A new pathogenic spotted fever group rickettsia from Africa

A spotted fever group (SFG) rickettsia was isolated in Zimbabwe from a patient with tick-bite, fever, headache and regional lymphadenopathy. A further six isolates were obtained from Amblyomma hebraeum ticks collected in Zimbabwe. These human and tick isolates were indistinguishable from each other, and from an Ethiopian SFG rickettsia, by microimmunofluorescence (MIF), sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), Western blotting and polymerase chain reaction followed by restriction fragment length polymorphism analysis (PCR-RFLP). They were, however, distinguishable from other SFG rickettsiae by MIF serotyping and in the case of the most closely related serotypes, Rickettsia conorii and the Israeli SFG rickettsia, by SDS-PAGE and Western blot. PCR-RFLP failed to distinguish between the Zimbabwean and Israeli SFG rickettsia, though each of these gave different digestion products from R. conorii. The Zimbabwean human and tick isolates and the Ethiopian SFG rickettsiae therefore represent a previously undescribed rickettsial serotype which apparently is pathogenic in human beings. It is proposed that the new serotype be named the agent of African tick-bite fever in order to distinguish it from R. conorii, which until now has been recognized as the only SFG rickettsia to infect man in Africa.