Determination of duration of immunity of calves vaccinated with the Theileria annulata schizont cell culture vaccine

Bovine tropical theileriosis caused by Theileria annulata is a serious haem oprotozoan disease of cattle affecting exotic cattle, their crossbreeds and young indigenous calves. Cell culture vaccines have been developed and use d effectively in various countries for the control of this disease. However , the duration of immunity provided by these vaccines is poorly understood. The present experiments were planned to study the duration of immunity in animals after vaccination with the T. annulata (Hisar) schizont cell cultur e vaccine. Two groups of calves were vaccinated and challenged after a peri od of 3 and 6 months, respectively. There was no fever in any of the vaccin ated calves after challenge. However, the vaccinated animals exhibited mild to moderate enlargement of lymph nodes and parasitological reactions. The parasitological reactions were very mild in calves challenged after 3 month s and moderate in calves challenged after 6 months. There was a mild but si gnificant decrease in the haematological values of calves after challenge. A significant rise in the anti-theilerial antibody titres was observed in a ll calves after vaccination, which increased further, by many folds after c hallenge. On the other hand, all the challenge control calves showed sympto ms of acute theileriosis and died. The observations suggested that the T. a nnulata (Hisar) schizont cell culture vaccine provided immunity in vaccinat ed animals for at least 6 months in the absence of field tick challenge. Ho wever, there was some decline in immunity after 6 months, if the animals ar e not exposed to ticks during this period. (C) 2000 Elsevier Science B.V. A il rights reserved.