|Photo of E. aedis spores from here|
Edhazardia aedis is a microsporidian parasite that specialises on infecting Aedes aegypti - also known as the mosquito that can act as the main vector for a variety of viruses include those that causes degnue fever, yellow fever, and Chikungunya. Edhazardia aedis can spread through the mosquito population via two methods; (1) the parasite can proliferation throughout the mosquito's body until it ultimately overwhelms the host, which dies and dissolves into a cloud of infective spores, or (2) if an infected female mosquito survives the ordeal to adulthood and still manage to produce offspring, her mosquito babies will inherit E. aedis from her (gee, thanks a lot mum!).
But E. aedis can sometimes run into a competing species - Vavraia culicis. It is also a microsporidian, but unlike E. aedis, it is a generalist that can infect many different species of mosquitoes. It is also a mosquito-killer which has the same general modus operandi as E. aedis, where the parasite's spores are released when the host finally succumbs. This study found that mosquito larvae which have less access to food and are infected by both parasites tend to die earlier - and when the host dies, the spores are dispersed for both E. aedis and V. culicis - so everyone wins, right? Well not quite.
While host death does release the spores which allow them to infect more mosquito larvae, the parasites get more spores for their bucks by keeping their host alive for longer - so a host that ends up keeling over too early is not very cost effective. This applies to both E. aedis and V. culicis. Even before host death, the cost of co-infection starts manifesting itself. Regardless of whether the host dies sooner or later, both parasites produce less spores in co-infections. If E. aedis is sharing a host, it produces half as many spores as it would have if it had the host all to itself. But co-infection is even more costly for V. culicis, which manages to produce only a bit over a quarter of the spore it would have in single infections.
It is unknown how these two parasites duke it out in the mosquito, or why E. aedis has a competitive edge over V. culicis. Perhaps by being a specialist of A. aegypti, E. aedis has some sort of home ground advantage when it comes to getting the most out of its host. So it seems that some parasites just don't like sharing, and when it comes to living with others, sometimes it pays for a parasite to be a specialist.
Duncan, A. B., Agnew, P., Noel, V., & Michalakis, Y. (2015). The consequences of co-infections for parasite transmission in the mosquito Aedes aegypti. Journal of Animal Ecology 84: 498-508.