Kinetics of antibody responses in Rickettsia africae and Rickettsia conorii infections

Abstract

African tick-bite fever, caused by Rickettsia africae, is the most common tick-borne rickettsiosis in sub-Saharan Africa. Mediterranean spotted fever due to Rickettsia conorii also occurs in the region but is more prevalent in Mediterranean countries. Using microimmunofluorescence, we compared the development of immunoglobulin G (IgG) and IgM titers in 48 patients with African tick-bite fever and 48 patients with Mediterranean spotted fever. Doxycycline treatment within 7 days from the onset of disease significantly prevented the development of antibodies to R. africae. In patients with African tick-bite fever, the median times to seroconversion with IgG and IgM were 28 and 25 days, respectively, after the onset of symptoms. These were significantly longer by a median of 6 days for IgG and 9 days for IgM than the times for seroconversion in patients with Mediterranean spotted fever (P < 10(-2)). We recommend that sera collected 4 weeks after the onset of signs of patients with suspected African tick-bite fever should be used for the definitive serological diagnosis of R. africae infections.

FIG. 1.

(A) Comparative kinetics of IgM antibodies to R. conorii (gray) and to R. africae (gray cross-hatch) in Mediterranean spotted fever patients and to R. africae (white) and to R. conorii (black cross-hatch) in African tick-bite fever patients. The standard deviation for each median titer is shown. (B) Comparative kinetics of IgG antibodies to R. conorii (gray) and to R. africae (gray cross-hatch) in Mediterranean spotted fever patients and to R. africae (white) and to R. conorii (black cross-hatch) in African tick-bite fever patients. The standard deviation for each median titer is shown.

FIG. 2.

(A) Cumulative percentage of IgM seroconversion against R. conorii (gray curve) and R. africae (black curve) in patients with African tick-bite fever. (B) Cumulative percentage of IgG seroconversion against R. conorii (gray curve) and R. africae (black curve) in patients with African tick-bite fever.