Prospects for subunit vaccines against tick-borne diseases.

Tick-borne parasites are a serious impediment to the improvement of live-stock production in the developing world. The major parasites affecting cattle include Theileria parva, T. annulata, Babesia bigemina, B. bovis, Anaplasma marginale and Cowdria ruminantium. The control of these infections is dependent on the use of acaricides to decrease transmission by the tick vectors, and immunization of susceptible animals with live vaccines. The use of acaricide is hampered by the development of resistance, and live vaccines require cold chain facilities, which are generally unreliable in developing countries. There is therefore a need for improved vaccines that can circumvent these problems. There is a subunit vaccine being developed for T. parva based on the major surface antigen of the sporozoite (p67). A similar antigen, SPAG 1, has been identified as a candidate for T. annulata. Although several candidate antigens have been identified for Babesia spp., progress towards development of a subunit vaccine based on these antigens has been hampered by polymorphism among isolates and between species, and lack of knowledge of the immune effector mechanisms responsible for protection. The search for protective antigens of A. marginale has focused on outer membrane proteins; immunization with a variety of these antigens alone or in combination, has yielded promising results. As with Babesia, further definition of immune effector mechanisms is needed to optimize immunization strategies. The work on identifying the protective antigens of C. ruminantium is in its embryonic stages; however, two antigens have been identified and are currently being evaluated. There is high expectancy for subunit vaccines for all these diseases; however there is need for further work to elucidate the immune mechanisms in order to select appropriate antigen delivery systems.