Mixed leishmanial infections in Rhombomys opimus: a key to the persistence of Leishmania major from one transmission season to the next

An important feature of the foci of zoonotic cutaneous leishmaniasis (ZCL) in Turkmenistan and Uzbekistan is a 6–10-month break in transmission when Leishmania parasites persist in great gerbils (Rhombomys opimus)—the main host for three species (L. major, L. turanica and L. gerbilli). Almost all (95%) of the laboratory-maintained R. opimus experimentally infected with L. major cured their infections within 6 months, a situation which, if mirrored in field conditions, cannot provide reliable persistence of the infection to the next transmission season. However, infections with L. turanica alone persisted for a mean of 15 months, and mixed infections of L. major and L. turanica persisted even longer (mean = 25 months), parasites of both species remaining detectable in the skin for at least 18 months. Isoenzyme identification of 664 isolates obtained from wild-caught R. opimus, and of 58 cloned strains developed from them, showed that L. turanica, which is non-pathogenic for humans, tends to predominate in the gerbils from all types of natural ZCL foci, including those which are hyper-endemic; in June, L. turanica may be present in 80%–100% of the R. opimus in the foci. In contrast, infections with L. major alone occur far less commonly, and are especially hard to find at the beginning of the transmission season. However, 5%–25% of great gerbils in these foci are each infected with a mixture of L. major and L. turanica. In hyper- and meso-endemic foci, the proportion of L. major within mixed infections of Leishmania increases significantly towards the end of transmission season (August-September). It would appear, therefore, that mixed L. major/L. turanica infections in R. opimus promote the persistence of L. major between transmission seasons.