Leishmania braziliensis and Leishmania mexicana: Experimental cutaneous infections in golden hamsters

Abstract

The effect of certain inoculation conditions on the incubation period and development of cutaneous Leishmania braziliensis and Leishmania mexicana lesions was determined in golden hamsters infected in multiple sites. These conditions included: use of the nose, ear, forepaw, and flank as sites of inoculation; use of the intradermal and subcutaneous routes of inoculation; and use of amastigotes and promastigotes as inocula. Intradermal inoculation of 10 6 amastigotes obtained by trypsin digestion of donor hamster lesions produced median onset times (the time for lesions to develop in 50% of the sites inoculated) of 3.4 days for a South American strain of L. braziliensis braziliensis, 4.8 days for a South American strain of L. mexicana amazonensis, and 5.2 days for a Costa Rican strain of L. mexicana mexicana. Intradermal inoculation of 10 6 promastigotes from primary passage in insect cell culture media produced median onset times of lesions of 11.0, 13.5, and 22.0 days for the respective strains. Inoculation of < 10 6 parasites of all Leishmania strains resulted in extended median onset times of lesions and, in certain instances, fewer lesions developing. Additional experiments with the L. braziliensis strain demonstrated a direct relationship of median onset times of lesions and the dose of amastigotes inoculated over a range of 10 0 to 10 6 organisms. Intradermal and subcutaneous routes of inoculation were compared in the shaved flank. The greater efficacy of the intradermal route was demonstrated when subcutaneous inoculation of 10 6 amastigotes resulted in an approximate fivefold increase in the developmental period of L. braziliensis braziliensis and L. mexicana amazonensis lesions. Intradermal inoculation required only 10 2 amastigotes of these two strains to produce developmental periods of lesions comparable to those produced by 10 6 amastigotes inoculated subcutaneously. When 10 6 amastigotes of L. mexicana mexicana were inoculated subcutaneously, no lesions developed during 56 days of observation. L. braziliensis lesions resulting from intradermal inoculation exhibited rapid growth only for the first 28 to 35 days, while lesions of both L. mexicana strains exhibited rapid growth during most of the 56-day observation period. Objects consistent with Schaumann bodies were found in L. braziliensis braziliensis lesions; otherwise the histologic appearance of lesions conformed with previously reported descriptions. The influence of the site of inoculation on the developmental period of lesions was minimal, although fewer lesions tended to develop in the flank; growth of lesions on the dorsal surface of the forepaw was more consistent than at other sites. Reproducible growth of L. braziliensis braziliensis lesions was obtained for amastigote and promastigote inocula which were injected intradermally in the forepaw. Inoculation by the intradermal route and use of amastigote inocula are proposed as standard conditions for the characterization and maintenance of L. braziliensis and L. mexicana strains in vivo in the hamster.