Not a Typical Spring Weekend: Seven Salmonella Investigations

Posted on by Mark Sotir
Under a very high magnification of 12000X, this colorized scanning electron micrograph (SEM) revealed the presence of a large grouping of Gram-negative Salmonella Typhimurium bacteria that had been isolated from a pure culture.
Under a very high magnification of 12000X, this colorized scanning electron micrograph (SEM) revealed the presence of a large grouping of Gram-negative Salmonella Typhimurium bacteria that had been isolated from a pure culture.

Foodborne illnesses occur throughout the year and summers tend to be busy with outbreaks. I work in CDC’s Enteric Diseases Epidemiology Branch and we just spent a few busy weeks on an investigation linking E. coli 0157 illnesses to raw cookie dough. See Karen Neil’s blog about that process.

Still, even in our busiest times, we don’t generally find ourselves spending entire weekends working on outbreaks. But on Saturday and Sunday, March 28 and 29, 2009, our branch found itself tracking the investigations of seven Salmonella outbreaks concurrently – six in the United States and one in Uganda. I was Acting OutbreakNet Team Lead that weekend.

Several people in our branch were away. Some were out for much-needed vacation time since we had just come off the investigation of Salmonella serotype Typhimurium infections from peanut butter. Others were teaching training courses or attending meetings in other countries that help us to develop and maintain professional relationships around the world. It fell to me to keep tabs on all that our team was doing.

Each outbreak had its own rhythm and questions to be answered.

That weekend, Adam Langer, our preventative medicine resident, was leading a call center for an outbreak of Salmonella serotype Carrau. This involved four day and evening shifts of CDC volunteers who came in over the weekend to interview people living near those who became ill, but who hadn’t become sick themselves. We were trying to find out whether the ill people ate a different food than the people who didn’t become sick.

Our laboratory colleagues had identified cases of Salmonella serotype Rissen in four western states. Earlier that week, our state partners in Oregon had collected ground pepper samples from a restaurant on our hunch that imported pepper might be carrying the bacteria. Our hunch came from CDC’s PulseNet Team, which collects and analyses subtype patterns of strains of bacteria that cause foodborne illness to identify potential clusters — they had an old report of this strain of Salmonella being isolated from pepper. The samples from the Oregon restaurant had just tested positive, reinforcing our suspicions that ground pepper might be causing the illnesses.

After 24 hours, this inoculated XLD agar culture plate cultivated colonial growth of Gram-negative, rod-shaped and facultatively anaerobic Salmonella sp. bacteria.
After 24 hours, this inoculated XLD agar culture plate cultivated colonial growth of Gram-negative, rod-shaped and facultatively anaerobic Salmonella sp. bacteria.

What was particularly interesting to us was that both these serotypes of Salmonella – Carrau and Rissen – are rarely seen in the United States.

We wondered if there was any link between the Salmonella Carrau illnesses and another outbreak we were investigating, due to Salmonella serotype Anatum illnesses in the southwestern United States. Both outbreaks involved mostly women and girls of similar ages. A CDC field team was working with Arizona officials to help investigate the Salmonella Anatum cases.

In addition, Casey Barton Behravesh was heading up a group of CDC and state investigators working on an outbreak of Salmonella serotype Saintpaul infections that turned out to be associated with raw alfalfa sprouts. There were also cases of Salmonella serotype Oranienburg associated with passengers on a cruise ship that one of our Epidemic Intelligence Service (EIS) Officers, Achuyt Bhattarai, was investigating under the direction of Samir Sodha. And Eric Mintz was leading EIS officer Karen Neil and others who were assisting in investigating over 300 suspect cases of Salmonella serotype Typhi in western Uganda. They were working with the Ugandan Ministry of Health, UNICEF, and non-profit organizations.

As if that wasn’t enough, the Food and Drug Administration (FDA) had just informed us that they had learned that cultures of pistachio nuts were yielding Salmonella serotype Montevideo and they were interested in finding out if there were any associated human cases. Working with other regulatory agencies like FDA is a critical part of what we do.

And as always, we were keeping our eyes on several other clusters of illnesses to see what, if anything, was happening with them.

We haven’t really seen a break since that weekend. Right now we have several active E. coli and Salmonella outbreak investigations underway. A key tool for all this work is PulseNet, a laboratory-based surveillance system for enteric diseases which is a cooperative effort between the Enteric Diseases Laboratory Branch (EDLB), led by Peter Gerner-Smidt, and laboratories across the country. The EDLB PulseNet database staff continually review data submitted by PulseNet member laboratories to try and identify similarities in bacteria strains. PulseNet member laboratories across the US and around the world also monitor their respective databases for pattern similarities. This allows us to detect laboratory clusters of foodborne illnesses so that we can investigate the causes as quickly as possible.

And now we’ve entered summer, traditionally our busiest time of year. We’re definitely not slowing down.

Posted on by Mark Sotir

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Page last reviewed: October 9, 2009
Page last updated: October 9, 2009