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Multistate Outbreak of Human Salmonella Typhimurium Infections Associated with Exposure to Clinical and Teaching Microbiology Laboratories

April 28, 2011


CDC is collaborating with public health officials in many states to investigate a multistate outbreak of Salmonella Typhimurium infections associated with exposure to clinical and teaching microbiology laboratories. Investigators are using DNA analysis of Salmonella bacteria obtained through diagnostic testing to identify cases of illness that may be part of this outbreak.

As of April 20, 2011, a total of 73 individuals infected with the outbreak strain of Salmonella Typhimurium have been reported from 35 states: AK (1), AL (3),  AZ (2), CA (1), GA (5), IA (1), ID (2), IL (3),  IN (1), KS (1), KY (3), MA (2),  MD (2),  MI (2), MN (4),  MO (2), NC (1),  ND (1),  NE (2),  NJ (2),  NM (3),  NV (1), NY (1),  OH (1),  OK (1),  OR (1),  PA (6),  SC (2),  SD (1), TN (2),  TX (1),  UT (3),  WA (5),  WI (3),  WY (1).  Among persons with available information, illness onset dates range from August 20, 2010 to March 8, 2011. Infected individuals range in age from less than 1 year to 91 years-old, and the median age is 24 years-old. Sixty-three percent of patients are female. Fourteen percent of patients have been hospitalized. One death has been reported.
The outbreak can be visually described with a chart showing the number of persons who became ill each day. This chart is called an epidemic curve or epi curve. Illnesses that occurred after March 19 might not yet be reported due to the time it takes between when a person becomes ill and when the illness is reported. This takes an average of 2 to 3 weeks. Please see the Timeline for Reporting of Salmonella Cases for more details.

The numbers of new cases have declined substantially during the past several months, and reports associated with this outbreak strain appear to have returned to the expected baseline of approximately 0 to 4 cases reported per week.   

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Investigation of the Outbreak

In an epidemiologic study conducted during February and March 2011, 32 ill persons answered questions about exposures during the days before becoming ill. Investigators compared their responses to those of 64 persons of similar age previously reported to state health departments with other illnesses (controls). Preliminary analysis of this study has suggested exposure to clinical and teaching microbiology laboratories is a possible source of illness. Illnesses have been identified among students in microbiology teaching laboratories and employees in clinical microbiology laboratories. Ill persons (60%) were significantly more likely than control persons (2%) to report exposure to a microbiology laboratory in the week before the illness began. Additionally, multiple ill persons reported working specifically with Salmonella bacteria in microbiology laboratories. The New Mexico Department of Health found that the outbreak strain was indistinguishable from a commercially available Salmonella Typhimurium strain used in laboratory settings. This commercially available strain was known to be present in several teaching or clinical laboratories associated with ill students or employees infected with the outbreak strain. These data suggest this strain is the source of some of these illnesses. Additionally, several children who live in households with a person who works or studies in a microbiology laboratory have become ill with the outbreak strain.

As part of this ongoing investigation, CDC is working with state and local health departments, the American Society for Microbiology, and the Association of Public Health Laboratories to conduct a survey of laboratory directors, managers, and faculty involved with clinical and teaching microbiology laboratories to identify areas where improvements in biosafety and laboratory safety training can be made to prevent future illnesses.

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Clinical Features/Signs and Symptoms

Most persons infected with Salmonella develop diarrhea, fever, and abdominal cramps 12 to 72 hours after infection. The illness usually lasts 4 to 7 days, and most persons recover without treatment. However, in some persons, the diarrhea may be so severe that the patient needs to be hospitalized. Salmonella infection may spread from the intestines to the blood stream, and then to other body sites and can cause death unless the person is treated promptly with antibiotics. The elderly, infants, and those with impaired immune systems are more likely to have a severe illness from Salmonella infection.

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Advice to Students and Employees in Clinical and Teaching Microbiology Laboratories

  • Be aware that bacteria used in microbiology laboratories can make you or others who live in your household sick, especially young children, even if they have never visited the laboratory. It is possible for bacteria to be brought into the home through contaminated lab coats, pens, notebooks and other items that are used in the microbiology laboratory.
  • Persons working with infectious agents, including Salmonella bacteria, must be aware of potential hazards, and must be trained and proficient in biosafety practices and techniques required for handling such agents safely, including:
    • Wash hands frequently while working in and immediately after leaving the microbiology laboratory and follow proper hand washing practices. This is especially important to do before preparing food or baby bottles, before eating and before contact with young children.
    • Do not bring food, drinks or personal items like car keys, cell phones and mp3 players into the laboratory. These items may become contaminated if you touch them while working or if you place them on work surfaces.
  • Do not bring pens, notebooks, and other items used inside of the microbiology laboratory into your home.
    • Wear a lab coat or other protective uniform over personal clothing when working in a microbiology laboratory; leave it in the laboratory when you are finished. Remove protective clothing before leaving for non-laboratory areas (e.g., cafeteria, library, or administrative offices). Dispose of protective clothing appropriately or deposit it for laundering by the institution. Take it out of the laboratory only to clean it.
  • If you work with Salmonella bacteria in a microbiology laboratory, watch for symptoms of Salmonella infection, such as diarrhea, fever and abdominal cramps. Call your health care provider if you or a family member has any of these symptoms.

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Advice to Laboratory Directors, Managers, and Faculty involved with Clinical and Teaching Microbiology Laboratories

  • A comprehensive set of biosafety guidelines for work with Salmonella and other similar human pathogens can be found in the Biosafety Level 2 section of the CDC/NIH Biosafety in Microbiological and Biomedical Laboratories manual and the Guidelines for Biosafety Laboratory Competency, MMWR Supplement.
  • Non-pathogenic (attenuated) bacteria strains should be used when possible, especially in teaching laboratories. This will help reduce the risk of students and/or their family members becoming ill.
  • Persons working with infectious agents, including Salmonella bacteria, must be aware of potential hazards and trained and proficient in the practices and techniques required for handling such agents safely.
  • Advise persons using the laboratory to watch for symptoms of Salmonella infection, such as diarrhea, fever, and abdominal cramps, and to call their health care provider if they or a family member have any of these symptoms.
  • All students and employees using the laboratory should be trained in biosafety practices.
  • Ensure that handwashing sinks have soap and paper towels. Require students and employees to wash their hands before leaving the laboratory.
  • Do not allow lab coats to leave the microbiology laboratory, except to be cleaned.
  • Do not allow food, drinks or personal items like car keys, cell phones and mp3 players to be used while working in the laboratory or placed on laboratory work surfaces.
  • Place dedicated writing utensils, paper, and other supplies at each laboratory station. These items should not be allowed to leave the laboratory.

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