The insect pests commonly referred to as “flies” belong to the order Diptera, which also includes mosquitoes, gnats and midges. The house fly (Musca domestica) is the most commonly encountered and heavily controlled fly pest of homes and buildings in the US, and the species has become well established in most human-populated areas worldwide. Other fly species that are common indoor pests include fruit flies, drain flies, phorid flies, and blow flies, all of which are associated with the spread of disease-causing microorganisms. These flies are often referred to as “filth flies” because they breed on organic waste materials in order to provide their larval offspring (maggots) with plenty to eat after hatching. After becoming smeared with pathogen-rich organic waste, filth flies enter homes where they contaminate everything they make contact with, most importantly food. According to a collection of academic studies, filth flies carry numerous pathogens that can be transmitted to humans in three disgusting ways.

Unlike arthropods that serve as biological disease vectors like mosquitoes and ticks, flies mechanically transmit pathogens that they acquire from their breeding substrates, which include garbage, rotting food, drain scum, animal carcasses, excrement, and numerous other forms of organic waste. The disease-causing microorganisms that filth flies acquire from their breeding sites are mostly species of bacteria, but they also carry viruses, parasites, and fungal disease organisms. The disease organisms present on the exterior body of filth flies mechanically dislodge and fall onto every surface with which the pests make contact. The two other ways in which filth flies mechanically transmit disease pathogens is by defecating and vomiting on surfaces, and especially on human foods. While all filth fly species carry and spread disease pathogens, house flies carry the greatest number and variety of pathogens due to their habit of breeding on many different sources of organic waste, most notably excrement, which is particularly rich in harmful pathogens. Luckily, filth flies are unlikely to transmit disease to humans, but house flies and fruit flies are well associated with the spread of food-borne pathogens.

Do you think that house flies may contribute to disease more than experts currently realize?