Wild birds, including migratory species, commonly carry salmonella, but the strains of the bacteria they convey usually do not harbor antimicrobial-resistance genes that would make them a top-level threat to human health, according to a nationwide study led by Penn State researchers.
“While we’ve known for a while that wild birds can carry salmonella, the strains they carry appear to be of lesser concern to human health,” Ed Dudley, professor of food science at Penn State, said. “The assumption was that these Salmonella — like the bacteria we can isolate from domesticated farm animals — would carry large numbers of antimicrobial-resistance genes. We found the opposite to be true.”
Wild birds are common reservoirs of Salmonella enterica, a pathogen that sickens millions of people every year, he explained.
Scientists have worried that wild birds carrying antimicrobial-resistant Salmonella enterica pose a risk to public health because they can spread the resistant bacteria across large areas in a short time.
The new study indicates that wild birds do not serve as important reservoirs of resistant Salmonella enterica strains.
The researchers whole-genome sequenced 375 Salmonella enterica strains from wild birds collected in 41 U.S. states from 1978–2019 to examine bacterial resistance to antibiotics and heavy metals.
The study, which was spearheaded by Yezhi Fu, a postdoctoral researcher in Dudley’s research group in the College of Agricultural Sciences, answers important questions about the role migrating birds play in transmitting diseases to humans.
They found that Typhimurium was the dominant Salmonella enterica strain, accounting for 68 percent of the bird isolates. But, less than 2 percent of those isolates were identified as multi-antimicrobial resistant or resistant to heavy metals.
All the multi-resistant Salmonella enterica were isolated from waterbirds or raptors. None of them was isolated from songbirds.
The isolates tested in the study came from the National Wildlife Health Center, which is part of a U.S. Geological Survey lab.
“We worked with the National Wildlife Health Center because it has this genetically amazing collection of Salmonella isolates collected over more than 40 years from sick migratory birds,” he said. “It is an opportunistic collection for us, and somebody just needed to analyze it. It yielded information we couldn’t have gotten anywhere else.”
The researchers also discovered that certain strains of Salmonella were associated with specific hosts. Songbirds and waterbirds were likely to host the same strains, while gulls and terns carried distinct, different lineages of the bacteria.
The study indicates that Salmonella Typhimurium may have undergone evolution within wild birds in the United States.
Contact Marcus Schneck at mschneck@pennlive.com.