Molecular epidemiology of Haemophilus ducreyi infections.

Haemophilus ducreyi was isolated with an enriched chocolate agar containing vancomycin from seven patients with clinical evidence of chancroid. Four cases were imported from outside the United States; the other three cases were acquired in the United States from one of the imported cases. All strains of H. ducreyi contained ampicillin-resistance plasmids with molecular weights of 7.3, 5.7, or 3.6 megadaltons. Plasmid molecular weights were identical for isolates from the epidemiologically linked cases and differed according to the geographic origins of the strains. Findings of this study confirm that H. ducreyi is sexually transmitted and suggest that clinical chancroid develops frequently after acquisition of H. ducreyi.