Multidrug-Resistant Salmonella Newport Outbreak Linked to U.S. Beef, Mexican Soft Cheese

By Will Boggs MD

September 09, 2019

NEW YORK (Reuters Health) - A recent multistate outbreak of infections with multidrug-resistant Salmonella enterica serotype Newport with decreased susceptibility to azithromycin was linked to U.S. beef and Mexican cheese, according to an epidemiologic investigation.

"Finding the outbreak strain in both cheese and beef indicates that the human illnesses likely originated from presence of the bacteria in cattle in the United States and Mexico," Dr. Ian D. Plumb of the Centers for Disease Control and Prevention (CDC), in Atlanta, told Reuters Health by email.

"It also highlights how antibiotics are a precious resource - unnecessary use of antibiotics increases the risk of resistant bacteria spreading," he added. "Avoiding unnecessary use of antibiotics in cattle that are also used to treat human infections could help prevent the risk of resistant bacteria spreading from cattle to cause human illness."

Between June 2018 and March 2019, 255 cases of infection with the outbreak strain were identified in 32 states, including 10 cases with bacteremia and two deaths, Dr. Plumb and colleagues report in the August 23 issue of Morbidity and Mortality Weekly Report (MMWR).

Most patients (65%) were Hispanic, and 43% reported having visited Mexico in the seven days before the onset of their illness.

Among patients who traveled to Mexico, 87% reported eating beef and 63% reported eating soft cheese, most commonly queso fresco, a cheese typically made with raw, unpasteurized milk from cows or goats.

Among those who did not travel to Mexico, 29% reported eating Mexican-style soft cheese and 93% reported eating beef.

In September 2018, the outbreak strain was detected in samples from a steer at a slaughter and processing plant in Texas; in October 2018, it was detected in a mixture of queso fresco and Oaxaca soft cheese purchased in Tijuana, Mexico; and in November 2018 and March 2019, the outbreak strain was detected in beef samples at two Texas slaughter and processing facilities.

Among patients with treatment information, 75% received antibiotic therapy, but 33% received an antibiotic to which the outbreak strain was resistant or showed decreased susceptibility.

"For patients with invasive Salmonella or with risk factors for invasive disease, prompt antibiotic treatment is indicated," Dr. Plumb said. "When giving antibiotics, it's important to test to make sure that the antibiotics given will work in a particular patient. If a patient is suspected to have the outbreak strain and needs antibiotics, it's important for clinicians to know that some of the commonly recommended antibiotics may not work, and alternative injectable antibiotics may be needed."

He added, "There are measures that you can take to prevent infection with the outbreak strain. If you eat beef, make sure that the beef is cooked to a safe internal temperature, using a food thermometer. Ground beef, including hamburgers, should be cooked to at least 160 F, and steaks and roast to at least 145 F. After cooking, it's best to wait for 3 minutes before cutting or eating beef."

"If you eat soft cheese, make sure that the label says, 'Made with pasteurized milk,'" he said.


MMWR Morb Mortal Wkly Rep 2019.