CROSS-REACTIVITY BETWEEN ANTIBODIES IN THE SERA OF INDIVIDUALS WITH LEISHMANIASIS, TOXOPLASMOSIS, AND CHAGAS-DISEASE AND ANTIGENS OF THE BLOOD-STAGE FORMS OF PLASMODIUM-FALCIPARUM DETERMINED BY INDIRECT IMMUNOFLUORESCENCE

Seroepidemiologic studies using the indirect immunofluorescent antibod y test (IFAT) are valuable in malaria control programs in identifying local foci of malaria, in diagnosing malaria in asymptomatic, low-para sitemia blood donors in nonendemic countries, in detecting imported ma laria and preventing its introduction into new areas, and in excluding recurrent fever from causes other than malaria. Because other disease s may occur in areas where malaria is prevalent, the aim of this work, using the IFAT, was to determine the frequency of cross-reactions bet ween blood-stage antigens of Plasmodium falciparum and antibodies pres ent in the serum of individuals with leishmaniasis, toxoplasmosis and Chagas' disease: Since malaria transmission does not occur in the stud y area (State of Minas Gerais, Brazil) where these other diseases are present, we studied sera from individuals living in this area who had never been in the areas endemic for malaria in the Amazon region. Posi tive reactivity of sera with blood malaria antigens evaluated by IFAT at dilutions greater than or equal to 1:40 was detected in 19 (38%) of 50 individuals with cutaneous leishmaniasis, five (62%) of eight indi viduals with visceral leishmaniasis, 14 (32%) of 44 individuals with C hagas' disease, four (11%) of 36 individuals with toxoplasmosis, and i n none of the 14 uninfected controls. All 23 of the control malaria se ra from the endemic area (State of Mate Grosso, Brazil) were positive at high dilutions. We found no correlation between titers of the IFAT with malaria and the specific antigens used for serodiagnosis of the o ther protozoan infections studied. At dilutions of 1:20 and 1:40, the sensitivity of the IFAT test was 100% and specificity was 52% and 72%, respectively. At a dilution of 1:80, the sensitivity was 86% and the specificity was 90%. Thus, the choice of the final dilution of serum f or use for the diagnosis of malaria should take into account the study population and the type of parasites prevalent in the area. At the pr esent time, leishmaniasis is endemic in many areas of the Amazon regio n where malaria is endemic. Thus, we recommend the use of sera diluted greater than or equal to I:XO to exclude cross-reactive antibodies to this parasite in seroepidemiologic studies of malaria.