Experimental superimposed infection of the hamster with Leishmania mexicana and L. braziliensis.

Hamsters inoculated subcutaneously with Leishmania mexicana (L. m. amazonensis or L. m. mexicana) and with L. braziliensis panamensis, either simultaneously or with an interval, in different parts of the body (right front paw and nose), showed an independent course of infection for either parasite with the typical clinical and histopathological characteristics produced by these organisms. In one group of animals, metastases from the paw (L. m. amazonensis) to the nose (L. b. panamensis) was proven and the partial transformation of the granuloma from epithelioid and giant multinuclear cells (tuberculoid) to histiocytes full of parasites (lepromatous), was observed. These two specific contrasting reactions, are confined to the place of infection and influenced by the type of parasite. The histopathological picture normally observed in hamsters as a reaction to L. mexicana s.1., occurs only occasionally in humans. It is suggested, on the basis of this animal model, that DCL in the New World is only produced by parasites of the "mexicana" complex and that both parasite characteristics and host immune status are responsible for this phenomenon.