Leishmania parasites produce a thick gel that maximises their transmission from fly vector to host
Responsible for leishmaniasis, a disease that can cause severe skin lesions or affect internal organs, Leishmania parasites use biting phlebotomine sand flies as vectors to move between hosts. To maximise transmission into a new victim, they produce a thick fluid, the promastigote secretory gel (PSG), which obstructs the flies’ digestive system. Looking inside an infected fly, using micro-computed tomography, reveals how the mass of parasites and PSG distends the midgut (in green, with ingested blood in red), and forces a valve, lying between the foregut and midgut, to open more widely. This makes it more difficult for flies to take in blood, causing them to feed for longer than usual when they next bite, and so increasing the chance of parasites being regurgitated. If infected flies do manage to get another meal, parasite numbers grow and the volume of the PSG plug increases, further boosting transmission during later bites.
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