The declining effectiveness of antibiotics may make tens of thousands more U.S. patients vulnerable to potentially fatal infections after surgery or chemotherapy, researchers estimate.

Up to half of pathogens causing surgical site infections and more than a quarter of bugs leading to infections after chemotherapy are already resistant to antibiotics commonly used in the U.S., the researchers calculate based on an analysis of previously published studies.

As antibiotic resistance worsens, those percentages may climb, threatening the safety and effectiveness of many surgical procedures and medications commonly deployed to battle cancer, the study team concludes.

"What we estimate in the paper are the additional infections and deaths for these patients who need antibiotics in order to safely get through surgery and chemotherapy," senior study author Ramanan Laxminarayan of the Center for Disease Dynamics, Economics and Policy in Washington, D.C., said by email.

If, for example, antibiotic effectiveness drops by another 30 percent from current levels, that might result in an additional 120,000 infections for U.S. cancer and surgery patients and 6,300 infection-related deaths each year, according to the results published in The Lancet Infectious Diseases.

In this scenario, most of the additional deaths would occur for patients undergoing colorectal surgery, total hip replacement and chemotherapy for blood cancers.

Even a 10 percent reduction in antibiotic effectiveness could mean 40,000 more infections and 2,100 additional deaths a year after surgery and chemotherapy, the researchers calculate.

A 70 percent decline in antibiotic effectiveness might lead to 280,000 more infections and 15,000 more deaths.

These projections are based on an analysis of infection rates found in previous studies of surgeries and chemotherapy treatments that often involve patients receiving what's known as prophylactic, or preventive, antibiotics to ward off infections before they develop.

Researchers focused on the ten most common surgeries that involve prophylactic antibiotics: hip fracture surgery, pacemaker implantation, surgical abortion, spinal surgery, total hip replacement, cesarean section, transrectal prostate biopsy, appendectomy, abdominal hysterectomy and colorectal surgery. They found infection data for all of the procedures, but lacked data on infection-related death rates for some operations.

For prostate biopsies alone, they estimate that 42 percent of post-procedure infections - 13,320 cases - every year in the U.S are currently caused by bacteria resistant to a single class of widely used antibiotics, fluorquinolones.

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They also examined chemotherapies that are often accompanied by prophylactic antibiotics in the treatment of the blood cancers leukemia, lymphoma and myeloma. One study included in the analysis found nearly 27 percent of infections in these patients were caused by resistant microbes.

The previous studies were done from 1968 to 2011, mostly in Europe, Canada and other countries outside the U.S., and the definitions of infections were not uniform across all of these studies, the authors acknowledge.

It's also possible that growing awareness of antibiotic resistance and more fine-tuned efforts to prevent health-care associated infections might mean the impact of resistance on infections for cancer and surgery patients is smaller than projected in this analysis, the researchers note.

Even so, the findings offer more evidence of the impact of antibiotic resistance on outcomes for patients who rely on these treatments to prevent infections after surgery and chemotherapy, Joshua Wolf, an infectious disease researcher at St. Jude's Children's Research Hospital in Memphis, Tennessee, writes in an accompanying editorial.

"As antibiotic resistance rates rise, it is inevitable that prophylaxis will become less effective," Wolf told Reuters Health by email.

"It is very likely that surgical procedures will become less safe for patients," Wolf added. "Treatment for cancer would also become more challenging as antibiotic resistant bacteria become more prevalent."

To slow the trajectory, doctors can avoid prescribing antibiotics for coughs and colds unless they have evidence of a bacterial infection, Wolf added. Patients can also avoid pressuring doctors to give them these prescriptions to treat a common cold.

Consumers, too, can help by refusing to buy meat that was raised with antibiotics for growth promotion, Wolf noted.

"The broad solution is for everyone to do a better job on infection control and conserving the effectiveness of the antibiotics we have," Laxminarayan said.