To evaluate the role of heterosexual contact as opposed to the sharing of contaminated needles for HTLV-III transmission among intravenous (IV) drug abusers, between May and October 1985 we fully evaluated 30 couples in which both persons were IV drug abusers and 21 couples in which only one was an IV drug abuser (19 male and two female) while each had a heterosexual partner not belonging to known groups at risk for AIDS and a stable, long-term heterosexual relationship (>1 year; median duration, 2.5 years). All IV drug abusers admitted frequent sharing of syringes. Sexual behavior, evaluated through a questionnaire, was similar in both groups. In particular, the frequency of sexual contacts was low (less than one sexual contact per week). The control group consisted of 20 age-matched couples evaluated in a concomitant study on sexual behavior prior to and after the diagnosis of malignant disease in one of the partners (two or more sexual contacts per week). Of 30 couples composed of two IV drug abusers, seven couples were concomitantly seropositive and 18 were concomitantly seronegative. The remaining five couples had discordant serological test findings. On the other hand, of 21 couples composed of an IV drug abuser and a non-IV drug abuser, antibodies to HTLV-III were detected in only one heterosexual partner of the-12 seropositive IV drug abusers and in none of the heterosexual partners of the remaining nine seronegative IV drug abusers. Therefore, among the overall 24 couples in which one or both partners were seropositive, concordance in HTLV-III seropositivity was present in seven (58%) of 12 couples composed of IV drug abusers and in only one (8%) of 12 couples composed of an IV drug abuser and a non-IV drug abuser. Taking into consideration the general practice of sharing of syringes among our IV drug abuser population, we can conclude that the sharing of contaminated needles plays a major role in the transmission of HTLV-III infection among IV drug abusers.
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