Detection of antibodies to Plasmodium vivax by indirect immunofluorescence: influence of the geographic origin of antigens and serum samples.

The results of a double-blind serological study of 15 sera sampled in a residual focus of vivax malaria transmission in Algeria, and of 7 sera from patients with slide-proven P. vivax infections acquired in India, are analyzed. The reactivity of each of these serum samples was tested by indirect immunofluorescence using 6 different batches of antigen, including 3 batches of P. vivax antigen prepared with isolates from Zaire (Africa), India and the Solomon Islands, respectively. The geometric mean of reciprocal titers (GMRT) calculated on the 7 sera from proven vivax infections fell from 289.8 using the homologous antigen from the same geographic origin (India) to 48.7 using a homologous (vivax) antigen originating from a different continent (Africa). Among the 15 samples from Algeria, the percentage of seropositives decreased from 100% using the homologous P. vivax antigen originating from the same continent (Africa) to 53.3% using a homologous antigen from India. Two aspects are included in the discussion: in seroepidemiological studies, sensitivity could be improved by the use of a homologous antigen from the same geographic origin; in detection of clinical cases of malaria and species identification based on serology, our results stress the need for caution in interpreting serological titers and for taking into account the geographic origin of the isolates used as antigen.