The serological response to heartwater immunization in cattle is an indicator of protective immunity.

A significant correlation was demonstrated in Friesian-cross steers between the serological response to previous vaccination with the Ball 3 strain of Cowdria ruminantium and the development of protective immunity against the Kalota isolate from Malawi. Of 10 animals which seroconverted after vaccination, all were completely or partially immune to challenge. Ten of the 14 animals which failed to seroconvert were immune but the proportion was not significantly different from that in the unvaccinated controls (4/10). Of 29 animals vaccinated and treated simultaneously with a slow-release doxycycline implant, 13 failed to seroconvert, and of these, four were completely susceptible to challenge.