Infections from antimicrobial-resistant strains of Salmonella pose a serious threat to public health, and according to new research published by the Centers for Disease Control and Prevention, these types of infections are on the rise.

In a study published by the CDC in Emerging Infectious Diseases, researchers estimated a 40 percent increase in the annual incidence of infections with clinically important resistance during 2015–2016. They estimated 222,000 infections, compared with the 2004–2008 estimate of 159,000 infections. This is an estimated 63,000 more infections a year. The study says that these estimates and further analysis of changes will help in setting targets and priorities for prevention. 

The CDC estimates that nontyphoidal Salmonella infections cause an estimated 1.2 million illnesses, 23,000 hospitalizations and 450 deaths each year in the United States. Although most infections result in self-limiting illnesses, antimicrobial treatment is recommended for patients with severe infection or at high risk for complications. Antimicrobial-resistant Salmonella infections can cause adverse clinical outcomes, including increased rates of hospitalization, bloodstream infections, other invasive illnesses, and death. 

The study, led by Felicita Medalla, MD, used Bayesian hierarchical models of culture-confirmed infections during 2004–2016 from two CDC surveillance systems to estimate changes in the national incidence of resistant nontyphoidal Salmonella infections.  

The study did not examine the causes of the rise of antibiotic resistance in Salmonella, the authors did provide a few possible sources — animal antibiotic use and international travel.

Animal antibiotic use
The use of medically important antibiotics in U.S. pork production is widespread. A 2019 report from the U.S. Department of Agriculture showed that just more than 90 percent of swine operations gave pigs medically important antibiotics. The study notes that pork products have been linked to resistant infections in the West. The CDC also cites a study that identified multidrug-resistant strains in swine in the Midwest, some of which carried plasmid-mediated resistance genes.

The study notes that the increase in ampicillin-only and multidrug-resistant Salmonella infections was highest in the West and Midwest regions. These two areas of the country have high pork production and consumption.

International travel
Nontyphoidal Salmonella infections can be acquired during international travel, from contaminated food and water, through animal contact and from environmental sources, such as wetlands and irrigation water. Antimicrobial-resistant infections have been linked to various food and animal sources.

Chicken and eggs have been the main domestic sources of Salmonella Enteritidis infections. About 20 percent of Enteritidis infections are linked to international travel, which is an important source of ciprofloxacin-nonsusceptible Salmonella Enteritidis infections.

International travel could have contributed to an increase in the incidence of ciprofloxacin-nonsusceptible Salmonella Enteritidis infections, which increased in three regions and was highest in the Northeast. International travel has increased since 2014, and residents of northeastern states accounted for more than one-third of U.S. travelers during 2015–2016.

In the United Kingdom, an increase in these infections has been linked to international travel and imported foods. In the United States, ciprofloxacin-nonsusceptible strains of Salmonella Enteritidis and other serotypes have been isolated from imported seafood.

The full study can be read here.

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