Multistate Outbreak of Multidrug-Resistant Salmonella Heidelberg Infections Linked to Contact with Dairy Bull Calves

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.

November 28, 2016

CDC is working with Wisconsin health, agriculture, and laboratory agencies, 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 may have been part of this outbreak. PulseNet, coordinated by CDC, is the national subtyping network of public health and food regulatory agency laboratories. PulseNet performs DNA fingerprinting on Salmonella bacteria isolated from ill people by using techniques called pulsed-field gel electrophoresis (PFGE) and whole genome sequencing (WGS). CDC PulseNet manages a national database of these DNA fingerprints to identify possible outbreaks.

Twenty-one people infected with an outbreak strain of Salmonella Heidelberg have been reported from eight states. A list of states and the number of cases in each can be found on the Case Count Map page.

Among 19 people with available information, illnesses started on dates ranging from January 11, 2016 to October 24, 2016. Ill people range in age from less than 1 year to 72, with a median age of 21. Sixty-two percent of ill people are female. Among 19 ill people with available information, 8 (42%) reported being hospitalized, and no deaths have been reported.

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

This outbreak can be illustrated with a chart showing the number of people who became ill each day. This chart is called an epidemic curve or epi curve. Illnesses that occurred after October 24, 2016 might not be reported yet because reporting takes an average of 2 to 4 weeks. Please see the Timeline for Reporting Cases of Salmonella Infection for more details.

Investigation of the Outbreak

Epidemiologic, traceback, and laboratory findings have identified dairy bull calves from livestock markets in Wisconsin as the likely source of infections. Dairy bull calves are young, male cattle that have not been castrated and may be raised for meat. Dairy bull calves in this outbreak have also been 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 19 people interviewed, 15 (79%) 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 ill or died.

One ill person’s dairy calves were tested for the presence of Salmonella bacteria. This laboratory testing identified Salmonella Heidelberg in the calves. Further testing using WGS showed that isolates from ill people are closely related genetically to isolates from these calves. This close genetic relationship means that the human infections in this outbreak are likely linked to ill 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 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 fifteen ill people and eight cattle. This correlated with results from standard antibiotic resistance testing methods used by CDC’s National Antimicrobial Resistance Monitoring System (NARMS) laboratory on clinical isolates from two ill people in this outbreak. The two isolates tested were susceptible to gentamicin, azithromycin, and meropenem.  Both were resistant to amoxicillin-clavulanic acid, ampicillin, cefoxitin, ceftriaxone, chloramphenicol, nalidixic acid, streptomycin, sulfisoxazole, tetracycline, and trimethoprim-sulfamethoxazole and had reduced susceptibility to ciprofloxacin. Antibiotic resistance limits treatment options and has been associated with increased risk of hospitalization, bloodstream infections, and treatment failures in patients.

Traceback information available at this time indicates that most calves in this outbreak originated in Wisconsin. Wisconsin health and agriculture officials continue to work with other states to identify herds that may be affected.

This investigation is ongoing. CDC will provide updates when more information is available.

Advice for Livestock Handlers

This outbreak is a reminder to use a One Health approach to preventing illness, which recognizes that the health of people is connected to the health of animals and the environment. Follow these steps prevent illness when handling any livestock, like dairy bull calves:

• Always wash hands thoroughly with soap and water right after touching livestock, equipment for animals, or anything in the area where animals live and roam.
  • This is especially important to do before preparing or consuming food or drink for yourself or others.
  • Adults should supervise hand washing for young children.
  • Use hand sanitizer if soap and water are not available right away.
• Use dedicated shoes, work gloves, and clothing that you only use when working with livestock. Keep these items outside of your home.
  • Do not eat or drink in the areas where livestock live and roam.
  • Do not allow toys, pacifiers, spill-proof cups, baby bottles, strollers, or similar items in livestock areas.
  • Wash hands after removing any clothes and shoes you wore while working with livestock.
• Work with your veterinarian to keep your livestock healthy.
  • If you think your livestock are sick, talk to your veterinarian as soon as possible and take extra care to wash your hands after working with the animals and use separate clothes when caring for them.
  • Children, adults over age 65, and people with compromised immune systems should limit their contact with sick animals.

Advice for Veterinarians

• If veterinarians recognize ill dairy bull calves with laboratory-confirmed Salmonella Heidelberg, they should report the illness to their State Animal Health Official.
  • Laboratory testing, to include antimicrobial susceptibility testing, is recommended among dairy bull calves diagnosed with Salmonella Heidelberg, especially those associated with human illness.
  • A list of State Animal Health Officials can be found here. [PDF – 15 pages]
• If you suspect that a calf has a Salmonella infection, collect a fecal sample and submit it to a state or university veterinary diagnostic laboratory for culturing and pulsed-field gel electrophoresis (PFGE) testing.
  • For testing, the American Association of Veterinary Laboratory Diagnosticians (AAVLD) recommends submitting fecal samples to a state or university veterinary diagnostic laboratory for Salmonella culturing and PFGE.
  • If the laboratory isolates Salmonella but cannot perform PFGE, the isolate may be forwarded to a laboratory that can perform the procedure.  This may be one of the AAVLD labs in your area. To locate an AAVLD laboratory in your area, go to AAVLD Accredited Labs, or go to the AAVLD’s home page and click on the “Accreditation” link on the top menu bar.
  • Isolates may also be sent to USDA National Veterinary Services Laboratories (NVSL). To submit isolates to NVSL, complete Form VS 10-3 indicating whether serotyping, PFGE, or both are requested.
• Talk to your clients about reducing the risk of transmission of Salmonella illness from cattle to their family.
  • Be sure to tell clients that Salmonella infections are a zoonotic disease, meaning that the infection can spread between animals and people. If the client or any of their family members are ill, encourage them to contact a health care provider immediately.
  • Direct clients to the Advice for Livestock Handlers above.

Information for Health Care Providers

Salmonella bacteria live in the intestinal tracts of animals, including cattle. Cattle may become ill from Salmonella, but usually have no signs of illness. Young calves are more susceptible to being ill. The primary mode of transmission to humans is fecal-oral and is associated with contact with animals or their environments.

Clinicians should consider multidrug-resistant (MDR) Salmonella Heidelberg infection in the differential diagnosis of patients with exposure to cattle, farms, or farm workers and symptoms compatible with salmonellosis (e.g., diarrhea, fever, and abdominal cramps).

Guidance for clinicians whose patients have suspected or confirmed Salmonella Heidelberg infection related to this outbreak:

• Testing
  • Obtain a culture (stool or blood, as indicated by symptoms and signs).
  • Request antimicrobial susceptibility testing.
• Treatment
  • Most patients with nontyphoidal Salmonella do not require antibiotic treatment (exceptions may include patients <6 months, >50 years, and those who are immunocompromised or severely ill; please refer to treatment guidelines [PDF – 21 page] for additional information).
    • If treatment is indicated and MDR Salmonella Heidelberg infection is suspected, clinicians should begin treatment with azithromycin until susceptibility results of the patient’s isolate are available.
• Follow up
  • Obtain follow-up stool cultures for patients who have culture-confirmed MDR Salmonella Heidelberg and who are:
    • Food handlers
    • Health care workers
    • Childcare workers or attendees
• Patient counseling
  • Counsel patients to follow prevention practices. Key messages for counseling patients with salmonellosis:
    • Wash hands carefully with soap after going to the bathroom.
    • Don’t prepare food for others while ill. After you recover, wash hands carefully with soap before preparing food for others.
  • Before returning to work or childcare, these patients should have two consecutive negative stool cultures taken at least 24 hours apart and at least 48 hours after resolution of symptoms.