House flies are such a common insect that it is considered part of everyday life for humans. Most of us see it as just an irritation, buzzing around and trying to feed on any food it can access within the home but flies can also transmit diseases. In fact many common infections are transmitted by house flies. Some diseases are very serious and even potentially deadly. Although it can never be eradicated entirely, reducing fly populations and limiting its contact with food in particular is an important part of hygiene.
It is important to understand the life cycle of the house fly (Musca domestica) in order to prevent disease transmission. Most of us think of maggots (one stage the house fly’s life cycle) as being more filthy than the adult fly. However, it is the adult fly that is more likely to spread diseases by its feeding habits and movement patterns.
House flies mainly spread infectious diseases. These are diseases caused by viruses, bacteria, protozoa and even nematodes (worms like the roundworm or threadworm). There are over 100 pathogens (disease-causing organisms) that are associated with house flies. Unlike other insects, such as mosquitoes or ticks, these pathogens do not specifically require an insect vector. The house fly plays no specific role in the life cycle of these pathogens. The fly is simply a carrier in some instances.
Diarrheal illnesses are some of the more common diseases spread by house flies. This includes bacteria such as E.coli, Shigella, Campylobacter, Enterococcus and related bacteria which commonly cause diarrheal illnesses and are found in the stool of people with these illnesses. Some of the diseases spread by house flies include:
It is evident that flies can spread many infectious diseases indiscriminately.
The house fly is able to spread disease through several routes. It does not bite like the horse fly or tsetse fly in order to inject the pathogen into a person. Instead disease-causing agents are spread on its body, in its mouth parts or through its vomit and faeces. House flies feed indiscriminately on a wide range of organic matter, from faeces to food (fruits, vegetables and meat). It is through this contact with the item it is feeding upon and even direct contact with people that disease-causing agents are acquired and passed on.
The infective dose for each pathogen varies greatly. Sometimes just a few microbes are required to cause serious disease. The contaminated matter containing these microbes, and even just the microbes itself, that are acquired from one source may adhere to the fly or be passed out in its vomit and faeces. The contaminated matter and microbes are then passed onto food once the fly lands and/or feeds on it. The situation is further exacerbated if the food is not refrigerated allowing the inoculation dose of microbes to multiply before the food is eaten.