Salmonella bacteria, a common cause of food poisoning, invade an immune cell. CREDIT: National Institute of Allergy and Infectious Diseases, National Institutes of Health

Researchers with the Centers for Disease Control and Prevention and the Minnesota Department of Health late last week reported an increase in infections caused by a multidrug-resistant (MDR) strain of Salmonella linked to pigs.

In a study published in Emerging Infectious Diseases, the researchers looked at data from five national surveillance systems to describe the epidemiology, antimicrobial resistance, and molecular genetics of infections caused by Salmonella enterica I serotype 4, [5], 12:i:-, which is the fifth most commonly reported illness-causing Salmonella serotype in the country. Infections caused by this strain have been rising since the 1990s, and recent years have seen the emergence of isolates in this strain with resistance to ampicillin, streptomycin, sulphamethoxazole, and tetracycline (ASSuT).

Resistance pattern

Among Salmonella isolates reported to the Laboratory-based Enteric Disease Surveillance System from 2009 to 2018, 19,212 (4.3%) from 37 states were 4, [5], 12:i:-. Forty-nine percent of the 4, [5], 12:i:- isolates included in National Antimicrobial Resistance Monitoring Systems surveillance had a resistance pattern that included ASSuT, and 35% had only ASSuT resistance. From 2009-2013 to 2014-2018, the frequency of 4, [5], 12:i:- infections rose from 3.7% to 4.9% of reported Salmonella infections, and the percentage of 4, [5], 12:i:- with ASSuT resistance rose from 1.1% to 2.6%.

Among isolates sequenced by PulseNet from 2015 to 2018, 69% were in the same MDR phylogenetic clade, and 77% of the isolates within that clade had genetic determinants of ASSuT resistance, while 16% had genetic determinants of decreased susceptibility to ciprofloxacin, ceftriaxone, or azithromycin.

Likely from swine

Among MDR 4, [5], 12:i:- outbreaks reported from 2009 to 2018, 63% were associated with pork consumption or contact with swine.

“Our findings indicate that 4,[5],12:i:- infections have increased in the United States because of an MDR clade that has expanded since 2010,” the study authors wrote. “Illness is likely to have resulted from transmission from swine that carry it. Further selection for this strain in swine might be prevented by limiting unnecessary agricultural use of classes of antibiotics to which the strain has resistance and by limiting unnecessary use of heavy metals in feed.”

Visit University of Minnesota Center for Infectious Disease Research & Policy for more.