Infection Parameters in the Sand Fly Vector That Predict Transmission of Leishmania major

Abstract

To identify parameters of Leishmania infection within a population of infected sand flies that reliably predict subsequent transmission to the mammalian host, we sampled groups of infected flies and compared infection intensity and degree of metacyclogenesis with the frequency of transmission. The percentage of parasites within the midgut that were metacyclic promastigotes had the highest correlation with the frequency of transmission. Meta-analysis of multiple transmission experiments allowed us to establish a percent-metacyclic “cutoff” value that predicted transmission competence. Sand fly infections initiated with variable doses of parasites resulted in correspondingly altered percentages of metacyclic promastigotes, resulting in altered transmission frequency and disease severity. Lastly, alteration of sand fly oviposition status and environmental conditions at the time of transmission also influenced transmission frequency. These observations have implications for transmission of Leishmania by the sand fly vector in both the laboratory and in nature, including how the number of organisms acquired by the sand fly from an infection reservoir may influence the clinical outcome of infection following transmission by bite. Many infectious diseases are initiated when pathogenic organisms are deposited into the skin of the human host by the bite of an insect. In the case of the parasite Leishmania, the causative agent of Leishmaniasis, factors associated with the bite of the infected sand fly vector influence infection outcome, suggesting that this is the most relevant means to initiate disease in experimental hosts. However, transmission frequency, and the dose of parasites delivered when transmission occurs, can vary enormously both within and between populations of experimentally infected flies. This variability represents a major obstacle to the widespread use of sand flies to study Leishmaniasis. We identified a parameter of Leishmania infection in the sand fly vector that predicts the degree of transmission competence within a group of experimentally infected flies. We also identified environmental and biological factors that influence transmission frequency. This information will make experiments relying on infected sand fly challenge more manageable, thereby increasing the likelihood that infected sand fly challenge, rather than needle challenge, will be used in future experimentation. Lastly, we demonstrated that the number of organisms acquired by the sand fly can influence subsequent sand fly infection intensity, and that this infection intensity has implications for disease outcome.