics.  The species of tick vectors involved vary with location, but should the parasites invade Canada, Dermacentor variabilis, D.albipictus, and Rhipicephalus sanguineus could function as vectors for both species.  Clinical signs in horses include icterus, anaemia and fever, but many infected animals show no clinical signs.  Diagnosis is by the examination of erythrocytes in stained blood films, serology (CFT, cELISA and IFA – all of which have problems), and PCR – which is still in the developmental stage for clinical diagnosis.  Effective treatment of B. caballi and T. equi is problematic, particularly if the goal is to clear the parasites from the horses.

Important species of Babesia and Theileria in other hosts elsewhere in the world include B. bovis, B. bigemina (Texas fever), B. divergens (redwater fever), Theileria parva (east coast fever), and T. annulata (Mediterranean coast fever) of cattle, other species in sheep and pigs, and B. canis canis and B. canis gibsoni in dogs.  In addition, in North America and elsewhere, small rodents are infected with Babesia microti, which is transmitted by Ixodes species ticks and is zoonotic.  In the US B. microti occurs primarily in New England and in the upper mid-west.  There have been occasional cases in people in Canada, including a report of transmission by blood transfusion.

Note: With the increasing use of molecular techniques the taxonomy of Babesia and Theileria appears to be becoming more complex, with the use of phrases such as "B. divergens-like" and "B.microti complex".  Hopefully these issues will eventually be resolved.

Traub-Dargatz JL et al. (2010) Equine piroplasmosis. AAEP Proceedings 56: 1-7.

Homer MJ et al. (2000) Babesiosis. Clinical Microbiology Reviews 13: 451-469.

Hunfeld K-P et al. (2008) Babesiosis: recent insights into an ancient disease. International Journal for Parasitology 38: 1219-1237.