Serious problems with Salmonella Newport strains that have decreased susceptibility to azithromycin have been documented by scientists in a review of an outbreak involving beef from the United States and some raw milk cheeses made in Mexico. 

Seventeen researchers authored the report that is posted in a CDC publication. It details a deadly 32-state outbreak that was confirmed to have lasted from June 2018 through March 2019 in the United States. Less than half of the victims reported traveling to Mexico before becoming ill.

The 255-patient outbreak involved multidrug-resistant Salmonella Newport. Azithromycin is a preferred treatment for salmonellosis, according to the report in the Morbidity and Mortality Weekly Report posted by the Centers for Disease Control and Prevention.

Specific properties of the outbreak strain make it rare and more dangerous than other forms of Salmonella that cause illness in humans, according to scientists. The more dangerous Salmonella is also linked to two other U.S. outbreaks. 

“The presence of resistance genes on a plasmid is concerning because of the potential for spread to other bacteria,” according to the published research. “The outbreak strain appears to have emerged recently because Newport with decreased susceptibility to azithromycin was not detected in animal, retail meat, or human isolates in NARMS (National Antimicrobial Resistance Monitoring System) surveillance before 2016. 

“During 2016-2017, two smaller multistate clusters of MDR Newport infections with decreased susceptibility to azithromycin were investigated among U.S. residents; isolates were within 11 alleles (forms of a gene that are responsible for hereditary variation) of the current outbreak isolates.”

In addition to the Salmonella Newport’s reduced susceptibility to azithromycin, researchers reported 252 isolates with resistance information, 90 percent had predicted resistance to trimethoprim-sulfamethoxazole, tetracycline, and chloramphenicol. In 57 percent of the isolates, there was additional predicted resistance to ampicillin and streptomycin, and nonsusceptibility to ciprofloxacin. 

Without the frontline antibiotics detailed in the research report, treatment of a growing number of human Salmonella infections will be difficult and could result in a higher percentage of fatalities. 

While the outbreak strain was found in soft cheeses made with raw milk and purchased in Mexico were responsible for some of the outbreak illnesses, the researchers reported beef from the U.S. also sickened people with the outbreak strain of Salmonella Newport. Many of the infected people had visited Mexico, but more than half had not.

“The genetic similarity between isolates from beef in Mexico, beef in the United States, and a steer in the United States strongly suggest that the outbreak strain is present in cattle in both countries,” according to the research report. “Because use of antibiotics in livestock can cause selecion of resistant strains, the reported 41 percent rise in macrolide use in U.S. cattle from 2016 to 2017 might have accelerated carriage of the outbreak strain among U.S. cattle.” 

Outbreak investigation
The researchers reported among patients who did not travel to Mexico, 29 percent reported eating Mexican-style soft cheese, and 93 percent reported eating beef. In 2018, an investigation of antibiotic-susceptible Salmonella Newport infections led to a U.S. ground beef recall.

“The outbreak strain (in the research report) was detected in beef samples collected in November 2018 and March 2019 at two Texas slaughter and processing facilities,” according to the research report. 

“In October 2018, the outbreak strain was detected in a mixture of queso fresco and Oaxaca soft cheese purchased in a market in Tijuana, Mexico. The cheese had been brought into the United States by a patient who became ill with a strain that was indistinguishable from the strain isolated from the cheese.”

For patients who had been to Mexico in the week before illness onset and for whom the information was available, 87 percent reported eating beef, and 63 percent reported eating soft cheese. Of several types of artisanal cheese reported, the most frequently recalled cheese was queso fresco.

Patient profile details
The researchers relied on CDC data regarding the demographics and other patient data. It included:

• From June 2018 through March 2019, 255 cases were identified in 32 U.S. states; 
• 29 percent of patients for whom information was available were hospitalized, 6 percent were admitted to an intensive care unit, 4 percent had Salmonella bacteremia, and two died; 
• Median patient age was 36 years with a range of less than 1 to 90 years;
• 58 percent of patients were female;
• 65 percent were Hispanic; 
• Among patients who traveled to Mexico with information on food consumption, 87 percent reported eating beef and 63 percent reported eating soft cheese; and
• 79 percent recalled obtaining the cheese in Mexico.