Hospital-acquired infections are very common and they pose a serious risk to patients who are already weakened by another medical condition. Although hospitals take several steps to avoid these sorts of infections, there is one thing they often overlook: insects.
Scientists have looked at the problem of insects in hospitals before, but research has mainly focused on bugs that can breed inside a hospital, such as ants and cockroaches. Our research suggests that we should be more worried about flying insects because several flies we trapped at English hospitals carried drug-resistant bacteria.
Using ultraviolet-light flytraps, electronic fly killers and sticky traps, we captured about 20,000 flies from seven English hospitals. We placed the traps in areas where food was prepared or stored as well as wards, neonatal units and maternity units.
Most of the insects we collected over the 18 month period were Diptera, or true flies, accounting for the 76% of the flies we caught. The Diptera species includes house flies, bluebottles, drain flies and mosquitoes.
Almost half of the true flies caught in our traps were non-biting midges. These flies are typically outdoor species that don’t breed inside buildings. But non-biting midges were not the only intruders in the hospitals. We also found several other species that live and feed on plants, such as aphids, froghoppers and leafhoppers. This suggests that hospitals could do more to prevent flying insects from entering the building.
Bugs on bugs
We determined how many and which bacteria were on the flies we trapped and whether those bacteria were drug resistant. We analysed several families of true flies and found 86 bacterial strains. Enterobacteriaceae, a family that includes E. coli and Klebsiella, were the most commonly isolated, accounting for 41%, followed by Bacillus (which includes the food poisoning bug B. cereus) at 24% and staphylococci (which includes S. aureus, a cause of skin infections, abscesses and respiratory infections) comprising 19%.
When we tested the bacteria for susceptibility to antibiotics, we found that 53% of the strains were resistant to one or more class of antibiotic. Of these, 19% were resistant to several antibiotics, referred to as multi-drug resistance.
Multi-drug-resistant bacteria are, naturally, harder to treat. And infections caused by multi-drug-resistant Enterobacteriaceae are associated with a high risk of death. Fortunately, the Enterobacteriaceae found on the flies in this study were susceptible to most of the antibiotics tested.
We found penicillin to be the least effective antibiotic, especially in Staphylococcus species, which is not surprising given that penicillin-resistant bugs are widespread. We also found bacteria that were resistant to other commonly used antibiotics, including tetracycline and clindamycin.
The link between flying insects in hospitals and hospital-acquired infections is not yet fully understood, but the ability of certain insects to spread bacteria that are attached to their legs or that are in their faeces has been described before. Even when the number of bacteria on insects is not enough to cause an infection directly, they can still act as a reservoir of pathogenic bacteria. This means that when a fly lands on a surface rich in nutrients for bacterial growth, even if only a few bacteria are delivered, it is a matter of hours before the cells multiply to sufficient level to be able to cause an infection.
The good news is that even though more than half the bacteria found on flies in our study were resistant to at least one antibiotic, they were susceptible to other antibiotics and hence treatable. Still, English hospitals could do more to improve pest control – it might help to reduce the risk of infection in already vulnerable patients.