In the current CDC’s Morbidity and Mortality Weekly report, some bacteria in a Salmonella Newport outbreak that was linked to beef and to a cheese imported from Mexico had decreased susceptibility to azithromycin and nonsusceptibility to ciprofloxacin. This susceptibility has emerged recently, because that serotype with decreased susceptibility to azithromycin wasn’t found in any isolates in the National Antimicrobial Resistance Monitoring System (NARMS) surveillance before 2016.
A genetically distinct group of MDR Newport isolates was identified in the two outbreaks. This resistance is rare among Salmonella serotypes that cause illness in the United States. Azithromycin is recommended to treat Salmonella infections orally; until 2017, decreased susceptibility to this antibiotic occurred in less than 0.5% of Salmonella isolates taken from U.S. patients.
The report states that the outbreaks, linked to Mexican-style soft cheese obtained in Mexico and beef obtained in the U.S., sickened 255 people with this antibiotic-resistant pathogen, and hospitalized 60 of them. Forty-three percent of the patients reported recent travel to Mexico.
While 29% of patients who provided information were hospitalized, 6% were admitted to an intensive care unit. Four percent developed Salmonella bacteremia, and two patients died.
Among patients who ate cheese from Mexico, the most frequently consumed cheese they could recall eating was queso fresco, a soft cheese that is usually made from raw, unpasteurized milk from cows or goats. The outbreak linked to cheese was not reported to the public.
The outbreak strain was fond in a cecal sample (a pouch at the beginning of the large intestine) taken from a steer collected at a slaughter and processing plant in Texas as part of surveillance. In October 2018, the outbreak strain was also found in a mixture of queso fresco and Oaxaca soft cheese that was purchased in a market in Tijuana, Mexico. Then the outbreak strain was found in beef samples collected in November 2018 and March 2019 at two Texas slaughter facilities.
Since consumption of cheese and consumption of beef were both associated with illness, the report states that dairy cattle are a likely source of these infections. In addition, the genetic similarity between beef in Mexico, beef in the United States, and a steer in the U.S. “strongly suggests that the outbreak strain is present in cattle in both countries.” And because the use of antibiotics in livestock can cause selection of resistant strains, the reported 41% rise in macrolide use in U.S. cattle from 2016 to 2017 may have accelerated carriage of the outbreak strain among U.S. cattle.
Officials think that the contamination happened from “carriage by cattle,” rather than contamination from poor hygiene during cheese production. Dairy cattle are often used as a source of ground beef.
The report states that consumers should not eat soft cheese that could be made from unpasteurized milk. In addition, when preparing beef, always use a food thermometer to check to make sure a safe final internal temperature is reached. Those temperatures are 145°F for steaks and roasts followed by a 3 minute rest time, and 160°F for ground beef. Finally, watch out for cross-contamination between raw beef and other meats and foods that are eaten uncooked.
