Wisconsin’s Salmonella Mystery and the Surprising Culprit

Apr 14 2011 :: Published in Food Safety, Member News

(This post was originally written for The “Germ Guy” Blog: Confessions of a Mercurial Microbiologist. Thanks to Jason Tetro for having us as guest bloggers.)

By Michelle M. Forman, Senior Media Specialist, Association of Public Health Laboratories
Contributor: Tim Monson, MS, Advanced Microbiologist, Food- and Waterborne Disease Program Coordinator, Wisconsin State Laboratory of Hygiene

Salmonella – we’ve all heard the news and have avoided the recalled foods.  Did you ever think you could get Salmonella without eating these tainted foods?  One Wisconsin infant’s illness sparked an international investigation that led to a surprising source.

In April 2010, a case of salmonellosis (an infection caused by the bacterial pathogen Salmonella) in an infant was reported to the Wisconsin Department of Public Health (WDPH).  The hospital where the baby was seen forwarded the Salmonella isolate to the Wisconsin State Laboratory of Hygiene (WSLH), the state’s public health and environmental lab.  Once there, WSLH laboratorians serotyped the isolate, subtyped the isolate using Pulsed Field Gel Electrophoresis (PFGE) and uploaded the PFGE subtyping data to the PulseNet database. (PFGE is a laboratory method for looking at the DNA fingerprints of bacteria.  The DNA is cleaved or cut into pieces using a specific enzyme, the DNA pieces are separated by size, and the pattern of DNA bands is visualized under UV light.  Bacteria that are likely to be related to each other will have similar looking or identical fingerprints (band patterns).  PulseNet is the national subtyping network of public health and food regulatory agency laboratories coordinated by the Centers for Disease Control and Prevention (CDC). This national subtyping network helps detect foodborne disease case clusters.)

Later that month, routine interviews were conducted by a Wisconsin public health nurse to try to figure out where this baby picked up Salmonella.  During the nurse’s visit, she discovered that there were multiple bearded dragons (ie, really big lizards) in the household.  The news did not come as a shock to Wisconsin epidemiologists and WSLH laboratorians.  Cases of reptile-associated Salmonella were not unusual, but it still required confirmation.  The epidemiologists requested a supplemental interview and sent a sanitarian to culture the lizards. Open wide, lizards! While the initial interview may have included questions on the baby’s diet, the supplemental interview included questions on the lizards’ diet. Upon looking at the supplemental questionnaire, a WDPH epidemiologist noticed that the lizards were fed, among other things, frozen mice.

Who was responsible for making this baby sick?  Was it the lounging lizards or their frosty mouse entrees?  The epidemiologists were determined to find out.

They asked the public health lab to search state and national databases for other clinical isolates both in and out of the state to see if there was a match.  The baby’s isolate had a particular PFGE pattern and because it was tested in a way that other state and regulatory agency laboratories use, it is possible to find other PFGE pattern matches.  Another case in Wisconsin was identified as having the same PFGE pattern; they had a match.  This case was in an adult from February 2010 who reported exposure to a pet snake and its environment in a friend’s home.  Several other out-of-state isolates matched as well.  Something fishy – er, mousy – was going on here.

The laboratorians in Wisconsin uploaded the PFGE pattern to PulseNet and notified the baby’s family.  Because there was not another matching cluster already posted to PulseNet, they soon closed the investigation in Wisconsin.

In June of that year, PulseNet posted a multi-state cluster of Salmonella to the PulseNet listserv called “CDC Team”.  Thanks to that post, thirteen clinical isolates and one reptile isolate from eight states were recognized.  Additionally, PulseNet began working with investigators in the United Kingdom who had been looking at a cluster of Salmonella infections with the same PFGE pattern as the U.S. cluster.  Soon after, a connection was made.  The U.K. was also supplied feeder mice by the same company that supplied the mice in the U.S. cluster of illnesses.   PulseNet then started working with the U.K. investigators to review the PFGE in the Wisconsin cases.

Later that same month, a third case of Salmonella with the implicated PFGE pattern was identified in Wisconsin.  To no one’s surprise, the investigation revealed – you guessed it – exposure to a snake in the household who was fed frozen mice.

Everything was starting to come together.

In July 2010, the FDA began collecting environmental isolates from the suspected source of the Salmonella, an animal wholesale company in Georgia that specializes in small feeder animal sales.

That same month, the Georgia company recalled the frozen reptile feed because of potential Salmonella contamination.

From December 2009 through June 2010, the Salmonella cluster grew to 34 clinical cases from 17 states.

So how exactly did the Wisconsin baby get sick?  Hard to say for sure.  CDC does offer some advice to prevent others from getting sick the same way:

  • Wash your hands thoroughly with soap and water immediately after handling frozen rodents used as food for reptiles, or anything in the area where they are stored, thawed, prepared, and fed to reptiles.
  • Wash your hands thoroughly with soap and water immediately after handling live rodents and reptiles, or anything in the area where they live and roam.
  • Running water and soap are best. Use hand sanitizers if running water and soap are not available. Be sure to wash your hands with soap and water as soon as a sink is available.
  • Keep frozen rodents away from areas where food and drink are stored, prepared, served, or consumed.
  • Avoid using microwave ovens or kitchen utensils used for human food to thaw frozen rodents used for reptile feed. Any kitchen surfaces that come in contact with frozen rodents should be disinfected afterwards.
  • Do not let children younger than 5 years of age or people with weakened immune systems handle frozen rodents.
  • Use soap or a disinfectant to thoroughly clean any surfaces that have been in contact with frozen rodents. Children older than 5 years old should perform this task only under adult supervision.
  • Recalled frozen rodents used as food for reptiles may still be in stores and in consumers’ homes, including in the freezer. Any recalled product should be thrown away to prevent Salmonella infections in humans, pets, or other animals. This product should be disposed of in a closed plastic bag and placed in a sealed trash can to prevent people or animals, including wild animals, from eating it.

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