Antibodies to Human T Lymphotropic Virus Type I in a Population from the Philippines: Evidence for Cross-Reactivity with Plasmodium falciparum

Residents of two areas in the Philippines were tested for human T lymphotropic virus type I (HTLV-I): Napsan, a site endemic for malaria, and a nonmalarious site, Bacolod. By ELISA, 30*^ of the 1743 Napsan residents were positive. Western blot immunoreactivity with two or more HTLV-I proteins was present in 8196 of the ELISA positives; however, there was no reactivity with the env proteins. The frequency of this indeterminate immunoreactivity increased with the malaria antibody titer. None of the 200 Bacolod sera were Western blot immunoreactive. In competitive serologic assays, an HTLV-I lysate blocked the HTLV-I immunoreactivity of the Napsan sera but did not reduce malaria antibody. A Plasmodium falciparum lysate blocked both the HTLV-I and malaria immunoreactivity of the Napsan sera. These results show that indeterminate HTLV-I immunoreactivity can be caused by P. falciparum antibody. Serosurveys conducted in malaria endemic areas should consider this possibility.