High prevalences of antibodies against human T-lymphotropic virus type I (HTLV-I), as confirmed by Western immunoblot, were found in several remote indigenous populations of the Solomon Islands and Vanuatu and in some isolated populations of New Guinea that had no contact with Japanese or Africans and little contact with Caucasians prior to our bleedings. By contrast, zero or very low prevalences of HTLV-I infection were found in Guamanians and Carolinians, despite more than 30 years of intense contact with the Japanese. A total of 1,601 sera, collected between 1963 and 1981 from 21 population groups in the Western Pacific, was tested by enzyme-linked immunosorbent assay (ELISA) for IgG antibodies to HTLV-I. By ELISA, prevalences of antibodies against HTLV-I ranged from zero to 50%. Seropositivity could be confirmed in only 12.5% of 48 ELISA-positive sera selected for testing by Western immunoblot. However, the confirmed HTLV-I seroprevalences in some Melanesian populations were still as high as those found in HTLV-I-endemic regions, such as southwestern Japan and the Caribbean basin. HTLV-I prevalences were similar among males and females, and acquisition of antibodies increased with age. Our data indicate that infections with HTLV-I or a related retrovirus have been widespread in the southwestern Pacific for over 25 year in populations with minimal outside contact, while some populations which had extensive Japanese contact have no evidence of infection. Furthermore, based on the high frequency of indeterminate Western immunoblots, we conclude that in Melanesia this may represent either incomplete specific reactivity to HTLV-I or the existence of an antigenic variant of HTLV-I, distinct from prototype Japanese, American, and European HTLV-I strains.
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