Immunoblot cross-reactions among Rickettsia, Proteus spp. and Legionella spp. in patients with Mediterranean spotted fever

Sera from patients suffering from Mediterranean spotted fever (i.e. an infection due to Rickettsia conorii) were studied by immunoblot to investigate cross-reactivity. A prevalence of IgM antibodies to Proteus OX 19, Proteus 0X 2, to the Rickettsia typhus group, to Legionella pneumophila serovars 4 and 5, to L. bozemanii Wiga and to L. micdadei Tatlock was found. Western blot confirmed that the antibodies were directed against the lipopolysaccharide as demonstrated by proteinase K digestion of the antigens. Cross-adsorptions showed that there is a common cross-reacting epitope among L. bozemanii Wiga, R. typhi and Proteus OX 19 but cross-reacting antibodies to L. micdadei and OX 2 were distinct and independent. This IgM cross-reaction could lead to a misdiagnosis.