Epidemiology

HTLV-1

Transmission

HTLV-I transmission takes place from mother to child (18), sexual contact (19), blood transfusion (20), and exchange of hypodermic syringe. Mother to child transmission takes place mainly through breastfeeding (21); in HTLV-I endemic regions, approximately 25% of breastfed children born from HTLV-I-seropositive mothers acquire the infection.

Recent studies suggest that HTLV-I transmission through breastfeeding could be associated to the presence of maternal antibodies for transactivated HTLV-I protein (22), or with elevated maternal titles of total antibodies for HTLV-I (23). However, the clinical use for these indicators has not been established yet. The HTLV-I perinatal or intrauterine transmission takes place, but it seems to be less frequent than the breastfeeding transmission; approximately 5% of children born from infected mothers, but not breastfed, acquire the infection (24).

The HTLV-I sexual transmission seems to be more efficient from men to women than from women to men. In a study conducted with couples in Japan, it was calculated the efficiency of sexual transmission from men to women in 61% in a period of 10 years, compared to less than 1% from women to men (25). Another study showed that the presence of antibody for the tax protein in a male pair was linked to the sexual transmission for the female pair (26). In a study conducted in Jamaica, the male genital ulcer disease was identified as a risk for female-male sexual transmission (27). In the USA, approximately 25%-30% of sexual partners and HTLV-I/II-seropositive blood donors are also seropositive. HTLV-I transmission through blood transfusion takes place with transfusion of cell blood products (red cells platelets, and total blood), but not with the plasma fraction or plasma derivates from HTLV-I infected blood. A seroconversion rate from 44% to 63% was detected in carriers of HTLV-I-infected cellular components in endemic regions (20, 30).

Lower rates (approximately 20%) were verified in the USA in carriers of contaminated of cellular components (31). The probability of transmission through total blood or stocked red cells seems to decrease with higher duration of the storage of the product. Sharing blood contaminated needles or syringes are the most likely form of transmission among intravenal drug users. HTLV-I is not transmitted through casual contact.

Health workers who deal with HTLV-I-infected people could only be infected through percutaneous exposure with blood contaminated by the HTLV-I. In Japan, a health worker who accidentally was inoculated with blood from a patient with adult T cells leukemia (ATL) suffered seroconversion (33). However, there was not a report of seroconversion taking place among the 31 other laboratory workers exposed to the HTLV-I through puncture lesion (34). General precautions, subscribed for contact with all patients, are adequate against HTLV-I transmission among health workers (35).