Immunization against diseases caused by Theileria parva: a review.

Theileria parva is the causative agent of three epidemiologically different diseases, East Coast fever (ECF), Corridor disease and January disease, caused by 3 types of T. parva, T. p. parva, T. p. lawrencei and T. p. bovis, respectively. The history of immunization against these diseases has been marked by salient discoveries such as the immune status in recovered animals, the activity of tetracyclines during the incubation period, the possibility for cryopreserving supernatant of prefed ticks and the development of useful serological tests. The possibility of simultaneous administration of stabilate and long-acting tetracycline have greatly contributed to making the infection and treatment method operational. The importance of antigenic diversity in T. parva has been reflected in the difficulties related to the selection of the immunizing stock or combinations of stocks: a 'cocktail' of East African isolates may give broad protection against field challenge by ECF (T. parva parva), but Corridor disease is more problematic. On the other hand, certain single isolates may give equally good protection against ECF field challenge. Studies on the immunology of T. parva infection and the application of molecular tools have led to the discovery that sera of recovered animals neutralize sporozoites of various isolates, and to the p67 molecular vaccine; yet so far the only available method of immunizing against T. parva infections is the infection and treatment method or, in the case of T. parva bovis, the use of sublethal stabilate doses. Infection and treatment is applied on a fairly large scale in Zambia, and on a more limited scale in a few other countries. Immunity by this rather crude method is long-lasting and solid, but cross-immunity problems against some field strains remain. Furthermore, as immunized animals remain carriers, immunization may contribute to attaining and improving endemic stability in endemic areas in indigenous breeds with an adequate level of genetic tolerance to ECF. On the other hand, carrier animals may constitute a risk for spreading the disease into ECF-free areas where the vector is present. Other disadvantages of the method are that immunization of cattle during the incubation of naturally contracted East Coast fever will not prevent the disease and jeopardize its reputation. Furthermore, stabilates have to be cryopreserved, often a technical drawback, and contamination with undesirable pathogens may occur in tick-derived material. Therefore the need remains for the development of effective molecular vaccines and it must be remembered that immunization must be cost-effective and sustainable and it is only one aspect of integrated control of theileriosis and other tick-borne diseases. There is no universally valid strategy. Several factors have to be considered: value and susceptibility of cattle to theileriosis and to other tick-borne and tick-associated diseases, infestation by various ticks present in the area, the type of theileriosis (ECF, Corridor disease or January disease) and the epidemiological situation where immunization is taking place. The optimal age for immunization of the calves in endemic areas needs to be determined: when calf mortality by naturally occurring theileriosis is a problem, the sooner calves are immunized the better, but a proportion will have contracted natural infection before they can be reached, and immunization of very young calves might not be fully effective.