The parasite Leishmania major infects many vertebrates including humans, dogs and rodents. In humans, infection can cause skin sores and boils,fever, organ damage and even death. Leishmania is transmitted to humans via biting sand flies from Central and South America, Africa and Asia. Although flies suffer poor health when carrying the parasites,infected flies are common in these areas. Does this mean that there is some advantage associated with Leishmania susceptibility in sand flies?
Yosef Schlein and Raymond Jacobson at the Hebrew University in Israel recently tested this idea. As hunger-tolerance is an advantage to flies, the scientists wondered if this tolerance was somehow related to the flies'susceptibility to infection. Sand flies have a voracious appetite for plants that are packed with nutritious sugars. But by the end of the dry season,plant sugars are in short supply so that flies that can `do without' have a real advantage. A low sugar diet limits the life expectancy of sand flies and few survive long enough to deposit their eggs and transmit their parasites. So, the flies that are best able to `do without' are therefore most likely to produce offspring and to spread infection. Could these hunger-tolerant flies be targeted for Leishmania susceptibility?
The researchers investigated this possibility by exposing flies to different nutritional conditions, and infecting survivors with Leishmania. Flies that survived sugar-deprivation were far more likely to carry parasites than those with unlimited access to sugars. Hunger-tolerant flies were therefore most susceptible to infection. And that's not the end of the story. Schlein and Jacobson also showed that offspring from hunger-tolerant flies were just as prone to infection as their parents!
The researchers then investigated how hungry sand flies fared in the wild. They found that flies living in arid areas had empty bellies, whilst those living in irrigated sites were stuffed full of plant sugars. Offspring descended from arid area flies were much more susceptible to parasites than those from sugar-saturated parents. Again, their findings indicate a link between hunger-tolerance and Leishmania-susceptibility.
This study ultimately suggests that in sand flies, the success of Leishmania infection reflects the ability of some flies to survive periods of sugar deprivation. This association probably developed because hardy flies stand a better chance in reproduction and parasite transmission. The reason why hunger-tolerant flies are more susceptible to Leishmania will likely prove an appetising topic for future study.