VISCERAL LEISHMANIASIS IN VERVERT MONKEYS - IMMUNOLOGICAL RESPONSES DURING ASYMPTOMATIC INFECTIONS

Nine vervet monkeys (Cercopithecus aethiops) were infected intradermal ly with 8x10(7) virulent L. donovani promastigotes. Four animals devel oped clinical visceral leishmaniasis and died over a period of 18 mont hs. The remaining five animals have remained asymptomatic for a period of 3 years now. Attempts to isolate parasites from spleen and liver t hrough biopsies were fruitless. Immunological responses of these subcl inically infected animals were examined. Enzyme-linked immunosorbent a ssay (ELISA) and western blot analyses demonstrated Leishmania specifi c antibodies in these animals, but the antibody titres were low. When proliferation of peripheral blood monocytes (PBMC) to Concanavalin A ( Con A) of these animals was compared with control 'disease free animal s' there were no significant differences in response. However L, donov ani antigen (fixed promastigotes) specific proliferation was demonstra ted in the five subclinically infected animals. High and varying level s of interferon gamma (IFN-gamma) were secreted in PBMC cultures from the five vervet monkeys when stimulated with either Con A or L. donova ni antigens. In control animals, IFN-gamma was only detected when PBMC were stimulated with Con A. Marked delayed-type hypersensitivity (DTH ) responses were demonstrated in the five subclinically infected anima ls 48 h after injection with formalin fixed promastgotes. It was concl uded that the visceral Leishmania disease spectrum due to L. donovani observed in humans could be induced in vervet monkeys and that L. dono vani asymptomatic/cryptic infected animals have competent humoral and cellular responses to homologous parasites.