An Unsuspected Treat Contaminated with Listeria. How about them Caramel Apples?

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Caramel apples

By  Mandip Kaur and Brendan Jackson

Oh, how sweet it is to enjoy a caramel apple when autumn sweeps in! Maybe you like yours topped with nuts? Sprinkles? How about chocolate?

But who knew that this past fall, certain caramel apples would be contaminated with the dangerous Listeria monocytogenes bacteria (here, Listeria for short), and cause illnesses across the nation? This was quite a curveball for public health investigators: listeriosis (the disease caused by Listeria) outbreaks are often traced to soft cheeses and sometimes to produce, but no one had ever reported an outbreak linked to whole apples.

Cracking the case

Text 1We scratched our heads over this one, but with the help of the Listeria Whole Genome Sequencing (WGS) Project and patient food history information, public health officials at the state and local levels, the Centers for Disease Control and Prevention (CDC), and the Food and Drug Administration (FDA) were able to identify the source of this outbreak and prevent more people from getting sick.

Public health investigators must work with accuracy and speed to stop an outbreak in its tracks and prevent illnesses–especially true in this Listeria outbreak. Thirty-five people who lived in 12 states became sick and sadly, Listeria contributed to at least three of the seven reported deaths. It was the largest US Listeria outbreak since the one linked to cantaloupe in 2011.

We used a three-pronged approach to solve the outbreak:

  • Laboratory: CDC, FDA, and state labs tested samples from patients, foods, and the environment for related strains of Listeria.
  • Epidemiology: Local, state, and CDC investigators interviewed patients about foods they ate before getting sick.
  • Traceback: FDA, state, and local officials investigated sources of the suspect food and its ingredients.

It all started in mid-November 2014, when lab scientists at CDC raised a flag after noticing that certain Listeria infections across the United States—out of the hundreds that had occurred that year—were genetically related, suggesting an outbreak.

The laboratory sideDNA

We used whole genome sequencing, or WGS, to help determine the scope of this outbreak. WGS provides high-resolution genetic information about the strains of Listeria causing illness. Since the 1990s, we’ve tested patient samples from across the country using pulsed-field gel electrophoresis, or PFGE, and then compared them in PulseNet (a national network of public health labs) to see which ones appear related.

In the past year, we’ve started using WGS, too, because it gives a much more detailed look at Listeria strains than PFGE. Thanks to WGS, we were able to detect related cases across the country a week faster than if we relied on PFGE alone. WGS also gave us a clearer picture of which illnesses ought to be included in the outbreak, and which were likely due to other sources.

The epidemiology front

How could we figure out what food was making people sick? We asked patients what they ate, of course. First, we used a standard questionnaire that asks about dozens of foods that could be linked to listeriosis, but when epidemiologists examined their responses, no food jumped out as the likely culprit.

Text 2Over the following weeks, epidemiologists re-interviewed patients and their families—sometimes multiple times—about all the foods they had eaten in the month before getting sick. Just try to remember everything you ate a month or two ago—no easy task! As you can imagine, some patients had several foods in common, and we chased a number of false leads.

Then we got a call from our colleagues in Texas about two patients who had eaten caramel apples. They thought they might be on to something, but could caramel apples really be a source of listeriosis? The quickest way to find out was to ask patients in other states. We were amazed as one patient after another answered “yes” to eating caramel apples.

By December 19, 2014, we learned that 15 of the 18 ill people interviewed had eaten commercially produced, prepackaged caramel apples. When we compared this information with data on how often people in general eat caramel apples, we could tell it was no coincidence—caramel apples were almost certainly to blame.

With evidence from the WGS findings and the food history information, we informed the public about the outbreak that day. We recommended that consumers not eat commercially produced, prepackaged caramel apples until we had more specific information about brands or stores–patients were reporting multiple brands, although most couldn’t remember a brand at all. 

The trace back tale

FDA and state officials traced back the caramel apple brands to multiple manufacturers. They even traced back all the ingredients in the caramel apples including the apples, caramel, toppings, and sticks—a difficult and time-intensive task.

As it turned out, all of the manufacturers in the investigation used apples from Bidart Bros., a California apple supplier. That was the only common denominator. After learning about the investigation, Bidart Bros. issued an initial recall of certain apples on December 22, 2014. Over the next week, three caramel apple manufacturers that received apples from Bidart Bros. issued recalls of their own.apples_crop

FDA and California health officials inspected the Bidart Bros. apple packing facility, which was closed for the season, and swabbed surfaces. On January 8, 2015, tests from the Bidart Bros. facility found strains of Listeria that were indistinguishable from the ones making patients sick. We had the third piece of the puzzle.

Soon after, the company issued a public recall of all apples produced in 2014, and CDC and FDA were able to narrow our guidance to consumers and inform them that they should avoid only caramel apples made from Bidart Bros. apples.

This outbreak investigation highlights how WGS, with its precision and speed, combined with detailed patient interviews about exposures can help identify the scope and source of an outbreak. CDC used these vital tools to keep the public informed and to advise people to not eat a contaminated caramel apple a day, to keep that Listeria away!


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15 comments on “An Unsuspected Treat Contaminated with Listeria. How about them Caramel Apples?”

Comments listed below are posted by individuals not associated with CDC, unless otherwise stated. These comments do not represent the official views of CDC, and CDC does not guarantee that any information posted by individuals on this site is correct, and disclaims any liability for any loss or damage resulting from reliance on any such information. Read more about our comment policy ».

    A beautiful, clearly defined example of the powerful impact that adding genomic knowledge to the Knowledge Base is having, and a faint hint of the profound impact to come.

    I would like to use your photograph of the caramel apples for an article on food safety for our company newsletter. The Media office said that images from the PHIL were available for reprinting, but I can’t tell whether this photo is from the PHIL. Can you tell me if it is available for my use?

    Thanks very much.
    Barb Wenda

    What is interesting about this blog to me is the fact that Listeria was involved with apples because that is something that the people in my family regularly eat, which concerned me.
    This relates to my life because as a nurse, I will need to deal with certain outbreaks such as this Listeria outbreak.
    The changes I can make from the knowledge gained in this blog is that I can pay more attention to outbreaks that are occurring around me because I had no idea that this outbreak had taken place which could have put me or my family in danger.

    The most interesting thing about this article is the fact that there was a Listeria outbreak about six months ago and I did not hear anything about it. I, being a caramel apple lover, would have been shocked to have gotten Listeria. This article relates to my life because I love eating caramel apples and do not like that I did not know about the outbreak and could have been affected. I will adjust my life by never eating prepackaged caramel apples. The fresh caramel apples are always better to eat anyways!

    @BarbWenda the caramel apples image from the blog is not a CDC PHIL image, but you may access the image through

    The interesting thing about this blog is that who knew that something so delicious could be so deadly. I would have never guessed that caramel apples could cause a bacteria. It’s something good to know because I absolutely LOVE caramel apples!
    This can relate to my life because I always get caramel apples for a snack when I walk the mall so now I know to not choose that particular type of snack.
    Some adjustments or changes that I could make is to be more aware of the foods that may cause bad bacteria because I would have never known if I did not read this. I need to become aware of the outbreaks caused from foods.

    1. What I found interesting in this blog is that my favorite snack has been attacked by microbes! I cannot believe this occurred because I am always eating apples; they are my favorite fruit.
    2. I do not think this relates to my life other than I eat it often, but now I’m more cautious when I purchase apples or caramel apples.
    3. The adjustment I can make regarding this article it to watch what I am eating. I should do research about the microbes and how it can be spread so people I care about or I do not get sick.

    This is interesting to me since I love eating caramel apples. I will definitely be paying more attention to where my apples come from if I even bother to get them at all. I never would have thought about caramel apples causing listeria.

    What I thought was very interesting was that apples could even become contaminated with Listeria. From what I have learned in my studies was that Listeria contaminated mostly unpasteurized food like feta cheese and panel, and also meat products but never would I have thought apples. Something else I thought was interesting was how genome sequencing was used in order to find out what the outbreak led to and also blood tests by using electrophoresis. Surveys were done and this is what epidemiologists do in order to crack a case. This is something as nurses that we should be able to relate to. When a patient comes in with complaints we need to be able to analyze the situation (survey) ask questions, collect data. After reading this blog I think its safe to conclude that keeping yourself informed of worldwide outbreaks can be beneficial to you and others. Learning about public health and how to spread awareness can save many. As health care providers we should be cautious and educate our patients to do the same.

    As a father of children who happen to love candy apples I found this very interesting because this is something I didn’t even know was possible. After reading this article I decided its best not to buy the prepackaged candy apples and just wait for the local fairs or candy stores to procure my childrens candy apples. According to cnn Three companies — Happy Apple Company of Washington, Missouri; California Snack Foods, of El Monte, California; and Merb’s Candies of St. Louis, Missouri — have issued voluntary recalls of their caramel apples. This second company happens to be less than five miles from my house.

    This is very interesting because I don’t often think about the dangers I could be exposing myself to when I buy commercially produced products. I assume since they are under FDA regulations and other strict regulations that the food is safe and won’t make me sick. I think this relates to me because I do consume commercial products and produce. There aren’t really a lot of changes that could be made on my part because I depend on the company to provide food that is safe for consumption. The companies could make more of an effort to make sure their produce/product is safe for consumption and isn’t infected with a disease. Blogs like this are very helpful and can help in the future by recognizing the strategies the epidemiologists used to figure out the commercially produced caramel apples was the culprit and, more specifically, the company that provided the apples. According to the Prevention website, food such as beef, pork, poultry, dairy products, strawberries, raspberries, cherries, apples, pears, tomatoes, potatoes, greens, coffee, peaches, nectarines, celery, red and green bell peppers, and grapes are commonly contaminated(Loux, 2011).

    I love learning more about the epidemiological process and how it can be used to help save peoples lives! This was a very interesting case!

    Thanks for sharing. It would make me think better before making a decision over what to eat.

    I appreciate this article for explaining more about foodborne disease outbreak investigations. This is important because it increases trust for results of future investigations if the public knows more about how they are conducted. They can begin to understand why answers aren’t always concrete or quick.

    I have heard of Listeria outbreaks, but never knew from where they came. In reading the links you included, the sources for listeria are many, and apparently could show up anywhere. It seems there is no full avoidance of its possible contact, other than basic food safety of washing and cooking properly. Appreciate the information.

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Page last updated: March 23, 2015