THEILERIA PARVA - KINETICS OF REPLICATION

The kinetics of replication of Theileria parva (Muguga) in the bovine host were studied using infections produced by 101, 102, and 103 ticks. It was found that the growth of the organism was exponential from at least days 11-21 and probably from the start of growth in the bovine host. The growth rate was one ten-fold increase in macroschizont numbers every three days. Extrapolation backwards of the growth curves gave results for input of infective units compatible with those computed directly from counts on tick salivary glands. The major exponential phase of replication was carried out by small macroschizonts with 1-6 nuclei. Later, schizonts with up to 2-4 nuclei appeared and their rise in number was followed sequentially by microschizonts and piroplasms. The appearance of large nuclear-numbered macroschizonts and piroplasms was timedependent and not dose-dependent, suggesting that the switch to the maturation phase, from the exponential division phase or phases, occurs after a fixed number of divisions. This quantitative description of the life cycle is discussed in relation to the pathogenesis, immunology, and epidemiology of the disease. The relevance of different growth rates to the taxonomy and behavior of different strains or so-called species" of Theileria is pointed out. © 1969."