Retrospective Immunodiagnosis of Malaria in Nonimmune Travelers Returning From the Tropics

Background: Since travelers are now frequently advised to carry emergency self medication for the treatment of suspected malaria, the issue of retrospective diagnosis of malaria in nonimmune patients has recently gained more importance. Reliable methods for evaluating the frequency and justifying the use of self medication in clinically suspected malaria are warranted.
Methods: One hundred and eighty-five sera from 132 nonimmune travelers returning to a nonendemic area, and with a microscopically confirmed diagnosis of malaria, were investigated for the presence of antibodies against blood stages of Plasmodium falciparum or Plasmodium vivax, by the indirect fluorescence antibody test (IFAT). Eighty-eight patients suffered from infection with P. falciparum and 44 from the infection with P. vivax. In falciparum malaria, 97.1% of patients had positive reactions during a period of 15–60 days after onset of symptoms. During the same period after onset of symptoms, significant titers were demonstrated in 88.2% of patients with vivax malaria. Extended cross-reactions between the antigens used and a wide range of interindividual differences in antibody titers were observed. One hundred sera from Germans recovering from nonmalarial febrile illnesses were used as negative control group in the investigation, of which none resulted in a positive IFAT.
Conclusions: We conclude from these results that the IFAT is a specific and sensitive tool for the retrospective confirmation of malaria in the differential diagnosis of fever imported from endemic areas by nonimmune travelers. Nevertheless, when dealing with the individual patient, a careful interpretation, with inclusion of all available clinical data, is mandatory. However, by using blood stage antigens, the IFAT can be considered a sensitive tool in epidemiologic surveys. The tool can be used with a high degree of reliability, even without access to additional clinical data.