Not to be confused with the fungi genus Chytridium.
Crithidia are members of the trypanosome protozoa. They are parasites that exclusively parasitise arthropods, mainly insects. They pass from host to host as cysts in infective faeces and typically, the parasites develop in the digestive tracts of insects and interact with the intestinal epithelium using their flagellum. They display very low host-specificity and a single parasite can infect a large range of invertebrate hosts. At different points in its life-cycle, it passes through amastigote, promastigote, and epimastigote phases; the last is particularly characteristic, and similar stages in other trypanosomes are often called crithidial.
Crithidia bombi is perhaps the most well documented species and is the most prevalent parasite of bumblebees, including common species like Bombus terrestris, Bombus muscorum, and Bombus hortorum. The parasites negatively impact reproductive fitness of Bombus queens, as they affect their ovarian development as well as early colony establishment after the queens emerge from hibernation. Crithidia mellificae, is a parasite of the bee. Other species include C. fasciculata, C. deanei, C. desouzai, C. oncopelti, C. guilhermei and C. luciliae. C. deanei is atypical of the Crithidia genus, and it has been argued not a member of the Crithidia at all. It is not typical of trypanosomatids because of its unusual shape and it harbours endosymbiotic bacteria. C. luciliae is the substrate for the antinuclear antibody test used to diagnose lupus and other autoimmune disorders
These parasites may be at least partially responsible for declining wild bee populations. They cause the bees to lose their ability to distinguish between flowers that contain nectar and those that don't. They make many mistakes by visiting nectar scarce flowers and in so doing, slowly starve to death. Commercially bred bees are used in greenhouses to pollinate plants, for example tomatoes, and these bees typically harbor the parasite, while wild bees do not. It is believed that the commercial bees transmitted the parasite to wild populations in some cases. They escape from the greenhouses through vents; a simple mesh could help prevent this.