March 20, 2017

CDC worked with the Wisconsin Department of Health Services; Wisconsin Department of Agriculture, Trade and Consumer Protection; Wisconsin Veterinary Diagnostic Laboratory; Wisconsin State Laboratory of Hygiene; several other states; and the U.S. Department of Agriculture Animal and Plant Health Inspection Service (USDA-APHIS) to investigate a multistate outbreak of multidrug-resistant Salmonella Heidelberg infections.

Public health investigators used the PulseNet system to identify illnesses that were part of this outbreak. Thirty-six people infected with the outbreak strains of Salmonella Heidelberg were reported from ten states. A list of states and the number of cases in each can be found on the Case Count Map page.

Among 36 people with available information, illnesses started on dates ranging from January 27, 2015 to January 16, 2017. After the initial announcement on November 28, 2016, investigators identified seven people in four states who were infected with the outbreak strains in 2015. Based on interviews with the ill people and whole genome sequencing (WGS) data, these people were added to the total case count for the outbreak.

Ill people ranged in age from less than 1 year to 72, with a median age of 18. Of ill people, 60% were female. Among 31 ill people with available information, 13 (42%) were hospitalized, and no deaths were reported.

WGS showed that isolates from ill people were closely related genetically to one another. This close genetic relationship means that people in this outbreak were more likely to share a common source of infection.

Investigation of the Outbreak

Epidemiologic, traceback, and laboratory investigations identified sick calves, including dairy bull calves from livestock markets in Wisconsin, as the likely source of most of these infections. Dairy bull calves are young, male cattle that have not been castrated and may be raised for meat. Some calves in this outbreak were also purchased for use with 4-H projects.

In interviews, ill people answered questions about any contact with animals and foods eaten in the week before becoming ill. Of the 36 people interviewed, 25 (69%) reported contact with dairy bull calves or other cattle. Some of the ill people interviewed reported that they became sick after their dairy bull calves became sick or died.

One ill person†™s dairy calves were tested for the presence of Salmonella bacteria. This laboratory testing identified Salmonella Heidelberg. Additionally, environmental samples were collected from a livestock market in Wisconsin, and these samples also identified Salmonella Heidelberg. Further testing using WGS showed that isolates from ill people were closely related genetically to both the isolates from these calves and to the livestock market. This close genetic relationship means that the human infections in this outbreak were likely linked to sick calves.

As part of routine surveillance, the Wisconsin State Laboratory of Hygiene, one of seven regional labs affiliated with CDC†™s Antibiotic Resistance Laboratory Network, conducted antibiotic resistance testing on clinical isolates from the ill people associated with this outbreak. These isolates were found to be resistant to multiple antibiotics and shared the same DNA fingerprints, showing the isolates were likely related to one another.

WGS identified multiple antimicrobial resistance genes in outbreak-associated isolates from 26 ill people, 43 cattle, and 10 from animal environments. Resistance genes correlated with results from standard antibiotic resistance testing methods used by CDC†™s National Antimicrobial Resistance Monitoring System (NARMS) laboratory on clinical isolates from five ill people in this outbreak. All five isolates tested were resistant to amoxicillin-clavulanic acid, ampicillin, cefoxitin, ceftriaxone, streptomycin, sulfisoxazole, and tetracycline, had reduced susceptibility to ciprofloxacin, and were susceptible to azithromycin, gentamicin, and meropenem. Four of the five isolates were also resistant to trimethoprim-sulfamethoxazole, and two of these were resistant to chloramphenicol and nalidixic acid. Antibiotic resistance limits treatment options and has been associated with increased risk of hospitalization, bloodstream infections, and treatment failures in patients.

Traceback information collected during the outbreak indicated that most of the calves originated in Wisconsin. Multiple farms and animal markets in Wisconsin were identified during the investigation, including one market where environmental samples were collected and demonstrated presence of the outbreak strain of Salmonella Heidelberg. Wisconsin health and agriculture officials continue to work with other states to identify other herds that may be affected. They also are identifying ways to work with farms and livestock markets to reduce the risk of Salmonella contamination and infection in dairy calves.

This outbreak investigation is over. However, infections in calves continue to be reported and people can still get a Salmonella infection from contact with these livestock. Read more information about Salmonella and livestock and how people can reduce the chance they will get an infection. Livestock owners should continue to monitor for increased morbidity and mortality in dairy calves and consult their veterinarian if noted.