Laboratory diagnosis of boutonneuse fever by enzyme-linked immunosorbent assay

Little is known about the use of the enzyme-linked immunosorbent assay (ELISA) in the diagnosis of boutonneuse fever, and the reports which do exist have concerned the use of a strain of African origin and a peroxidase-labelled conjugate. We have used, as antigen, a Sicilian strain of Rickettsia conorii recently isolated from a patient with boutonneuse fever and anti-human immunoglobulins G and M labelled with alkaline phosphatase as conjugate. 432 (84.6%) of 500 sera from patients with boutonneuse fever, 4.4% of 384 sera from patients with various other pathologies, and 2.9% of 204 sera from blood donors gave positive reactions. The ELISA was highly sensitive and specific in the diagnosis of boutonneuse fever, and seems to be the test of choice for sero-epidemiological investigations, and when a large number of specimens must be examined. The test is more sensitive than the indirect immunofluorescence test, at least in the early stage of the disease (44.9% vs 21.3% positive respectively), and a long time after an acute attack.