Crithidia

Crithidia is a genus of unicellular, flagellated parasitic protozoa in the family Trypanosomatidae, within the class Kinetoplastea. Members are obligate parasites of insects (monoxenous trypanosomatids) and are typically confined to the gut of their arthropod hosts. The genus comprises several species found in diverse insects, including bees, flies, and mosquitoes. Some species have been studied as model organisms for kinetoplastid biology, while others are ecologically important due to their impact on pollinators.

Morphology and anatomy: Crithidia cells are small, typically about 7–12 micrometers long, with a single anterior flagellum and a kinetoplast near the base of the flagellum. They possess a cytostome for endocytosis and a relatively simple cell body, with life stages that are adapted to the gut environment of their insect hosts.

Life cycle and transmission: Crithidia species complete their life cycle within a single insect host. They divide by binary fission in the gut and are typically excreted with feces or released into the gut lumen, where they can be transmitted when another insect ingests contaminated material. There is no known vertebrate reservoir for most Crithidia species, making infections largely restricted to invertebrate hosts.

Ecology and significance: Crithidia bombi infects bumblebees and can affect foraging behavior and colony performance. Crithidia fasciculata is frequently found in mosquitoes and has been used in laboratory studies as a model kinetoplastid. Crithidia luciliae is notable in clinical diagnostics for the Crithidia luciliae immunofluorescence test, used to detect anti-dsDNA antibodies. The genus is important for understanding insect–parasite interactions and broader kinetoplastid biology.