THEILERIA-PARVA - KINETICS OF INFECTION IN THE LYMPHOID SYSTEM OF CATTLE

The kinetics of infection with T. parva in cattle were studied by examining the total cellularity and numbers of parasites in a range of lymphoid organs from animals killed at intervals during the course of infection. With the dose of T. parva stabilate used, macroschizonts were initially detected in the drainage lymph node about 7 days after inoculation and death of the host resulted on day 18-19. Associated with the initial detection of parasites, there was a marked increase in cellularity of the drainage lymph node and a more gradual and less pronounced increase in cellularity of the other lymphoid organs. From about day 12 onward, there was a gradual decrease in the cellularity in all of the lymphoid organs, so that in animals examined in the terminal stages of the infection there was often cellular depletion. The pattern of these cellular changes was similar in groups of Boran and Friesian cattle, although increase in cellularity and terminal depletion were more marked in the Friesians. Blood leukocyte counts in infected Boran started to drop as early as day 7 of infection and by day 14 had reached values < 25% of normal. Quantitation of parasitic schizonts indicated that the numbers of parasites in the lymphoid organs do not increase in a simple exponential manner. Rather, there appears to be an early rapid increase in parasite numbers followed by a phase of less rapid multiplication. Because of the marked changes which occurred in total cellularity of the lymphoid organs during the course of the infection, a significant discrepancy was found between the replication rate of the parasite as calculated using total numbers of parasites and that obtained using schizont index (SI). Results indicated that the use of SI, as described in previous studies, is not a reliable method of determining the replication rate of the parasite.