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 6 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), 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, R. 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.<new para>ADDITIONAL ABSTRACT:<new para>The suggestion that rickettsial tick-bite fever in Africa might have 2 distinct causes dates from a lecture by A. Pijper in 1934. Modern serological methods pursued by the group headed by P.J. Kelly and D. Raoult in Zimbabwe have indicated that a rickettsia distinct from Rickettsia conorii causes human illness in Africa. This paper confirms in a single case that this distinct pathogen was the cause; it was also isolated from specimens of the veld tick Amblyomma hebraeum and may have a reservoir in cattle. The organism, provisionally named R. africae, is serologically separate from the "classical" agent of African tick-bite fever, R. conorii, which is transmitted by dog ticks. In world terms, it is likely that many other tick-borne strains and species of rickettsia remain to be discovered. newline˜G.O. Cowan