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The purpose of this review is to present some concepts on the etiology of tropical spastic paraparesis or human T-cell lymphotropic virus-I (HTLV-I)-associated myelopathy (TSP/HAM). The large number of syndromes that have been associated with HTLV-I (60 to date), the existence of TSP/HAM cases associated with other retroviruses (human immunodeficiency virus-2 [HIV-2], HTLV-II), the existence of many TSPs without HTLV-I, and the evidence of clear epidemiologic contradictions in TSP/HAM indicate that the etiopathogenesis of TSP/HAM is not yet clear. Tropical spastic paraparesis/HAM affects patients of all human ethnic groups, but usually in well localized and relatively isolated geographic regions where HTLV-I has been endemic for a long time. Environmental factors and geographic locations appear to be critical factors. Because the neuropathology of TSP/HAM suggests a toxometabolic, rather than a viral cause, it is proposed that an intoxication similar to neurolathyrism could account for some of TSP/HAM cases, mainly in tropical and subtropical countries. If this were the case, HTLV-I could be a cofactor or act as a bystander. it is possible that co-infection with another agent is necessary to produce TSP/HAM and most of the syndromes associated with HTLV-I.

To explore the HTLV-I-carrying groups among the indigenous inhabitants in South America, a sero-epidemiological study on HTLV-I focusing on hinterland villages isolated from others in the Andes and Amazon regions was conducted. Five (2.9%) out of 171 subjects showed positive for HTLV-I antibody in the gelatin particle agglutination (PA) test. Two out of 5 positives with high antibody titer (greater than or equal to x 1024) in the PA test also showed a positive immunofluorescence (IF) test and anti-HTLV-I-specific protein products, p19, p24, p28, gp46, and p53 in sera by the Western blotting (WB) test. One of three negatives in the IF test showed positive antibodies to p19 and p24 by the WB test. Finally, two were confirmed as HTLV-I carriers and one was suspected of being a carrier. All three are Paez Indians from the central Andes; 53- and 34-year-old women and a 35-year-old man. The results show that HTLV-I carriers exist among isolated indigenous people in South America.