Babesia bovis: computer simulation of the relationship between the tick vector, parasite, and bovine host.

Components contributing to the survival of Babesia bovis in its tick vector (Boophilus microplus) and bovine host were identified and their relationship simulated with an interactive BASIC computer program. Estimates for major independent variables (host-finding success, feeding success, filial infection rate, recovery rate) were derived from published laboratory and field data. Values for dependent variables (tick burden, inoculation rate, tick and bovine infection rates) were calculated during successive tick generations until equilibrium conditions were reached. The simulation was used to predict the effects of alternate disease control strategies. Babesiosis outbreaks were associated with simulated daily infestations of approximately two to eight engorged ticks. Below this range, B. bovis parasites disappeared, while above it all cattle were infected during the period of calfhood resistance (through 9 months of age). The babesial inoculation rate was most unstable over the above range due to instability in tick populations and tick infection rates. The risk of babesiosis outbreaks among older, less resistant cattle was also greatest over this range. The optimum daily infestation was eight to nine engorged ticks. This level would maintain herd immunity to babesiosis without causing significant physiologic stress to the bovine host. The difficulty in maintaining ecological and immunological stability at low infestation rates discourages "strategic dipping" as a means of maintaining herd immunity to babesiosis. Introduction of tick-resistant cattle provided a more effective means of reducing the risk of babesiosis outbreaks. The economic implications of computer-simulated babesiosis control alternatives should be analyzed. The validity of simulation parameters should be confirmed by field studies on the relationship between tick burdens and the babesial inoculation rate. These studies would be greatly facilitated by development of an assay system for the detection of babesiae in field-collected ticks.