Houseflies have evolved in close association with people and their environment. While this bugs are plentiful and annoying, of greater importance is their role in possible transmission of organisms, which cause diseases and illnesses such as typhoid, cholera, dysentery and diarrhea. They can also mechanically transmit parasitic worms such as pinworms, roundworms and hookworms.
The houseflies’ filthy habits make them efficient in picking up disease causing organisms from garbage, sewage and fecal matter. They regurgitate, defecate or simply transfer those organisms by leg or mouth to human and animal food.
The housefly (Musca domestica) is well known around homes, farms and ranches. It is the most abundant of the domestic “filth fly” species in the U.S. Musca sorbens, a closely related species found in Africa, Asia, and the Pacific, is important in the spread of eye infections.
The origin of the housefly is unknown, but its biology and worldwide distribution suggest it began in a subtropical or tropical part of the world such as East Africa. M. domestica, troublesome because it is dependent on human settlements and domestic animals, frequently enters houses.
M. domestica, as an adult, is 6-8 mm long with a 13-15 mm wingspan. It has a gray thorax with four longitudinal dark stripes. The front half of the abdomen is buff-colored and occasionally transparent at the sides with a central dark band broadening to cover the last abdominal segments.
The housefly passes through complete metamorphosis as egg, larva, pupa and adult. Development is temperature dependent, but under most conditions the life cycle is complete in 10-20 days. In the third of three larval stages, the mature larvae, or maggots, crawl away from their breeding place and burrow into loose ground nearby where they transform to the next stage, the pupa. When pupal development is complete and the adult fly is ready to emerge, it pushes the end of the pupa open with an extrudable saclike structure on the head. The adult continues to alternate expansion and contraction of this structure to clear a path through the soil to the surface.
M. domestica is extremely prolific and has tremendous potential for population increases. For example, in 1911 C.F. Hodge made the following estimation. Assume that one adult female deposits 120 to 150 eggs per “lot,” with at least six lots at intervals of three or four days. A pair of flies beginning “operations” in April may be progenitors, if all were to live, of 1.91 x 10 flies in August.
That’s almost 200 million trillion flies, or 191,000,000,000,000,000,000 flies.
That many flies would cover the earth with a layer at least 2.5 feet deep. While Hodge’s estimation is unlikely to happen, it demonstrates how rapidly a small population of houseflies can increase in size. It also illustrates the importance of early season fly control measures.
The housefly is well adapted by structure and behavior to transmit disease organisms. Its body is covered with fine hairs and bristles that readily pick up filth particles. At the base of each leg is a cushion-like structure covered with glandular hairs. The sticky secretions from the glandular hairs gather bacteria and other organisms.
The housefly excretes and regurgitates whenever it comes to rest. Regurgitation is the process of digestion during which food is brought up bit by bit from the fly’s crop and mixed with saliva before being passed on to the digestive tract. Because the maximum flight range of houseflies exceeds five miles, they have ample geographic opportunity to spread disease pathogens prior to landing on, and contaminating human food.
Housefly larvae breed in accumulations of waste, garbage and manure. Adult flies, which often use buildings for shelter, feed on human and animal food and waste material. If undeterred, they commonly fall into and contaminate people’s food. They also land on the exposed skin of people and animals. These habits potentially lead to disease transmission.
While houseflies are not necessary for the transmission of several important diseases and are rarely the most important agents, M. domestica is one means of transmission under conditions favorable to the flies. The intestinal diseases, which can be transmitted to humans, include:
Shigellosis – Bacillary dysentery and other diarrheal diseases.
Salmonellosis – Typhoid, paratyphoid, enteritis, food poisoning and others. While flies play a role in inoculating food with Salmonellosis-type microorganisms, they are generally less important here than in transmitting Shigellosis.
Cholera – Transmission by flies is possible but probably of minor significance.
Amoebic Dysentery – Flies may transmit cysts but this seems to be uncommon.
Parasitic Worms – Flies can carry eggs and cysts of many intestinal worms such as pinworm (Enterobius), roundworms (Ascaris), whipworms (Trichiuris), hookworms (Ancylostoma), tapeworms (Taenia, Dipylidium) and others may contribute to the spread of worms to people and animals.
Polio – Houseflies are able to transmit viruses such as Poliomyelitis and related viruses to human foods in quantities high enough to cause infection in some susceptible persons.
Eye Diseases – Eye diseases such as Trachoma (viral) and epidemic conjunctivitis (bacillary) can be spread by houseflies, Musca sorbens, found in Africa, Asia, and the Pacific, has an important role in spreading eye infections. M. sorbens is strongly attracted to infected eyes and feeds on eye secretions. M. domestica is also attracted to infected eyes but seems to be less important in disease transmission.
In years past, the fly swatter was the consumer’s primary if not very sanitary weapon against houseflies. Effective management of housefly populations today however, involves effort in a number of areas. Fly breeding habitats should be limited by minimizing exposed garbage and pet excrement. Adult flies can be excluded from structures by proper building design and the use of tight fitting screening on all windows and doors. Cleanliness overall is an important part of controlling houseflies, particularly because of their filth disseminating habits. Household cleaning products help maintain a clean home that will discourage flies. Many insecticidal products are available for combating houseflies, including flying insect killers in aerosol and liquid form and various fly strips and traps. The sprays include direct contact (knockdown) products; spatial controls that when sprayed in the air allow the active ingredient to permeate all areas of the room and search out flies in their hiding places; and residual sprays that leave the insecticide behind to kill other flies. Aerosol delivery makes the spray particles just the right size to be effective with the least amount of active ingredient for effectiveness. Other mechanical means include light traps and electrically charged devices (“zappers”) that can reduce the numbers of adults in localized areas.