The Centers for Disease Control and Prevention (CDC) reported today that human infections caused by a strain of multidrug-resistant (MDR) Salmonella linked to Mexico rose significantly in 2021 and 2022.
In a report published today in Morbidity and Mortality Weekly Report, investigators with the CDC, the US Department of Agriculture, and several state and local public health departments said the number of clinical isolates of an MDR strain of Salmonella enterica Newport doubled in 2021 compared with the 2018 to 2020 baseline and remained high in 2022. The strain, which was first identified in the United States in 2016, is linked to travel to Mexico, consumption of cheese obtained in Mexico, and beef from Mexico and the United States.
The strain—named REPJJP01— has now been detected in all 50 states and the District of Columbia and has caused several multistate outbreaks.
Strain linked to Mexico
From June 2018 to March 2019, a multistate outbreak caused by the REPJJP01 strain led to 255 infections and 60 hospitalizations. Investigation of that outbreak revealed that the infections, 43% of which involved people who had traveled to Mexico, were linked to Mexican-style soft cheese obtained in Mexico.
But the investigators also found links to beef products from Mexico and the United States, suggesting the strain was present in cattle from both countries.
The authors of today's report said an increase in reports of REPJJP01 to the CDC's PulseNet database—the national subtyping network for foodborne bacterial disease surveillance—in 2021 prompted another investigation, which involved whole-genome sequencing of clinical isolates and interviews with case-patients to obtain travel and food-exposure information.
The 641 human isolates obtained in 2021 and 2022 were more than twice the annual baseline number of cases detected from 2018 through 2020 (315). But the authors say the number of illnesses is likely higher, with an estimated 29 cases of Salmonella for each culture-confirmed case.
The increase in infections from this MDR strain is concerning because it limits treatment options, has more severe outcomes, and creates opportunities for resistance genes to spread.
Of the 1,282 people with culture-confirmed infections caused by REPJJP01 in 2021 and 2022, 56% were Hispanic or Latino. Of the 721 patients who had known travel history, 48% reported traveling to Mexico in the month before illness onset. Eleven case-patients who did not report any travel said they had eaten foods, including queso fresco and dried beef, bought in Mexico by family or friends.
Of the 721 patients with hospitalization data, 247 (33%) were hospitalized and 2 died. The vast majority of patient isolates (1,141, 89%) were resistant or had reduced susceptibility to at least one antibiotic recommended for treatment, and 1,110 (87%) were MDR.
"The high hospitalization rate is consistent with studies indicating that patients with antimicrobial-resistant Salmonella infections are more likely to be hospitalized," the authors wrote. "The increase in infections from this MDR strain is concerning because it limits treatment options, has more severe outcomes, and creates opportunities for resistance genes to spread."
Multiple pathways for transmission
Additional investigation of two multistate outbreaks in 2021 found that beef, including dried beef, was a suspected vehicle in one of the outbreaks. A sample of ground beef containing a strain of Salmonella Newport that was genetically indistinguishable from the clinical isolates was the confirmed vehicle in the other outbreak. Most of the 25 isolates analyzed from cattle (beef products and cecal samples) were resistant, and 65% were MDR.
The findings indicate the strain has multiple pathways for transmission.
"This strain might spread to the United States through returned travelers from Mexico, cattle born or raised in Mexico and slaughtered in the United States, or beef or cheese imported from Mexico," the authors wrote. "The REPJJP01 strain might also spread within the United States through animals or beef products."
Salmonella causes an estimated 1.35 million illnesses and 26,500 hospitalizations in the United States each year.
The CDC says it is continuing to work with local and state health departments to identify sources of infection. In the meantime, it urges clinicians to be aware of the potential for multidrug resistance in travelers to Mexico with salmonellosis, and warns consumers to follow food safety practices while abroad, such as avoiding beef or other foods sold by street vendors.