After years studying the dust that blows across the southern Great Plains, Phil Smith no longer looks at the dark dust storms that routinely rise over Lubbock without a healthy dose of apprehension.
In a study slated for publication next month, he and his Texas Tech University colleague Greg Mayer may have made their biggest discovery yet: DNA from antibiotic-resistant bacteria in cattle feedlots is airborne.
For years, scientists have known that people can contract antibiotic-resistant bacteria by consuming contaminated meat or water. Smith and Mayer’s findings indicate that humans could also be exposed to “super bugs” or “super bacteria” traveling through the air.
“This is the first test to open our eyes to the fact that we could be breathing these things,” said Smith, an environmental toxicologist at Texas Tech.
Antibiotics are commonly used on cows in industrial feedlots to treat disease and, more controversially, as a growth promoter added to cattle feed. An estimated 80 percent or more of antibiotics sold in the United States are used on livestock.
The study may help explain how bacteria that no longer respond to antibiotics could be spreading and causing hard-to-treat infections in humans. Two million Americans are infected by antibiotic-resistant bacteria each year, and 23,000 die as a direct result of the infections, according to the federal Centers for Disease Control and Prevention.
The cattle industry is pushing back, saying the report misrepresents the risk of super bacteria to human health. Dr. Sam Ives, a veterinarian working with the Texas Cattle Feeders Association, said antibiotic use in the industry is “judicious.”
“If I truly thought that the usage of these products was putting anyone at danger, I wouldn’t be using them,” Ives said.
The study focused on feedlots in Texas’ Panhandle and South Plains, each holding tens of thousands of cattle, where animals spend the last few months of their lives gaining weight before slaughter. Feedlots are an ideal place to study antibiotic resistance because bacteria there are exposed to high levels of antibiotics.
Last year, Mayer and Smith collected dust samples upwind and downwind from 10 commercial cattle feed yards over six months. They set up high-tech vacuums on the nearest county road, collected particulate matter on mesh screens and took the samples to a lab where sophisticated machinery helped them identify the gene sequences, which Mayer calls the “fingerprints” of antibiotic-resistant bacteria.
“The ‘aha’ moment came when we saw how much more prevalent resistant sequences were downwind than upwind,” said Mayer, a molecular biologist at Tech. “It was not just higher in some of them – it was 4,000 percent more. It made me not want to breathe.”
The study area is not only in one of the most windswept stretches of the country. It is also home to a high concentration of industrial cattle feedlots. In the Panhandle alone, 5 million beef cattle are sent to slaughter every year, according to the Texas Cattle Feeders Association.
“What we have demonstrated is that this mechanism of transport is completely viable,” Smith said of air travel by bacteria. “We believe that this bacteria could remain active for a long period of time and, given the wind that we have around Lubbock, it could be traveling for long distances.”
But he and Mayer are the first to acknowledge that their findings raise a number of questions. Among them: What happens when the DNA of antibiotic-resistant bacteria, traveling on the wind, reaches its destination?
One of their main concerns is that bacteria harboring antibiotic resistance genes will transfer antibiotic-resistant DNA to bacteria in the community where they settle, spreading resistance.
Difficult-to-treat super bacteria have been traced to hospitals – such as in the case of the recent endoscopy super bug – and, increasingly, to livestock raised on industrial feedlots. The well-known MRSA bacteria, which jumped from humans to cattle, where it grew resistant to antibiotics, now kills more than 11,200 people each year.
But representatives of the cattle feeding industry, which Texas A&M AgriLife estimates at 14 million beef cattle across the state, have rebuffed the Texas Tech findings, deeming them partial and inconclusive. Ives, a feedlot veterinarian who teaches at West Texas A&M University, said the study mischaracterizes the threat to humans of bacteria that have grown resistant to antibiotics used on cattle. Many of the antibiotics used on livestock are not used on humans, he explained.
“The judicious use of antibiotics as we are using them today has very little risk to human health,” Ives said. “My children work with me in the feedlot; my wife works with me. Am I concerned for their safety? No, I’m not.”
A growing body of evidence has linked antibiotic resistance in livestock bacteria to human infection, and antibiotic resistance has become an issue of national concern in recent years.
In 2013, the Food and Drug Administration issued guidelines for reducing antibiotic use for growth promotion in livestock, and this year, the Obama administration announced that it would double funding for fighting antibiotic resistance in its proposed 2016 budget.
In his 10 years treating patients at his Lubbock clinic, Dr. Randall Wolcott has seen bacteria grow increasingly resistant to antibiotics used to treat their infections. “We know that antibiotic-resistant genes are becoming more common,” Wolcott said. “We use too many antibiotics for animal health, and it keeps food prices down. Is it a good tradeoff? This research is a piece of the puzzle that will help decide that question.”
In the community of Cotton Center, about 40 miles north of Lubbock, Rocky Stone expressed mixed feelings about the Mayer and Smith’s study. “The cattle industry employs a lot of people in this area,” said Stone, superintendent of the Cotton Center school district. “Until someone shows me proof that there is a problem, I’m not going to worry about it.”
A mile down the road, Elba Saenz swept her front stoop. She has lived in Cotton Center for nearly 40 years and has always noted the smell coming from the Hale Center Feed Yard, 5 miles north of her house. “If someone told me that there were resistant bacteria floating in the wind from the feed yard, I would believe them,” Saenz said. “But even if I am not happy about the feed yard, what am I supposed to do?” she added. “I live here.”