Investigating the epidemiology of heartwater (Cowdria ruminantium infection) by means of a transmission dynamics model.

A mathematical model of the transmission dynamics of Cowdria ruminantium by the ixodid tick Amblyomma hebraeum in the bovine host is developed and used to investigate the epidemiology of heartwater across a range of vector challenge. The processes described are supported by empirical data. The pattern of outcome measures (incidence, case-fatality and proportion of infected hosts) predicted agrees with those described anecdotally from field experience and empirical observation, and demonstrates the concept of endemic stability. The underlying theory is explored and it is shown that endemic stability may be due principally to the protection of calves against disease by either innate or maternally derived factors. The role of vertical infection in the establishment and maintenance of endemic stability is also investigated. Although increasing the vertical infection proportion results in endemic stability occurring at progressively lower levels of tick challenge, the concomitant reduction in incidence and case-fatality predictions across the range of tick challenge means the endemically stable state simultaneously becomes less discernible. Model limitations and future developments are discussed. The essential role of a transmission dynamics model in assessing the impact of new vaccines in conjunction with vector control programmes is highlighted.