Babesiosis (Babesia bovis) stability in unstable environments

Enzootic stability (herd immunity) in bovine babesiosis occurs when the rate of transmission (inoculation rate) of Babesia spp by the tick vector is sufficient to immunize a majority of susceptible calves before the loss of calfhood resistance. The effect of three tick (Boophilus microplus) control strategies (none, threshold, and strategic) on enzootic stability and the likelihood of babesiosis (Babesia bovis) outbreaks was studied using a spreadsheet age-class computer simulation model. The model was driven by weekly bovine tick counts from Brazil and Uruguay. The Eldorado do Sul, RS, Brazil bovine population (30 degrees 05' South latitude) was found to be in a naturally occurring state of enzootic stability, corresponding to an inoculation rate exceeding 0.005 throughout the year. Threshold dipping strategies should not increase the risk of babesiosis in cattle so managed. Strategic dipping resulted in an extended period of enzootic instability lasting 30 weeks, which requires protection of the herd through immunization. Because of the more prolonged low winter temperature conditions, the Tacuarembó, Uruguay bovine population (31 degrees 40' South latitude) was found to be in a naturally occurring state of enzootic instability, characterized by a 28 week period in which the inoculation rate was below 0.005. Strategic dipping should lead to eradication of the babesial parasite from tick and bovine populations, but would not result in eradication of the tick vector. This could lead to subsequent outbreaks if Babesia carrier animals were to be introduced into the herd. In both populations, strategic tick control could be accompanied by concurrent babesiosis vaccination.