Transmission of human protozoan parasites by synanthropic insects is predominantly mechanical. In adult flies it occurs via mechanical dislodgement from the exoskeleton, fecal deposition, and regurgitation, i.e., vomit (14
). Flies can carry human pathogens on the sponging mouthparts, on body and leg hairs (i.e., setae), or on the sticky pads of the feet (i.e., tarsi). Fine hairs on the pads of a fly's feet are coated with a sticky substance which improves the fly's ability to adhere while resting or climbing on nonhorizontal surfaces. This substance also enhances the adhesion of particles, i.e., viruses, bacteria, and protozoan cysts, to fly legs, which then can be directly transported to the next visited surface and dislodged. Small particles readily adhere to a fly's exterior surfaces due to their electrostatic charge (14
). Fly exoskeletons have certain electrostatic charges, and any particle with a different charge or a neutral charge will adhere to the fly surface.
The effectiveness of feces in enhancing the transmission of infectious agents by house flies is much greater than that of any other substrate or medium (9). This is a result of fecal viscosity, which increases the efficiency of tarsi and bristles in trapping particles suspended in the feces (9). Protozoan parasites can pass through the fly gastrointestinal tract without alteration of their infectivity and can be subsequently deposited on visited surfaces in "fecal spots" (9). Alternatively, the parasites present in fly alimentary tracts can be regurgitated, i.e., vomit drops, on a surface perceived by a fly as a meal (regurgitation always precedes feeding). Frequent meals on contaminated substrates together with alternating regurgitation and ingestion cause progressive accumulation of human pathogens in the fly alimentary system (14). Human pathogens can also be transmitted as airborne particles for short distances from fly-electrocuting traps, as electrocuting traps do not alter the infectivity of pathogens transported by flies (22). There are size limitations regarding the transmittal from the contaminated sites. Bigger particles such as helminth eggs are transported by flies on their external surfaces, i.e., exoskeletons, while small cystic stages of human-infectious intestinal protozoa can be ingested as well as transported on the exoskeleton.
It is unlikely that human protozoan parasites are transmitted transtadially through the larval and pupal stages of synanthropic insects to the adult stages of species whose larval stages, e.g., maggots, breed in contaminated substrates (9
). Transtadial transmission is the passage of an infectious agent from the egg to the adult insect stage. This is because the pupation process involves intense reorganization of the digestive tract tissue, resulting in the development of a new digestive system and production of the meconium (9
), accumulated intestinal wastes that remain behind in the puparium. This has been confirmed in laboratory experiments. Toxoplasma gondii
oocysts were isolated from larvae and pupae of house flies reared in infectious cat feces but not from newly emerged adult house flies (31
). Also, Cryptosporidium parvum
oocysts were present in the alimentary canals of maggots reared on a contaminated substrate, inside the pupae, and in the meconium but not in or on adult flies (9
). However, even if flies are sterile when they emerge from the pupa, they will acquire pathogens rapidly from contaminated substrates in which they develop by direct contact (9