Houseflies live close to humans and domesticated animals and because they are so mobile they can easily spread bacteria that make people sick.
They carry these disease-causing agents on their body surfaces and in their gut.
Owing to their diverse habitats, ability to fly long distances and attraction to decaying organic materials, houseflies greatly amplify the risk of human exposure to harmful microorganisms.
A single housefly can carry enough of these to cause infantile diarrhoea, cholera, bacillary dysentery, tuberculosis and anthrax.
The growing threat of antimicrobial resistance
As microbiologists specialising in medical and veterinary bacteria and zoonosis, we collaborated on a study of bacterial communities harboured by houseflies in three hospices in South Africa. We used advanced methods to identify the bacteria.
Our research identified an additional health risk: some of the microbes that the flies carried in their gut may be resistant to antibiotics.
According to the World Health Organization, antimicrobial resistance occurs when bacteria, viruses, fungi and parasites no longer respond to antimicrobial medicines. Antibiotics and other antimicrobial medicines become ineffective and infections become difficult or impossible to treat.
Antimicrobial resistance is one of the leading global public health threats.
How we went about our study
We chose hospices for our study because they house terminally ill patients with weak immune systems.
Adult houseflies were collected at three hospices between September 2019 and February 2020. Two of the hospices were in Bloemfontein and one was in Johannesburg.
A minimum of three ribbon sticky traps were set for three to five days over a two week period. They were located in the kitchens and dining halls to capture houseflies near areas where food was prepared or consumed.
A total of 657 houseflies were collected from the three hospices.
Previously the microbiological culture method was one of the most broadly used tools to identify microbial organisms.
It was considered as a "gold standard" because of its ability to detect new bacterial species and test their susceptibility or resistance to antibiotics.
However, with this old method many bacteria couldn't be cultivated, especially those harboured by insects.
The bacteria are difficult to isolate, or they grow slowly in the culture.
We were able to use new DNA sequencing methods that get around the need to grow cultures. They help us better understand the diversity of microorganisms on the flies.
We found houseflies captured from hospices harboured highly diverse bacterial communities with antibiotic-resistant genes.
The hospice with the highest prevalence of antibiotic resistance genes was in Bloemfontein. The reason for this prevalence might be the hospice's proximity to a nursery which produces seedlings for vegetable production, broiler chicks and piggeries.
These types of agricultural activity often involve using antibiotics to prevent and limit the spread of diseases and as growth promoters.
Antibiotics are widely used in animal husbandry, and antibiotic resistance genes are frequently detected in livestock waste around the world.
Looking forward
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The presence of houseflies in hospice surroundings is of particular concern because the patients are frail and resistant bacteria are a health hazard.
It's important to take extra care to limit housefly breeding by following high standards of hygiene. Staff should be informed about the threat of antimicrobial resistance.
Staff should sanitise their hands at building entrances, for example, and kitchen surfaces should be kept very clean.
Waste management is also important. Any garbage kept inside and outside the building should be in sealed containers to avoid contact with flies.
Maropeng Charles Monyama, Senior Lecturer - Microbiology, University of South Africa
Jane Nkhebenyane, Senior lecturer, Central University of Technology
Oriel Thekisoe, Professor of Parasitology and Zoonosis, North-West University
