Infectious disease researchers investigating the cases traced the infections back to beef from the US and soft cheeses from Mexico (mostly queso fresco, which is typically made from unpasteurized milk). Genetic testing suggests that cows in both countries are carrying the germ.
a report published August 23 by the CDC, the investigators note that just a year earlier, the Food and Drug Administration recorded a spike in the use of antibiotics called macrolides by cattle farmers. From 2016 to 2017, cattle
farmers increased their use of macrolide antibiotics by 41%. Macrolides are a class of antibiotics that includes azithromycin. Because antibiotics within a class work to kill bacteria in similar ways, bacterial resistance to one drug in a class could lead to resistance to other drugs in the same class.
The investigators suggest that the surge in macrolide use could have encouraged the rise and spread of the azithromycin-resistant Newport strain.
“Because use of antibiotics in livestock can cause selection of resistant strains, the reported 41% rise in macrolide use in US cattle from 2016 to 2017 might have accelerated carriage of the outbreak strain among US cattle,” they wrote.
“Avoiding the unnecessary use of antibiotics in cattle, especially those that are important for the treatment of human infections, could help prevent the spread of [multi-drug resistant] Newport with decreased susceptibility to azithromycin.”
In recent years, around 70% of all medically important antibiotics in the US have been sold for use in animals. Public health advocates say agricultural use of antibiotics should be reduced significantly to preserve the effectiveness of the drugs.
To reduce the risk of infections—drug resistant or not—health officials advise consumers not to eat cheeses made from unpasteurized milk and to make sure beef reaches safe cooking temperatures: 145°F (62.8°C) for steaks and roasts followed by a 3-minute rest time, and 160°F (71.1°C) for ground beef or hamburgers.