Figure 1. Modeling outward transmission implicates skin of immunocompromised Leishmania donovani-infected hosts as a major source of parasites for vector sand flies.

In both immunocompromised and immunocompetent hosts, inward transmission by infectious sand flies results in persistence of sand fly-accessible parasites in host skin. If immunocompromised, parasites disseminate to the skin forming patches that vary in size and density giving rise to a heterogeneity in host infectiousness to sand flies, with large and/or more dense patches transmitting at higher rates than small and/or less dense patches. Dissemination of L. donovani parasites to the skin in an immunocompetent host remains to be determined (?).