Flinders Island spotted fever: a newly recognised endemic focus of tick typhus in Bass Strait. Part 2. Serological investigations.

Twenty-six cases of a spotted-fever-like illness have been identified on Flinders Island, Tasmania, over a 17 year period. These patients and 335 healthy persons from the island were investigated serologically using the Weil-Felix agglutination test (Proteus sp. antigens OX2, OX19, OXK) and rickettsia-specific microimmunofluorescence. The antigens used in these latter tests comprised one member of the typhus group (Rickettsia typhi) and three members of the spotted fever group (Rickettsia rickettsii, Rickettsia australis and Rickettsia conorii). Patients with Flinders Island spotted fever showed a higher prevalence of positive reactions to the Weil-Felix tests (with OX2 and OX19 antigens) and a higher prevalence of positive results to rickettsia-specific serological tests (with the exception of antibodies to Rickettsia typhi) than did healthy persons; OX2 (36% v. less than 1%); OX19 (36% v. less than 1%); Rickettsia rickettsii (42% v. 1%); Rickettsia australis (46% v. 1%); Rickettsia conorii (42% v. 1%); Rickettsia typhi (4% v. 4%). In seven of the 26 patients (27%) seroconversion was demonstrated by means of Weil-Felix tests, confirming recent infection. In six of these patients seroconversion was also demonstrated in rickettsia-specific tests. Although these results support the clinical evidence that the illness on Flinders Island is caused by a rickettsia of the spotted fever group, the aetiological agent remains to be isolated.