New CDC Study Shows an Increase in Antibiotic Resistance in Salmonella

The number of illnesses, hospitalizations, and deaths caused by Salmonella have been steadily rising each year, and now findings from a new report on nontyphoidal Salmonella enterica show that some 20% of blood isolates of the bacteria have achieved antibiotic resistance.

In a recent study conducted by the Centers for Disease Control and Prevention (CDC) and published in the Journal of Infectious Diseases, researchers looked at isolates of Salmonella collected by health departments around the United States from 2003 to 2013. The collection and tracking of these samples is part of the CDC’s National Antimicrobial Resistance Monitoring System (NARMS), which, since 1996, has been monitoring resistance patterns among pathogens in humans. Public health laboratories from all 50 states participate in NARMS by submitting every twentieth nontyphoidal Salmonella isolate to the CDC laboratory, along with information on the patient’s age and sex, the specimen source, and serotype.

Salmonella infection outbreaks have continued to make headlines, and in the United States, infections from the bacteria result in some 1.2 million illnesses, 23,000 hospitalizations, and 450 deaths each year, according to the CDC. Contaminated foods—and surfaces that have come into contact with them—cause most Salmonella infections, and the majority of such infections come from nontyphoidal Salmonella enterica. Individuals suffering from salmonellosis often develop a fever, diarrhea, and abdominal cramps within 12 to 72 hours of becoming infected, with illness typically lasting 4 to 7 days. While a typical Salmonella infection resolves without medical treatment, severe cases can result in hospitalization and the infection can become deadly without antibiotics. In the most dangerous of cases, the bacteria spreads from the intestines to the bloodstream, causing Salmonella bacteremia. Certain serotypes are more prone to cause such invasive infections. Typhoidal Salmonella serotypes lead to typhoid fever, but are less common than the nontyphoidal serotypes. The CDC states that children under 5 years old, adults ages 65 and older, and those with compromised immune systems are most at risk for severe infections.

The study by the CDC analyzed blood and stool isolates of Salmonella collected by NARMS, testing for susceptibility to more than a dozen antibiotic medications commonly prescribed by doctors. Antibiotics such as fluoroquinolones and third-generation cephalosporins are most commonly given to treat Salmonella infections. Cases of bacteremia are most common in drug-resistant serotypes and hospitalizations are more often the result of serotypes with ceftriaxone resistance. The 20,866 isolates studied in this CDC analysis included 19,677 from stool and 1,189 from blood; a little more than half of the samples came from females. Of the blood isolates, 868 were susceptible to all antibiotic agents tested, and among the 321 blood isolates resistant to at least one agent, 237 were resistant to first-line treatments. Most of those serotypes were Typhimurium, Heidelberg, and Dublin.

The researchers noted some new observations in comparing their findings to a previous study that had been conducted from 1996 to 2007. Resistance to ceftriaxone, a first-line treatment for Salmonella bacteremia, has doubled since that study, indicating that treatment recommendations for these infections may soon need to be updated. Fluoroquinolone resistance also doubled. “It is likely that the overall distribution of Salmonella serotypes in blood and stool is constantly evolving and serotypes that are commonly resistant may be increasing disproportionally in blood over time,” note the authors in the study. “Salmonella blood isolates were more likely than stool isolates to be resistant to more than one agent and to first-line treatment agents. Resistance to first-line treatment agents in patients with Salmonella bacteremia is a concern for public health and clinical outcomes and is important for informing clinical decisions regarding appropriate treatment.”

The authors note that antibiotics overprescribed for human infections as well as those added to animal feed are both large contributors to the emergence and spread of antibiotic resistance in Salmonella and other bacteria.

To prevent the risk of exposure to Salmonella, the CDC recommends some of the following precautions:

• Cook poultry, ground beef, and eggs thoroughly. Do not eat or drink foods containing raw eggs, or raw (unpasteurized) milk.
• If you are served undercooked meat, poultry or eggs in a restaurant, don't hesitate to send it back to the kitchen for further cooking.
• If you are served undercooked meat, poultry or eggs in a restaurant, don't hesitate to send it back to the kitchen for further cooking.
• Be particularly careful with foods prepared for infants, the elderly, and the immunocompromised.
• Wash hands with soap after handling reptiles, birds, or baby chicks, and after contact with pet feces.