In pseudomyiasis, dipteran larvae have been accidentally ingested and are found within an animal’s GI tract, where they are not able to continue their development. Dogs or cats infested with larvae of the facultative myiasis-producing flies in wounds or in the hair coat often ingest larvae while licking or grooming. These larvae pass through the GI tract and appear in the feces undigested. Dipteran larvae may also be passed in the feces when a roaming dog or cat ingests carrion that contains maggots; these maggots pass to the external environment undigested.
Pseudomyiasis can also occur if feces submitted for parasitologic examination are not fresh. Adult facultative-myiasis flies may have laid their eggs in these feces, and larval development may have begun.
Eristalis tenax, the rat-tailed maggot, may be seen in the gutter behind cows in dairy barns. These maggots are associated with liquid feces and with feces that have not been removed from the environment. The larvae are known as rat-tailed maggots because their breathing pores are found at the tip of a long, siphon-like breathing tube on their posterior end. Many farmers erroneously assume that the cows defecated these maggots. The adults are nonparasitic, free-living flies.