Leishmaniasis is a disease that has long vexed researchers. Spread by biting insects, it causes blood cell production to plummet, enlarges the liver and forces patients into bed for months with weakness and fever. Yet what is really strange is that leishmaniasis is caused by a parasite that really has no business in people. It belongs to the protozoan family trypanosomatida of which just about every member is a parasite of insects. Now new work with honey bees is suggesting that these unique insects may well be responsible for paving the way for parasites from this group to migrate into mammals.
Unknowingly passing parasites your way.
The new work came about in 2019 when a team working with bats in Brazil discovered a trypanosomatid parasite in these flying mammals. The parasite was Crithidia mellificae, a protozooan that was noted in the literature as being solely found in honey bees. They reported the find at the time but were left unsure of how the parasite had made the leap from insects to mammals. To study this, they studied the heat and acid tolerance of C. mellificae and its relatives in mosquitoes. They found that C. mellificae had extraordinary heat and acidity tolerance compared to the rest of its kin. This seems to be because the honey bees that it relies on as hosts, while ectothermic (cold blooded) individually, are colonial endotherms (meaning they create a body temperature through social activity). Hives literally give themselves fevers to drive off parasites but C. mellificae seems that have evolved traits to cope with this. The same is true for the acidity of the honey bee gut. It is much more acidic than the guts of mosquitoes.
That honey bees use these traits to stave off infection is unfortunate because, during the winter, many hives in regions with lots of human disturbance, are experiencing lower temperatures and suffering colony collapse as a result of parasite infection. The researchers reckon that this phenomenon paves the way for parasites in this family to adapt to warmer and more acidic conditions and ultimately make the leap into mammals that depend upon the heat and acidity of their bodies to keep many pathogens at bay. Whether this is the path that leishmaniasis took long ago remains to be determined, but the finding suggests that driving the evolution of new parasitic diseases now needs to be considered among the threats presented by the rampant colony collapse disorder taking place in honey bee hives around the world. You can read more in The Economist article that I wrote on this here.