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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Protista
- Phylum: Protozoa
- Subphylum: Apicomplexa
- Order: Piroplasmorida
- Family: Theileriidae
- Genus: Theileria
Pathogen CharacteristicsTop of page
Shared features of the life cycle and morphology of Theileria species
The life cycle and morphology of Theileria annulata and T. parva in their vertebrate hosts has been reviewed by Mehlhorn et al. (1994). After invading mononuclear cells, the sporozoites develop via trophozoites into multinucleate schizonts. This process is associated with activation of the host cell, which starts to proliferate, and at each cell division the parasite divides in synchrony with the transformed host cells (Hulliger et al., 1964). The infection is disseminated throughout the lymphoid tissues, by clonal expansion and metastasis of the schizont-infected cells. The schizonts then differentiate into merozoites. The mechanisms responsible for differentiation have been reviewed by Shiels et al. (1999, 2000). Released from the host cells, the merozoites enter the erythrocytes and, as intra-erythrocytic piroplasms, become available to ticks. The ultrastructure of the life cycle in their vertebrate hosts has been described and reviewed (Schein et al., 1978; Jura et al., 1983a, b; Mehlhorn and Schein, 1984; Shaw et al., 1991; Mehlhorn et al., 1994). Development and sexual recombination in ticks has been described for T. annulata (Schein et al., 1975; Voreb'eva, 1992) and for T. parva (Fawcett et al., 1982a, b).
Genetic diversity and relationships of Theileria species
The taxonomic relationships of the members of the order Piroplasmida have been controversial ever since they were discovered (Neitz, 1957; Markov, 1962; Krylov, 1978; Uilenberg, 1981; Norval et al., 1992). The species were first defined according to their morphology, hosts, tick vectors, distribution, antigenic relationships and ability/inability to cross-protect against other organisms. Molecular tools are now being used to confirm their identities and distributions, and to characterize their biological properties and host-parasite interactions (Allsopp et al., 1993; Morzaria et al., 1999; Sparagano, 1999; Sparagano and Jongejan, 1999). Molecular systematics have identified two monophyletic groups of Theileria: one group includes T. annulata, T. parva and T. lestoquardi [T. hirci]; the other group includes the members of the ‘T. orientalis/T. buffeli/T. sergenti' group and a new pathogen of small ruminants from China (Schnittger et al., 2000).
T. annulata has proved to be genetically diverse, both within regions and across its endemic range (Melrose et al., 1984; Ben Miled et al., 1994; Katzer et al., 1998; Gubbels et al., 2000a), as have the different stocks of T. parva isolated from cattle and buffalo (Conrad et al., 1987, 1989; Allsopp et al., 1993). However, the different stocks of T. annulata are generally accepted to represent one species, and sub-speciation of members of the T. parva complex into T. parva parva, T. p. bovis and T. p. lawrencei is also not justified (Anon, 1989; Allsopp et al., 1993; Lawrence et al., 1994).
The widespread members of the T. orientalis/T. buffeli/T. sergenti group are also genetically diverse (Matsuba et al., 1992; Chae et al., 1998; Kim et al., 1998; Chansiri et al., 1999; Kawazu et al., 1999; Gubbels, 2000b) and their relationships difficult to resolve. Many cattle are infected with mixed populations of geographically variable parasites and, as well as those parasites already described as T. orientalis, T. sergenti and T. buffeli, other as yet undefined species probably exist in East Asia (Kim et al., 1998). T. sergenti is an invalid name according to Uilenberg (1981). Some authors propose retaining the name T. buffeli for parasites of the Asian buffalo, until the two species are shown to be identical. They suggest using the name T. orientalis for cattle parasites and creating two subspecies to distinguish the Japanese, Ikeda stock (T. o. sergenti) from all other members of the species (T. o. orientalis) (Kawazu et al., 1999). Others would prefer to call all members of this group T. buffeli, until individual 'species' can be defined, as they may all have originated from a group of buffalo parasites, and the name T. buffeli takes precedence over T. orientalis (Stewart et al., 1996; Gubbels et al., 2000a). In contrast to both these views, OIE (2013) says that there are two species: T. sergenti (occuring in the Far East) and T. buffeli/T. orientalis (with a global distribution).
A number of other species exist, but most are not major pathogens.
For information on geographical distribution, see the datasheet on the disease theileriosis.
See the datasheets on Theileria annulata infections, Theileria orientalis/Theileria buffeli infections, Theileria parva infections, bovine theilerioses, and some of those on individual Theileria species.
Host AnimalsTop of page
Vectors and Intermediate HostsTop of page
|Haemaphysalis longicornis||Tick||New Zealand|
|Hyalomma scupense||Tick||Southern Russia|
|Rhipicephalus duttoni||Tick||Congo Democratic Republic|
ReferencesTop of page
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Anon., 1989. Classification of Theileria parva reactions in cattle. In: Dolan TT, ed. Theileriosis in Eastern, Central and Southern Africa. Proceedings of a workshop on East Coast Fever Immunization. Lilongwe, Malawi. 20-22 September 1988. Nairobi, Kenya: International Laboratory for Research on Animal Diseases, 187-188.
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