Determining the Incidence of Drug-Resistant Salmonella Infections in the US

Salmonella infection, often caused by consuming contaminated animal products such as beef, chicken eggs, and dairy, as well as produce, is fairly common in the US. Nontyphoidal Salmonella is responsible for about 1 million infections in humans every year, 19,000 of which are severe enough to require hospitalization and roughly 380 of which result in death. Unfortunately, an increasing number of Salmonella infections are developing resistance to one or more traditional drugs commonly used to kill the bacteria, namely ceftriaxone, ciprofloxacin, and ampicillin. What percentage of cases are resistant, and how can resistance be determined early enough to avoid prescribing the wrong medications that could possibly enable poor outcomes?

Researchers at the Centers for Disease Control and Prevention (CDC) examined records culled from the National Antimicrobial Resistance Monitoring System (NARMS), which tests isolate samples from Salmonella patients to come up with the percentage that display antibiotic resistance. The team took NARMS data that was collected from the lower 48 states between 2004 and 2012. That data was accompanied by information provided by the National Laboratory-based Enteric Disease Surveillance (LEDS) system, which compiles data on the number and location of Salmonella infections. The team used records from both sources, in addition to information from the US Census Bureau, to come up with estimates of the incidence of resistant Salmonella infection in the US.

The isolates that the team received were serotyped to determine what kind of nontyphoidal Salmonella infections they were, as most drug-resistant infections of this kind originate from the following Salmonella types: Typhimurium, Enteritidis, Newport, and Heidelberg. The team created three mutually exclusive categories of disease resistance that they wanted to explore: ceftriaxone and ampicillin resistant, ciprofloxacin nonsusceptible but ceftriaxone susceptible, and ampicillin resistant but ceftriaxone and ciprofloxacin susceptible. (“Nonsusceptible” incorporates both full resistance and intermediate resistance, according to the study authors.) They were then able to pinpoint the resistance rates of different Salmonella serotypes during that nine-year time period.

Antibiotic resistance was found in 12% of the isolates overall. The most common was ampicillin-only resistance, which applied to 6.5% of infections, most of which was of the strain Typhimurium. Next was ceftriaxone and ampicillin resistance, which was seen in 3.1% of the isolates, one-third of which were of the Newport strain, 27% of which were of the Typhimurium strain, and 15% of which were of the Heidelberg strain. Nonsusceptibility to ciprofloxacin was seen in 2.4% of the samples, of which 45% were of the Enteritidis strain; one-fifth of the samples nonsusceptible to ciprofloxacin were also resistant to ampicillin. In the study, the incidence of resistant infections was expressed in infections per 100,000 person-years. The researchers determined that any clinically significant resistance occurred in 1.93 out of 100,000 person-years. Broken down, ampicillin-only resistance was seen in 1.07 person-years, ceftriaxone/ampicillin resistance was seen in 0.51 person-years, and ciprofloxacin nonsusceptibility occurred in 0.35 person-years. It’s important to note that many of the isolates that were resistant to the antibiotics covered in this study are resistant to other agents as well.

Although the study team estimates that there are about 6,200 resistant culture-confirmed cases of Salmonella infection in this country every year, the CDC says the vast majority of Salmonella cases go undiagnosed; as many as 29 undetected cases occur for every single case confirmed by culture, and some of these may be resistant. Due to the fact that resistant infections make patients more susceptible to other serious illnesses, it’s important to determine whether a patient actually has Salmonella, what strain of Salmonella it is, and whether or not the infection is resistant. “As part of patient management, clinicians typically request antimicrobial susceptibility testing before or when antibiotic treatment is started,” said Felicita Medalla, MD, an epidemiologist at the CDC and an author of the study. “Drugs of choice are based on susceptibility of the organism (if known), knowledge of the antimicrobial susceptibility patterns of prevalent strains, site of infection, host, and clinical response.” However, according to Dr. Medalla this information is not always available early enough: “Clinicians often need to consider antibiotic treatment before susceptibility results are available. The antimicrobial susceptibility patterns of prevalent strains are among their important considerations. In addition to national data on resistance, more local data, including regional data, may help inform the selection of antibiotics for treatment.” His team’s study found resistant infections to be higher in certain regions of the US, a discovery that merits further investigation as it may have implications for future Salmonella treatment.

As always, the best treatment is prevention. Due to the fact that antimicrobial drugs fed to animals bred for human consumption are a major cause of Salmonella infection, the US Food and Drug Administration is working to tighten regulations in this area. Experts recommend avoiding raw eggs, undercooked beef or chicken, keeping refrigerators cold enough, and cooking foods to a safe internal temperature as prevention strategies for consumers to use.

Laurie Saloman, MS, is a health writer with more than 20 years of experience working for both consumer and physician-focused publications. She is a graduate of Brandeis University and the Medill School of Journalism at Northwestern University. She lives in New Jersey with her family.