PROSPECTS FOR SUBUNIT VACCINES AGAINST TICK-BORNE DISEASES

Tick-borne parasites are a serious impediment to the improvement of li vestock production in the developing world. The major parasites affect ing cattle include Theileria parva, T. annulata, Babesia bigemina, B. bovis, Anaplasma marginale and Cowdria ruminantium. The control of the se infections is dependent on the use of acaricides to decrease transm ission by the tick vectors, and immunization of susceptible animals wi th 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 s urface antigen of the sporozoite (p67). A similar antigen, SPAG 1, has been identified as a candidate for T. annulata. Although several cand idate antigens have been identified for Babesia spp., progress towards development of a subunit vaccine based on these antigens has been ham pered 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 ou ter membrane proteins; immunization with a variety of these antigens a lone or in combination, has yielded promising results. As with Babesia , further definition of immune effector mechanisms is needed to optimi ze immunization strategies. The work on identifying the protective ant igens of C. ruminantium is in its embryonic stages; however, two antig ens have been identified and are currently being evaluated. There is h igh expectancy for subunit vaccines for all these diseases; however th ere is need for further work to elucidate the immune mechanisms in ord er to select appropriate antigen delivery systems.