What Your Grandmother Taught You About Flies

You’ve always suspected that Grandma knew best when she insisted on keeping flies off the picnic food. Flies are ubiquitous in the summer and fall and they feed on just about anything, including manure and garbage. Grandma probably was visualizing the actual feces on the flies’ feet.

Now, it turns out that flies do more than just carry “dirty stuff” from place to place. They also carry the dreaded bacteria that are resistant to antibiotics. Everyone knows someone who has battled MRSA (Methicillin-resistant Staphylococcus aureus). And most know someone who died of the infection.

A study by Kansas State University has found conclusively that flies can carry the MRSA and other antibiotic resistant bacteria from one infected area to another.

And it’s not just house flies that are incriminated. German cockroaches are carriers also.

Ludek Zurek, K-State professor of microbial ecology and lead author of the study, stated “House flies are common where animal manure is produced, including in cattle, poultry and swine operations. Cockroaches, primarily German cockroaches, have become a common pest in confined swine operations.” Zurek and his colleagues collected house flies and cockroaches from food animal production locations, including swine and poultry farms, as well as wastewater treatment facilities that collect waste from multiple sources, including hospitals. The researchers then genetically analyzed the bacteria in the digestive tract of the insects and compared them to the bacteria present in the animal feces and wastewater. “We found these insects carry the same bacteria found in the animal manure. Then we started sampling insects found in surrounding urban areas, including fast food restaurants, and again, we found house flies with multi-drug resistant bacteria.” House flies collected several miles from the wastewater treatment plants in surrounding urban areas had a lower prevalence of the antibiotic-resistant bacteria than those examined from the facilities themselves, but the bacteria still existed in those urban house flies.

In addition, Zurek’s team showed that bacteria in the house fly digestive tract can exchange antibiotic resistance by horizontal gene transfer, that is, the resistant strains multiply in the fly and can be left behind on food by fly regurgitation or spitting, and defecation.

In addition to the insects, Zurek and his research team have also showed that wild birds, such as ravens and crows, carry multi-drug antibiotic-resistant bacteria. “Wild birds can pick up the antibiotic resistant bacteria from fields where animal manure was used as a fertilizer,” he said.

To help eliminate the potential connection to food animal production, the U.S. Food and Drug Administration last December released its first request to antibiotic manufacturers to voluntarily remove antibiotics from the list for animal growth promoters. The plan is to phase out antibiotics as a feed additive for growth promotion in United States in the next three years.

“Clearly, pest management and trying to minimize the pest populations on farms and outside of the farms is one way,” he said. “House flies aren’t just a nuisance. They can carry antibiotic resistant bacteria, so they should be taken seriously as a vector.”

Grandma was right!

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