Subhash P Morzaria, International Livestock Research Institute, Nairobi, Kenya

Theileria species are tick-transmitted intracellular protozoan parasites of domestic ruminants and wild Bovidae and the diseases they cause are collectively referred to as theileriosis. Of the several Theileria species infecting domestic ruminants (Table 1), the two most economically important are T. parva and T. annulata. There are approximately 250 million cattle at risk from these two parasites, which together are responsible for inhibiting livestock development in many parts of the world (Figure 1). The distribution of T. parva is limited to eastern, central and southern Africa where it is predominantly transmitted by the tick Rhipicephalus appendiculatus. Theileria annulata, transmitted by several species of Hyalomma ticks, is more widely distributed, extending from northern Sudan and the Mediterranean countries to the Middle East, India, southern Asia and China.

Theileriosis is mainly controlled by the application of acaricides to domestic ruminants. Apart from being expensive, these chemicals are toxic and hazardous to humans and the environment. Ticks also

Table 1 Important Theileria species of domestic ruminants


Main hosts

Vector genus Disease names

T. parva

T. annulata

T. mutans T. taurotragi

T. velifera

T. sergenti/orientalis/buffeii complex T. hirci T. ovis T. separata

Cattle, African buffalo3 Rhipicephalus

Cattle, water buffalo Hyalomma

Cattle, African buffalo3 Cattle, African eland3

Cattle, African buffalo


Sheep, goats Sheep, goats Sheep

Amblyomma Rhipicephalus

Amblyomma Haemaphysalis



East Coast fever Corridor disease January disease Mediterranean/tropical theilerlosis Benign theileriosis Benign theileriosis Tzaneen disease

Malignant theileriosis



Africa, Asia, Europe

Africa Africa


Asia, Europe, Africa,

Australia Europe, Asia, Africa Africa Africa

rapidly develop resistance to acaricides. Therefore, alternative methods of controlling theileriosis are sought, mainly focused towards the development of vaccines.

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