Myiasis is characterized as obligatory, facultative, or accidental, based on species‒host dependence2:
- Obligatory myiasis is caused by flies whose larvae require a living host for development. Cochliomyia hominivorax (ie, primary screwworm) and all bot flies are classic examples of flies that cause obligatory myiasis. In cats and dogs, the most commonly encountered bot flies are Cuterebra spp (eg, rodent bot fly, rabbit bot, warble).3 Flies causing obligatory myiasis may lay eggs in uninfected wounds that may be as small as a tick bite (C hominivorax) or penetrate through the skin of their hosts to invade healthy tissues (Cuterebra spp). Yorkshire terriers appear to be at higher risk of Cuterebra spp infestation than other breeds.4
- Facultative myiasis results when nonparasitic flies opportunistically infest dogs and cats. These flies usually lay eggs in decomposing organic matter (eg, carrion, feces) but may deposit eggs in animals with infected, open wounds, or hair coats soiled with feces, urine, or vomitus. Larvae typically feed on dead or decaying host tissue. Facultative myiasis in soiled or contaminated wounds most often is caused by species of Calliphora (ie, blow flies), Lucilia (ie, green bottle flies, blow flies), Musca (eg, M domestica, the common house fly), Phaenicia (ie, green bottle flies, blow flies), Phormia (ie, black blow flies), and Sarcophaga (ie, flesh flies).2,5 Dogs and cats with access to the outdoors and animals with weakened immune systems have a greater risk for infestation. Obesity in cats has been identified as a predisposing factor for infestation with Calliphora erythrocephala.6
- Accidental myiasis occurs when dogs and cats ingest fly eggs or larvae. Accidental myiasis is pseudoparasitism because further life-stage development of these larvae does not occur in the GI tract. Immature stages typically are eliminated without causing clinical disease.2 Fly eggs and larvae are found in vomitus or feces.