Advantages of measuring changes in the number of viable parasites in murine models of experimental cutaneous leishmaniasis.

Previously published studies of experimental cutaneous leishmaniasis in the mouse have relied almost exclusively on measuring changes in lesion size to follow the course of the infection. The purposes of the studies reported here were to develop a technique to quantitate the number of viable organisms in the tissues and to use the technique to follow the development and resolution of the primary infection as well as the development of acquired resistance to Leishmania tropica in a resistant (C3H/He) and a susceptible (BALB/c) mouse strain. It was found that individual L. tropica amastigotes derived from infected tissues would transform to promastigotes and repeatedly divide to form discrete, countable colonies on rabbit blood agar. The plating efficiency was approximately 88%. Using the blood agar plating technique to quantitate the organism against time of the infection, we obtained data that suggest that acquired resistance develops in C3H/He mice earlier than is suggested by reduction in lesion size. In addition, although this resistance eliminates the parasites from the primary lesion in 10 weeks, 1,000 to 10,000 parasites persist for months in the lymph node draining the lesion site. In these studies, we found no evidence of acquired resistance in the susceptible BALB/c mice. The organism grows progressively, and the infection can disseminate to the spleen within 2 weeks. These studies illustrate the advantages of quantitating viable parasites in studies of immunity in cutaneous leishmaniasis.