The feeding mechanisms and filthy breeding habits of synanthropic insects such as flies, cockroaches, and coprophagic beetles make them efficient vectors and transmitters of human enteric protozoan parasites (18). In particular, domestic filth flies (some species in the families Sarcophagidae [flesh flies], Muscidae [house flies and latrine flies], and Calliphoridae [blow flies and bottle flies]) have evolved to live in close association with humans (synanthropic flies) as annoying pestiferous scavengers (5, 15). Filth flies breed in animal manure and human excrement, i.e., coprophagic flies, and garbage, animal bedding, and decaying organic matter, i.e., saprophagous flies (5, 15).
Synanthropic insects are abundant in urban and rural areas where unsanitary conditions prevail and are usually scarce when sanitary conditions are enforced (14). Outbreaks and cases of food-borne diarrheal diseases in urban and rural areas are closely related to the seasonal increase in abundance of filth flies, and enforced fly control is closely related to reductions in the number of cases of such diseases (14). Over 50 species of synanthropic flies have been reported to be associated with unsanitary conditions and involved in dissemination of human enteropathogens in the environment. Of these, 21 species of filth flies have been listed by regulatory agencies concerned with sanitation and public health as causative agents of gastrointestinal diseases in people based on synanthropy, endophily (the preference of insects to enter buildings), communicative behavior, and strong attraction to filth and human food (22). These species include Hermetia illuscens, Megaselia insulana, Eristalis tenax, Piophila casei, Fannia canicularis, Musca domestica, Muscina stabulans, Stomoxys calcitrans, Calliphora viscina, Calliphora vomitoria, Chrysomya putoria, Cynomyopsis cadaverina, Cochliomyia macellaria, Phaenicia cuprina, Phaenicia sericata, Phormia regina, Sarcophaga crassipalpis, Sarcophaga carneria, and Sarcophaga haemorrhoidalis.
Cockroaches frequently feed on human feces, and therefore they can disseminate cysts of enteric protozoans in the environment (7, 16, 18, 24). Cockroaches have been epidemiologically involved in toxoplasmosis, giardiasis, sarcocystosis, and intestinal amoebiasis, (16, 18, 24, 27, 30).