The explosive human immunodeficiency virus type 1 epidemic among injectingdrug users of Kathmandu, Nepal, is caused by a subtype C virus of restricted genetic diversity

An explosive epidemic of human immunodeficiency virus type 1 (HIV-1) has be en documented among the injecting drug user population of Kathmandu, Nepal, whose seropositivity rate has risen from 0 to 40% between 1995 and 1997. B y using Catrimox to preserve whole-blood RNA at ambient temperature for tra nsportation, HIV-1 envelope V3-V4 sequences were obtained from 36 patients in this group. Analysis of the sequences indicated a homogenous epidemic of subtype C virus, with at least two independent introductions of the virus into the population, Viral diversity was restricted within two transmission subclusters, with the majority of variation occurring in V4. Calculation o f the synonymous-to-nonsynonymous mutation ratio (Ks:Ka) across this region showed that significant evolutionary pressure had been experienced during the rapid horizontal spread of the virus in this population, most strongly directed to the region between V3 and V4.