Salmonella showing more resistance to antibiotics

Increased resistance, linked to the use of antibiotics in animal feed, poses the greatest treatment challenges for young children.

By Victoria Stagg Elliott, amednews staff. Jan. 22, 2001.

Salmonella resistant to the antibiotic most commonly used to treat children severely infected by the bacterium has emerged in the United States, according to a study published in the Dec. 27, 2000, JAMA.

The Centers for Disease Control and Prevention study found that in 1996, 0.1% of all cases of salmonellosis nationwide were resistant to ceftriaxone. The rate increased to 0.5% in 1998. Early data from 1999 indicate that 1.9% of infections had reduced susceptibility, although testing is still incomplete.

Emergence of a tough bug
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"When you start to see emerging resistance, it is a great cause of concern because it's one less treatment option that will be open to you in the future," said Tamar Barlam, MD, director of the antibiotic resistance project at the national nonprofit Center for Science in the Public Interest, Washington, D.C.

According to additional research, significantly higher prevalence of ceftriaxone-resistant Salmonella infections have been found in at least one state. In Nebraska, the prevalence of drug-resistant Salmonella has increased to 6%.

The Nebraska data will be presented at this year's American Society of Microbiology annual conference in a paper by Paul Fey, PhD, one of the authors of the JAMA study and associate director of the clinical microbiology lab at the Nebraska Public Health Laboratory in Omaha.

Although antibiotic-resistant salmonellosis has appeared in the United States before, the cases mostly were imported from overseas. However, most of the cases found by the CDC between 1996 and 1998 are believed to have been acquired domestically. Nonetheless, salmonellosis resistant to antibiotics used to treat adults remains extremely rare here.

"For adults it's not an issue yet," said Dr. Fey. "For children it's becoming a major issue."

Like most patients with Salmonella infection, the subjects in the study generally were not treated with antibiotics -- which are used primarily in severe cases when the bacterium enters the bloodstream -- and were able to combat the infection on their own. But experts worry that it is only a matter of time before a drug-resistant strain causes a life-threatening infection that will be difficult to treat.

"When a patient gets a serious Salmonella infection and it's ceftriaxone resistant, that's going to be a huge problem," said Dr. Fey. "Invasive cases are pretty rare and resistant cases are pretty rare, but we predict that we will start seeing a combination of the two if the prevalence increases as it's showing right now."

Treatment options still exist for patients, but they are limited. Many antibiotics are not approved for use in those younger than 18.

"Ceftriaxone-resistant Salmonella takes away the commonly used antimicrobial for these sorts of infections," said Eileen Dunne, MD, MPH, primary author of the study and an officer with the CDC's epidemiology program office.

Infection with a drug-resistant strain can also result in delays in treatment and longer hospital stays. Ceftriaxone is the first-line treatment for children, and it takes a minimum of two days before the results from susceptibility tests, which reveal the presence of resistance, are available.

"Susceptibility testing takes time," said David Acheson, MD, associate professor of medicine at Tufts University in Boston. "The issue becomes: When you don't quite know what you're dealing with, what do you use? Once you know what it's susceptible to, it's easy. But in those two days, the patient can get pretty sick."

Growing resistance on the farm

Researchers say that unlike many incidences of organisms developing resistance that have been traced to human overuse or misuse of antibiotics, drug-resistant Salmonella is more likely a result of antibiotics used in the feed eaten by animals that produce human food.

"Food-producing animals are the source of most Salmonella infections, so the use of antimicrobial agents in these animals helps to select for drug-resistant Salmonella, which can be transmitted to humans," said Dr. Dunne.

The use of antibiotics on the farm is controversial.

"Antibiotics are useful to treat animals as well as humans, and I don't think it's appropriate to withhold reasonable therapy for sick animals," said Dr. Acheson. "Problems arise when we start to use antibiotics inappropriately either for growth promotion or flockwide treatment. And, OK, a sick animal needs an antibiotic. Is it appropriate to use an antibiotic that we would otherwise use in a human?"

To address the problem, the Food and Drug Administration Center for Veterinary Medicine issued a discussion paper last month outlining possible approaches for establishing thresholds for the use of antimicrobial drugs in food-producing animals. The center will also be holding a meeting this month about the issue.

ADDITIONAL INFORMATION:

Emergence of a tough bug

Objective: To summarize national surveillance data for ceftriaxone-resistant Salmonella in the United States.
Participants: Patients infected with Salmonella between 1996 and 1998.
Method: Human isolates were submitted to the Centers for Disease Control and Prevention from 17 state and community health departments participating in the National Antimicrobial Resistance Monitoring System for enteric bacteria. Patients with resistant strains were then interviewed.
Results: In 1996, 0.1% of patients with salmonellosis were found to be infected with the drug-resistant version of the bacterium. In 1998, that prevalence increased to 0.5%. Of those with resistant strains, 91% did not travel outside the United States before onset of illness.
Conclusion: Domestically acquired ceftriaxone-resistant Salmonella has emerged.

Source: JAMA, Dec. 27, 2000

Weblink

Abstract of JAMA report on "Emergence of Domestically Acquired Ceftriaxone-Resistant Salmonella Infections Associated with AmpC Beta-Lactamase" (http://jama.ama-assn.org/issues/v284n24/abs/joc00900.html)

CDC National Antimicrobial Resistance Monitoring System (http://www.cdc.gov/ncidod/dbmd/narms/)