Small ruminant theileriosis
Theileria parasites infect a large number of wild and domestic animals and are transmitted trans-stadially by various members of tick vectors of the family Ixodidae. Generally, the life-cycle of Theileria involves both the transmitting invertebrate tick vector, in which sexual reproduction and sporogony takes place, and the vertebrate host, in which asexual reproduction by schizogony and merogony occurs [1,2,3]. A number of Theileria species capable of infecting small ruminants have so far been described. Many of them are non-pathogenic such as Theileria ovis, T. separata , and T. recondita. There are three other species which are highly pathogenic for small ruminants:
In the case of T. lestoquardi, attenuated live vaccines based on inoculation of schizont-infected leucocytes have been utilised for the control of malignant theileriosis only in Iran and Iraq. It is envisioned that improvement and distribution of a live attenuated vaccine will contribute to the control of this important small ruminant disease.
Infection with the related bovine parasites, Theileria annulata and T. parva, is associated with the establishment of a solid immunity. It is well known that both antibody-dependent and antibody-independent mechanisms are involved. Antibodies are able to neutralise the infectivity of the sporozoites but on the other hand do not prevent initiation of an infection. However, T-cells play a crucial role in induction and maintenance of this immunity. Based on the cytokine profile and lytic effect of the T-cells, it seems that both CD4+/- and CD8+/-T cells are involved in the mediation of such immunity. Cytotoxic- and helper-T-cells recognise parasite-antigens, which are presented by the infected cells via MHC I and MHC II, respectively. To date, CD8+ T cells (cytotoxic T lymphocytes, CTL) are known to be the major anti-Theileria effectors and are activated in both T. annulata and T. parva infections. They offer immunity by directly lysing schizont-infected cells and their generation is closely related to the control of the infection. The innate immune response is also believed to play a role in protecting against T. annulata infection, involving the activation of cytokine-producing macrophages. However, little is known about the mechanisms of the immune response to T. lestoquardi or to T. uilenbergi infection. Experience gained from defining the response to bovine Theileria will be useful for addressing this knowledge gap in small ruminants. Moreover, understanding the immune response of small ruminants to Theileria may help in understand their immune responses to other pathogens [10,11,12,13,14,15,16,17,18,19,20].