Tick-bite fever in Zimbabwe. Survey of antibodies to Rickettsia conorii in man and dogs, and of rickettsia-like organisms in dog ticks.

As part of a study of tick-bite fever in Zimbabwe, the prevalence of antibodies to Rickettsia conorii in dogs and humans was examined. Blood samples were obtained from 184 dogs in different parts of the country and tested by indirect immunofluorescence. In all, 150 (82%) were positive at a titre of 1/40 or higher. Dogs from the south and east of the country showed very high seroprevalence compared with dogs from the main urban centre, Harare. Human serum was obtained from blood donors, from people attending clinics with non-febrile illness and from agricultural workers. Antibodies to R. conorii were found in a high proportion of samples from the south and east, with 100% of blood donor samples in one area being positive. As with the dogs, a much lower proportion of samples from Harare were positive. The exact role of dogs and their ticks (Rhipicephalus sanguineus, R. simus and Haemaphysalis leachi) in the epidemiology of human tick-bite fever remains unclear, since dog ticks were seldom found to harbour rickettsia-like organisms and man is not one of their preferred hosts. While dogs may be of little importance in disease transmission, their value as sentinels of spotted-fever group rickettsias in the environment was confirmed.