Do houseflies spread antibiotic resistance?

There was a one-day Congressional hearing last week on antibiotic resistance and industrial agriculture. Members of congress were looking for evidence that would link agricultural use of antibiotics to human illness. One Republican specifically asked for research done in the US, implying European studies would not be good enough. Medical experts from the CDC and NIH were evidently unable to cite an American study.

It’s almost impossible to connect the bacteria in a specific individual with an animal that received an antibiotic. But there’s certainly a case to made for the use of antibiotics in agriculture and the spread of antibiotic resistance. And that’s based on research done right here in the US.

A poultry house and its 30,000 flies

Chicken houses are very attractive to flies. A Danish study estimated that, over a six-week period, as many as 30,000 flies may enter a poultry house. The Johns Hopkins Bloomberg School of Public Health (Baltimore, MD) just released a study on antibiotic resistant bacteria found in houseflies who make their home on industrialized chicken farms.

The study collected houseflies and poultry litter from chicken farms in an area of Maryland, Delaware and Virginia. Poultry litter is the absorbent material on the floor of a chicken farm. By the time it’s been used, it consists primarily of chicken manure, which is rich in bacteria.

When scientists compared the antibiotic resistant bacteria found in both flies and litter, they found striking similarities. The bacteria from both sources had very similar resistance characteristics and very similar resistance genes. “Resistance characteristics” refers to exactly which antibiotics can no longer destroy the bacteria. Characteristics of the mechanism of resistance can now be identified on the genes themselves.

We can’t afford to squander antibiotics as growth promoters

Chickens are fed antibiotics to speed their growth. Bacteria, which are constantly mutating in response to their environment, develop resistance to those antibiotics. Previous studies have established a link between the antibiotics fed to chickens and antibiotic resistant bacteria associated with the farms. In particular, resistant bacteria have been found in the environment surrounding chicken farms, in farm workers, and in the chickens subsequently purchased by consumers. The Johns Hopkins study suggests that flies may play a role in spreading resistant bacteria and in exposing humans to these difficult-to-treat microorganisms.

According to the senior author of the study, Ellen Silbergeld:

Although we did not directly quantify the contribution of flies to human exposure, our results suggest that flies in intensive production areas could efficiently spread resistant organisms over large distances.

According to Dr. Robert Lawrence, director of the Center for a Livable Future at Johns Hopkins (emphasis added),

[C]onfined animal feeding operations—where thousands of animals are crowded together and are fed antibiotics for growth promotion—create the perfect environment for selection of bacteria that are resistant to antibiotics. “Antimicrobials are among the most important developments of the twentieth century in managing infectious diseases in people. We can’t afford to squander them by using them as growth promoters in industrial food animal production. The increase in antibiotic-resistant bacteria is a major threat to the health of the public, and policymakers should quickly phase out and ban the use of antimicrobials for non-therapeutic use in food animal production,” said Lawrence.

To see what the inside of a chicken “farm” looks like – technically known as a broiler poultry operation — check out the documentary Food Inc. The chickens are kept in the dark and packed so tightly they can’t move. For their own safety, say the “farmers.”

Resources:

Photo: Audilab, Photographs of invertebrates

Flies May Spread Drug-Resistant Bacteria from Poultry Operations, Johns Hopkins Bloomberg School of Public Health, March 16, 2009

Meat as Source of Antibiotic Resistance, Be Active: Asking Questions, Advocating Change, April 30 , 2010

Houseflies Collected In Fast Food Restaurants Found To Carry Antibiotic Resistant Bacteria, Medical News Today, June 17, 2006

Lilia Macovei and Ludek Zurek, Ecology of Antibiotic Resistance Genes: Characterization of Enterococci from Houseflies Collected in Food Settings, Applied and Environmental Microbiology, March 29, 2006