The U.S. Centers for Disease Control and Prevention (CDC) has recorded 21 human cases of the Heidelberg serotype of Salmonella in eight states between January and October 2016. Using DNA fingerprinting and whole-genome sequencing services from the PulseNet system, health officials have traced the source of the outbreak to dairy bull calves from Wisconsin.

The Wisconsin State Laboratory of Hygiene found evidence of antibiotic resistance in clinical isolates from people associated with this outbreak, and helped confirm a close genetic relationship between the isolates. Whole genome sequencing found antibiotic-resistance genes in isolates from 15 infected people and eight cattle, and testing of two isolates through the National Antimicrobial Resistance Monitoring System (NARMS) found resistance to amoxicillin-clavulanic acid, ampicillin, cefoxitin, ceftriaxone, chloramphenicol, nalidixic acid, streptomycin, sulfisoxazole, tetracycline, and trimethoprim-sulfamethoxazole and had reduced susceptibility to ciprofloxacin. The two isolates tested were susceptible to gentamicin, azithromycin, and meropenem. 

Of the 19 affected people CDC investigators interviewed, 15, or 79%, reported contact with dairy bull calves or other cattle. The CDC provides a list of precautions for veterinarians and animal handlers to reduce their risk of infection with Salmonella or other pathogens in livestock.

Read more about the outbreak from the CDC.