CDC Links Salmonella Outbreak to Contact with Dairy Calves

The ongoing outbreak appears to have originated in dairy calves in Wisconsin.


An outbreak of a multi-drug resistant Heidelberg strain of Salmonella has affected 54 people in 15 states, according to the U.S. Centers for Disease Control and Prevention (CDC). The ongoing outbreak has progressed for some time, with associated illnesses dating back to January 2015. Since the last update on August 2, 2017, the CDC has reported eight new cases in humans in six states. Eighteen of the human cases in this outbreak, or 33%, are children under the age of five years.

The outbreak appears to have originated with dairy calves from Wisconsin. Of 54 patients CDC investigators have interviewed, 63% reported contact with dairy calves or cattle. Ongoing surveillance in veterinary diagnostic laboratories meanwhile, has found calves in several states with sickness associated with the outbreak strains of multidrug resistant Salmonella Heidelberg. ‎Animal-health officials currently are working to trace the origin of calves associated with the most recent human cases.

Investigators have used whole-genome sequencing to identify multiple antimicrobial resistance genes in outbreak-associated isolates from 43 ill people, 87 isolates from cattle, and 11 isolates from animal environments. They’ve also verified those findings with standard antibiotic resistance testing methods used by CDC’s National Antimicrobial Resistance Monitoring System (NARMS) laboratory on clinical isolates from eight ill people in this outbreak. Those tests show:

  • All eight isolates were resistant to amoxicillin-clavulanic acid, ampicillin, cefoxitin, ceftriaxone, streptomycin, sulfisoxazole, and tetracycline, and had reduced susceptibility to ciprofloxacin.
  • Seven isolates were also resistant to trimethoprim-sulfamethoxazole.
  • Five were also resistant to nalidixic acid.
  • Three were also resistant to chloramphenicol.
  • All eight isolates tested were susceptible to azithromycin and meropenem.

The CDC recommends the following preventative steps for veterinarians or producers working with cattle.

  • Always wash your hands thoroughly with soap and water right after touching livestock, equipment, or anything in the area where animals live and roam.
  • Use dedicated clothes, shoes, and work gloves when working with livestock.
  • Keep and store these items outside of your home.
  • It is especially important to follow these steps if there are children under the age of five in your household. Young children are more likely to get a Salmonella infection because their immune systems are still developing.

The CDC offers updates and more detailed information on this outbreak on their website.