Prospective case-control analysis of the aetiologies of acute undifferentiated fever in Vietnam

Le-Viet, Nhiem; Le, Viet-Nho; Chung, Hai; Phan, Duc-Tuan; Phan, Quang-Duong; Cao, Thanh-Van; Abat, Cédric; Raoult, Didier; Parola, Philippe

The acute-and convalescent-phase plasma samples from all the patients with AUF were subjected to specific indirect immunofluorescence (IFA) assays to detect antibodies of Orientia tsutsugamushi serotypes Karp, Kato, and Gilliam; Rickettsia typhi; Rickettsia felis, Rickettsia conorii and another SFG rickettsiae; and Coxiella burnetii as previously described [26] . An IFA result was considered positive (acute infection) in any of the following cases: (i) detection of IgM, (ii) seroconversion between acute and convalescent sera, or (iii) 2-fold increased IgG and/or IgM titres between acute-and convalescent-phase sera. The cut-offs were those that had been validated and were used in our reference centre for the diagnosis of rickettsial diseases as follows: IgM titre >1:64 for R. conorii; IgM titre >1:32 for other rickettsial antigens; IgG titre >1:128; IgG titres >1:64 for other rickettsial antigens [61, 62] . Western immunoblotting was used to detect Rickettsia spp. in cases that were not differentiated by IFA due to cross-reactivity of relevant antigens of Rickettsia spp. [11, 61] .