Leishmania infecting man and wild animals in Saudi Arabia. 8. The influence of prior infection with Leishmania arabica on challenge with L. major in man.

A clinical trial is described of an attempt to protect against Leishmania major by prior vaccination with live L. arabica. After a single, previously leishmanin-negative, adult male volunteer was bitten by 8 Phlebotomus papatasi infected with L. arabica, no infected lesions were observed. He remained leishmanin-negative and his lymphocytes reacted weakly to antigens of L. arabica or L. major. Subsequently he and 3 other leishmanin-negative adult male volunteers were vaccinated with cultures containing 4 x 10(6) promastigotes of L. arabica. All remained leishmanin-negative but their lymphocytes showed some response to both L. arabica and L. major antigens. 96 d after vaccination these 4, and another, non-vaccinated, volunteer were challenged with 2 x 10(6) promastigotes of L. major. Active cutaneous, ulcerated lesions developed in all 5 volunteers. The lesions in 3 vaccinated volunteers were associated with marked lymphadenitis and beading, but the lesions started to heal spontaneously within 120-250 d after challenge. The lesion in the fourth vaccinated volunteer was less severe and lymphadenitis was not observed. The lesion in the unvaccinated subject developed more slowly and was smaller, but more chronic, than those in the vaccinated individuals. Marked cross-reactivity in terms of lymphocyte proliferation and interferon-gamma production was observed between L. major and L. arabica in both directions in subjects exposed first to one or the other organism. Although the procedure followed in this trial failed to give protection against L. major, further studies in volunteers should be considered.