As anyone who has ever eaten outdoors knows, house flies are very annoying and can ruin a picnic. We’ve developed methods for controlling them that range from bug zappers to screen shields that cover food. But now there’s even more reason for concern.

A new study conducted at Kansas State University and published in Applied and Environmental Microbiology has found that flies may be transferring antibiotic resistant bacteria from farms to people. This troubling finding turns flies from a nuisance into a health threat.

Since we know farm animals carry antibiotic resistant bacteria, primarily because they are fed sub therapeutic doses of antibiotics for growth promotion and disease prevention, it makes sense that insects could transfer those pathogens to people. And it turns out that flies literally vomit and poop when they land on you and on food, which makes for easy transfer of bacteria. While researchers aren’t positive how this happens, it makes sense. Investigating to find the precise mechanisms of bacterial transfer is necessary, but this research is in its early stages.

The lead author of the study, professor of microbial ecology Dr. Ludek Zurek, said in a statement, “there are a number of insects that are commonly associated with animals, such as house flies and cockroaches. House flies are common where animal manure is produced, including cattle, poultry, and swine operations. We found these insects carry the same bacteria found in animal manure.” The research team also found that the bacteria in the insect’s digestive tract can exchange antibiotic resistance by horizontal gene transfer.

Other studies have shown that flies around factory farms and in fast food restaurants carry antibiotic resistant bacteria, including E. coli, Salmonella, and Staphylococcus. In fact, Zurek’s study found the same pathogens in the flies around a restaurant and in the food served there.

So the next time you are eating outdoors, be especially vigilant about keeping flies and other insects away from your food. Dr. Zurek said, “house flies aren’t just a nuisance. They can carry antibiotic resistant bacteria, so they should be taken seriously as a vector.”