A Drug-Resistant Salmonella Newport Strain Tied to Travel to Mexico and U.S. Beef

Infections with a multidrug-resistant strain of Salmonella Newport, known as REPJJP01, roughly doubled in the United States in 2021 and stayed high in 2022. They are linked to travel to Mexico, to beef and cheese bought in Mexico, and to beef products bought in the United States.

Background

CDC identified the strain in 2016 and now monitors it as a persisting strain; it has been found in people in all 50 states and the District of Columbia. A 2018–2019 investigation tied it to travel to Mexico, beef products obtained in the U.S. and cheese from Mexico. A jump in reports to CDC's PulseNet database in 2021 prompted this look at 2021–2022.

Isolates were genome-sequenced at public health and federal labs; REPJJP01 was defined as S. Newport within 0–21 allele differences, and isolates were screened for resistance genes through the National Antimicrobial Resistance Monitoring System. Multidrug-resistant (MDR) meant resistance to three or more classes of antimicrobials.

Rising cases

Isolates of multidrug-resistant Salmonella Newport strain REPJJP01 by month of illness onset or sample collection and source, 2016–2022. CDC.

• Human isolates rose from an average of 315 a year in 2018–2020 to 641 in 2021, and remained high in 2022.
• Across 2021–2022 there were 1,308 isolates from patients, cattle and sheep, including 25 from cattle (beef products and gut samples) and one from a sheep, in 13 states.
• 1,282 U.S. residents had culture-confirmed infections. The true number is likely far larger: about 29 more cases are estimated for every confirmed one. The rise came even as overall Salmonella reports fell during the COVID-19 pandemic.

Who got sick

• Median age 37.5; 52% female; 56% Hispanic or Latino; 19% lived in California and 17% in Texas.
• Of 721 patients with data, one third (33%) were hospitalised and two died. Hispanic patients were hospitalised more often (40%) than non-Hispanic White patients (24%) or those of other races (22%).
• 87% of patient isolates were MDR, and 89% were resistant or less susceptible to at least one antibiotic recommended for treatment, most with decreased susceptibility to azithromycin. Most food and animal isolates were resistant too; 65% were MDR.

Sources of infection

• Travel to Mexico: of 721 patients with a known travel history, 48% had been to Mexico in the month before falling ill, far above the 9% of all non-typhoidal Salmonella patients reporting any international travel. Travel was more common in 2022 (53%) than 2021 (39%). Travellers were less often Hispanic (43% against 64%) and less often hospitalised (24% against 37%).
• Food from Mexico: eleven patients who hadn't travelled ate foods such as queso fresco or dried beef bought in Mexico by family or friends.
• U.S. beef: a 75-patient multistate outbreak in fall 2021, 80% of patients Hispanic, pointed to beef, including dried beef; a second, with 22 non-Hispanic patients in fall 2022, turned up the strain in a patient's leftover ground beef, but the beef couldn't be traced to a source, so no regulatory action was possible. Six small clusters each included a patient and a beef isolate.

The strain may reach the U.S. through returning travellers, through cattle born or raised in Mexico and slaughtered here, or through imported beef or cheese, and may also spread within the country: it has been found at federally inspected slaughter and processing plants nationwide and at import facilities on the southern border. Beef is hard to trace in outbreaks, because people eat it at home and out, receipts are rarely kept, and ground beef mixes many sources.

Why it matters

Patients with this strain were hospitalised more often (33% of all, 37% of non-travellers, 40% of Hispanic patients) than the 27% of non-typhoidal Salmonella patients in FoodNet in 2021–2022, consistent with resistant infections being more severe. Multidrug resistance narrows treatment options, worsens outcomes and lets resistance genes spread.

What to do

• Clinicians: expect possible multidrug resistance in recent travellers to Mexico with salmonellosis, and order susceptibility testing when treatment is needed.
• Travellers to Mexico: follow food and water safety advice and avoid beef, including jerky and dried beef, and other food from street vendors. CDC posted a Level 1 Travel Health Notice in September 2022.
• Consumers: eat beef only when thoroughly cooked, check temperatures with a thermometer, and wash hands and surfaces after handling raw meat.
• The beef industry: prevention is needed along the whole chain, from herd management and biosecurity to sanitary slaughter, antimicrobial treatments, processing and retail.

Limitations

Recorded cases undercount illness; not every patient was interviewed; and travel may be under-reported, since some patients were asked only about the 4 or 7 days before falling ill. The effect of reduced azithromycin susceptibility on treatment needs more research.

Sources

• Ford L, Ellison Z, Schwensohn C, et al. "Strain of Multidrug-Resistant Salmonella Newport Remains Linked to Travel to Mexico and U.S. Beef Products — United States, 2021–2022," MMWR. https://www.cdc.gov/mmwr/volumes/72/wr/mm7245a3.htm