Just how much of a health risk do houseflies pose to humans?
Certain pests are always going to be associated with disease. Rodents have been connected with disease and death for centuries – the plague or black death is etched in history. Cockroaches are always seen as dirty, as they are often seen in drains and bins, before making their way to foods and food preparation surfaces. Amongst the flying insects, mosquitoes are documented as known transmitters of serious diseases often causing death, but aren’t really seen as ‘dirty’ insects. Instead, it’s houseflies that are regarded as dirty and disease-ridden pests. But what diseases do they actually carry?
The housefly, Musca domestica, belongs to the group of flies known as the ‘filth flies’, due to their association with refuse, dead animals and faeces. They live and breed in decomposing organic material, so it’s neither surprising that they are seen as dirty nor that they carry disease. It’s when the adult flies make their way into homes and food businesses, and land on preparation surfaces and food items, that the alarm bells start to ring. And with good cause! A recent systematic review of 1718 research papers confirmed that over 130 pathogens had been associated with the housefly. The majority of these pathogens were bacteria, but there were also fungi, parasites and a few viruses.
The main group of bacteria isolated were enteric (gut) bacteria. This is perhaps not surprising as houseflies preferentially feed and lay their eggs in faeces and other animal waste. A number of these bacteria are very virulent and known to cause serious illness. A range of E. coli bacteria (which cause serious gastro problems) were common, as were bacteria that cause cholera and anthrax. Other infamous bacteria that they were found to carry included various Staphylococci, Streptococci, Pseudomonas and Clostridium species. Particularly concerning was that several studies reported antibiotic-resistant bacteria strains being isolated from houseflies, likely caused by the large housefly populations found breeding around intensive animal husbandry facilities which use large amounts of antibiotics in production.
Fungal species found on houseflies which are of medical importance to humans include Candida and Aspergillus. A range of species which are of veterinary importance were also isolated – another reason for pet owners not to leave pet food out! Most of these fungi were isolated from the cuticle of the housefly rather than from the gut.
Houseflies also carry a range of parasites, primarily gut parasites including hookworms. Again, perhaps this is not surprising given their diet.
Researchers in the US have demonstrated that houseflies can indeed acquire the SARS-CoV-2 virus by feeding or landing on infected material. By swabbing surfaces where the houseflies landed after feeding, they also established that houseflies were able to transmit the virus for up to 24 hours after exposure. Whether the amount of SARS-CoV-2 on surfaces could actually be picked up in sufficient quantities to cause infection in humans is somewhat unlikely.
Although houseflies can carry a wide range of diseases and are able to pass these pathogens on to the surfaces and food on which they land, actually demonstrating that humans subsequently become ill after eating infected food is somewhat problematic. Carrying out trials that involve feeding humans with infected food probably wouldn’t pass an ethics committee review!
To develop disease, a subject needs to ingest a large enough dose of the pathogen – the infectious dose. The number of pathogens found on the fly cuticle is a lot lower than the amount found in their guts. However, as houseflies regurgitate material when feeding, they can deliver large doses of gut pathogens. Indeed, the infectious dose for some E. coli strains is as little as 100 bacteria, so even though proving disease transmission from houseflies to humans is challenging, it is probably safe to assume it is likely. Indeed, a number of studies report the correlation between the incidence of diarrhoea and increasing fly populations, which provides some support for the idea that the housefly could be the cause of disease transmission, at least for the enteric bacteria diseases.
So, although housefly problems are common in Australia and many just view them as a nuisance and so “put up with them”, perhaps by highlighting the potential disease transmission they may be motivated to take action to control them.
Khamesipour, F., Lankarani, K.B., Honarvar, B. et al. A systematic review of human pathogens carried by the housefly (Musca domestica L.). BMC Public Health 18, 1049 (2018).
‘House flies can carry SARS-CoV-2 up to 24 hours after exposure, study finds’ by Angela Laguipo. News-Medical, April 22, 2021.
Science Direct, Infectious Dose information page.