EFFECTS OF IMMUNIZATION AGAINST THEILERIA-PARVA ON BEEF-CATTLE PRODUCTIVITY AND ECONOMICS OF CONTROL OPTIONS

Over 500 cattle of all age groups on 2 farms in Zimbabwe were immunise d against theileriosis using the ''infection and treatment'' method an d disease prevalence and their productivity assessed during a period o f 18 months. The immunising stock, Theileria parva (Boleni) was isolat ed in Zimbabwe. None of the immunised cattle suffered from theileriosi s upon natural exposure whereas 22 unimmunised cattle died of theileri osis and a further 48 required treatment for theileriosis. In the firs t year, some immunised cattle were maintained with minimal threshold d ipping (once or twice during the rainy season). During periods of very high tick challenge of 100 to 1,000 Rhipicephalus appendiculatus per animal from January to March, a transient decrease in liveweight gain was observed particularly in cows. However, by the end of the period o f observation, the weights had recovered so that intensively dipped an d immunised and threshold or strategically dipped groups of cattle sho wed no significant differences. From the results it was estimated that each engorging female R. appendiculatus caused a temporary depression in weight gain of 8 grams. In young stock the weight loss was exacerb ated by the presence of screw worm (Chrysomya bezziana) infestation. I t was then possible to define an economically attractive integrated ti ck and theileriosis control strategy based on these findings, whereby immunised cattle were dipped 6 times between mid-December and mid-Marc h. In this regimen, no weight loss occurred and no cases of screw worm were observed. For each of 3 herd sizes of 250, 500 and 1,000 cattle, comparisons were made of the costs of 4 different control options: (i ) Intensive dipping (40 times/year) (ii) Intensive pour-on acaricide t reatments (18 times/year) (iii) Theileria immunisation with strategic dipping (6 times/year) (iv) Theileria immunisation with pour-on treatm ent (4 times/year) It is concluded from these studies that, on farms w here theileriosis is a serious problem, immunisation coupled with a st rategic dipping programme is economically very attractive. In the year in which immunisation is carried out, costs will be higher than for i ntensive dipping, but from the second year on, the costs are decreased to approximately 50% of those for intensive dipping.