RISK OF HTLV-I INFECTION IN JAPANESE WOMEN WHO ARE LAST IN BIRTH-ORDER

The percentage of last-born women among pregnant women who were seropo sitive for human T-lymphotropic virus type I (HTLV-I) significantly ex ceeded that among HTLV-I seronegative women (119/258 (46.1%): 89/251 ( 35.4%); P < 0.05). The findings suggest that last-born women are susce ptible to HTLV-I infection. At least two possible interpretations of t his birth-order effect are: (i) these last-born women were born to mot hers who, on the average, were older than those of early-born women an d, as a consequence, were more likely to have been seropositive and to have passed on HTLV-I to their daughters; (ii) husband-to-wife transm ission of HTLV-I requires time to occur, so last-born women are more l ikely than early-born women to become infected.