- Mechanical Transmission of Human Protozoan Parasites by Insects

Synanthropic insects such as flies and cockroaches can significantly contribute to the spread of food-borne protozoan diseases in both developing and developed countries. With the capacities of modern synanthropic insect control, the detrimental impact of these insects on public health is minimized, which gives a false sense of security about the infectious disease threat from them. Populations of synanthropic insects can grow rapidly, and any temporary inattention to continuous and proper sanitation and control can allow transmission of human food-borne diseases associated with synanthropic insect infestations.

As synanthropic insects harbor human protozoan parasites acquired naturally from unhygienic sources, it is necessary to prevent these insects from gaining access to human food. A sanitary, insect-free environment is of the highest priority in modern food sanitation programs and in food-processing facilities. The proper venue for controlling the transmitters or vectors of food-borne pathogens is an effective sanitation and pest exclusion program which would control or eliminate pests from food-processing areas.

In areas where valid health statistics are not available, microbiological studies of synanthropic insects can provide essential epidemiologic information on human enteropathogens.

Not all nonbiting fly species are associated with unsanitary conditions and pathogen transmission or involved in the epidemiology of human diseases. Sanitation and pest control professionals should recognize the adult and larval stages of nonbiting flies that are potential public health threats and should be able to link the fly species with a potential hazard for food-borne disease. Only flies classified as filth flies pose potential health hazards and require regulatory actions or pest control programs to be effectively neutralized or eliminated. These programs and corrective actions instituted through them will help to prevent or correct potential microbial contamination or cross-contamination from these flies. Filth flies share certain attributes that have been recognized by entomologists and account for their strong association with human food-borne diseases, such as synanthropy, endophily, communicative behavior, and attraction to both excrement and human food products.

Fly populations can increase quickly in a relatively short time, particularly during the summer. The flies associated with transmission of human pathogens often exhibit clustering and swarming behaviors. As a result, the sites of attraction, filth or food items, are visited by large numbers of flies. This causes the cumulative effect of parasite deposition, which is much greater that the pathogen-carrying capacity of a single fly. High densities of flies proportionally increase the load of pathogens on surfaces visited by the flies.

Mechanical transfer of Cryptosporidium parvum oocysts by filth flies can be achieved through defecation, regurgitation, or mechanical dislodgement, and the vast majority of transported oocysts are viable and thus capable of infection. The biology and ecology of synanthropic filth flies indicate that their potential for mechanical transmission of Cryptosporidium parvum is high. The epidemiologic involvement of nonbiting flies in transmission of Cryptosporidium parvum is difficult to prove, as cases of cryptosporidiosis resulting from fly visitations on foodstuffs are classified as food borne. As fly eradication or control of synanthropic fly populations coincides with sharp reductions in outbreaks and cases of diarrheal diseases, it is most likely that food-borne cases of cryptosporidiosis could be even more drastically reduced by enforcing fly control.

Mechanical transmission of pathogens by nonbiting flies and epidemiological involvement of synanthropic flies in human food-borne diseases have not received adequate scientific attention. Further research is necessary to elucidate the mechanisms involved in retaining the infectivity of pathogens vectored by flies, the efficiency of various transport types, e.g., exoskeleton versus gastrointestinal tract, and the temporal and spatial dispersal of pathogens by flies from contaminated sites.

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