Resistance to Boophilus microplus (Canestrini) in Genetically Different Types of Calves in Early Life

Tick resistance and blood composition were studied in British (1/2 Shorthorn X 1/ Hereford) and zebu (? Brahman X ? British) calves from birth to 33 days of age in a tropical grazing area in which B. microplus is endemic. Calves of the 2 breeds were either naturally infested or were, in addition, artificially infested with 5,000 larvae at 2 and 9 days of age. Total numbers of mature female ticks carried from either type of infestation were significantly lower (P < 0.01) on zebu than on British calves. In the artificially infested calves of both breeds, the total number of ticks maturing between 20 to 26 days of age was significantly higher (P < 0.01) than the number maturing between 27 to 33 days of age emphasizing that a major component of resistance is acquired. However, in the naturally infested calves, breed differences in the numbers of ticks maturing during these 2 periods suggested the presence of genetic differences in innate resistance. Within breeds, total tick numbers carried during the study were negatively correlated with calf weight gain and with the concentrations of serum albumin, total protein, and cholesterol. It is now well established that there are marked differences between zebu and British breeds of cattle in their resistance to the cattle tick, Boophilus microplus (Canestrini), and also that within breeds there are wide animal variations in susceptibility to tick infestation (see, e.g., Seifert, 1971). While much work has been reported on the nature of resistance in terms of the number of mature ticks carried by individual cattle and also on its inheritance (Wharton et al., 1970), the actual mechanism of resistance remains largely undetermined. The presence of innate resistance was suggested by Riek (1962) but other workers have demonstrated that a major component of resistance is acquired and therefore assumed to be immunological (Roberts, 1968; Hewetson, 1971). All published work on tick resistance in cattle relates to the adult animal. A study of resistance and blood composition in the young animal may contribute to a more complete knowledge of resistance mechanisms and of host-parasite relationships. This paper reports the response to tick infestation of British (/2 Shorthorn X ' Hereford) and zebu (/2 Brahman X /2 British) calves from birth to 33 days of age and also changes in blood composition during this period. MATERIALS AND METHODS