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Salmonella Update: CDC Still Searching

Jim Prevor’s Perishable Pundit, November 2, 2006

The Centers for Disease Control are continuing to investigate the Salmonella outbreak. Although the authorities are comfortable saying it is produce-related, they are uncertain of the exact cause — although they report many patients reporting eating “red brown tomatoes.” We asked Pundit investigator Mira Slott to speak with Dr. Wences Arvelo, EIS Medical Officer in the CDC Foodborne Illness Division and try to learn more. A few key points:

  • Bulk product is much harder to tie to an outbreak. If you eat bagged spinach you might have the bag in the refrigerator; if you eat a tomato, you probably threw out the rest of the tomato. And even if you had the tomato, there is no traceback on the product level.
  • We need to find a better way than the kind of surveys that seem to get done with sick people. In the spinach case, there were many “false positives” with brands and types of spinach implicated by consumers that had nothing to do with the outbreak. This time it may be consumers not remembering that they had diced tomato on a taco somewhere. Better survey methodology might be an area of research investment.
  • This final quote from Dr. Arvelo provides important perspective: Food in the U.S. has never been safer than it is right now. People are more aware, and we have great technological systems in place. Outbreaks were going on before; they were just not detected. Consumers view news of all these outbreaks now and get concerned, but the truth is they were always occurring. We’re just finding them, and finding them earlier.

Here is the full interview:

Q: It has been reported that the CDC detected the Salmonella outbreak two weeks ago through Pulsenet. Why is CDC just alerting the public about it now?

A: The first time we were aware of cases related to this scenario was on September 28. Two cases in Vermont were identified, and those results were shared with Pulsenet. By October 27, CDC identified all the other cases at the multi-state level that were being followed.

Q: What steps occurred during that time frame spanning more than a month?

A: As the cases get reported to Pulsenet, people at Pulsenet look for patterns and matches and put the cases with the same fingerprint DNA into clusters. Three in Connecticut come in with the same DNA fingerprint, and it raised suspicions they are coming from same source.

Q: Does Pulsenet normally communicate this information to the Connecticutt health authorities?

A: Pulsnet contacts the states and raises their level of suspicion of outbreak. Where the state may not investigate two cases, now it’s 5, 10 or 171 people involved.

Q: At what point do you determine it’s an outbreak? Do you have a threshold of the number of people sick, or number of states involved?

A: There is no qualifying number. Once we notice a clustering of cases, a DNA fingerprint that keeps coming up, we act on it.

Q: That still doesn’t get to the heart of the problem; the more-than-four-week timeframe from initial discovery of cases to CDC reporting the news of the outbreak.

A: Usually it takes approximately two weeks from when a person becomes ill to when Pulsenet receives a report. Each case is different, but often it takes a while for the patient to go to the doctor, to have the problem diagnosed, the patient goes home, brings back a stool sample, which is then queued in the lab. Then the lab needs to culture and grow the Salmonella, which takes 24 hours, and then conduct DNA testing. We get the tip of the iceberg in cases, because some people that got sick didn’t get tested. There are many places where timing gets prolonged. If you look at the timeframe, that’s two cycles.

On October 27, we reached a peak in the large number of cases in 19 states. In the meantime, cases come up in Massachusetts, Connecticut, and other states. As soon as we know, we contact the states to start investigating to see if there’s a common source.

Q: And what investigative actions have taken place to that end?

A: Right now the source of the outbreak has not been identified. The preliminary investigations in several states have shown in many cases that people had been exposed to produce — lettuce and tomatoes. Once we get information in common like that, we want to look more closely into that product. In terms of public health, we want to implicate a particular source.

Q: Do you have any more specific information about the types of lettuce or tomatoes that could get us closer to the source?

A: All different kinds of lettuce, commodity and processed. Apparently, there is a commonality with many patients reporting they ate red brown tomatoes.

Q: Sounds like that opens up many possibilities.

A: The difference with the spinach scenario was that FDA could match the same fingerprint DNA from the product. If the consumers eat a tomato, they throw the leftovers away. Two weeks elapse from when the patient gets ill to the lab submitting DNA to Pulsenet and the likelihood of getting to that source becomes even more problematic.

Q: So is the risk of contamination at this point over?

A: At this point, very few cases are coming out and that would mean the source is not there anymore and risk is much less. Over the last few days, the incidents are a lot lower than before. Salmonella is very common. We always see it at a base level. E.coli pathogens lead to complications you don’t see in Salmonella.

Q: How frequently do you receive cases of Salmonella?

A: There are 6,000 to 8,000 cases reported each year of this particular Salmonella strain. Matching that exact same DNA and making the link happens at the patient level. The local health department has to interview the patient, and asks a series of questions. It’s hard because the patients might not report or even remember they ate lettuce in a burrito at a fast food chain one day. For the information to mean something, we need large numbers of people that remember the information correctly. We collaborate work with the states, local health agencies, CDC, FDA and sometimes USDA.

Q: There has been criticism that if Pulsenet were open on weekends, the process of discovery could be expedited, and in the end alleviate the number of people getting sick. What is your view?

A: This is a numbers game. Two cases on one day won’t make a difference. We need the cumulative effect of cases reported over a period of time. Being closed on the weekends won’t tip the scale one way or the other. That said, we are always striving to get to the source of the outbreak faster.

Q: How are efforts progressing?

A: Food in the U.S. has never been safer than it is right now. People are more aware, and we have great technological systems in place. Outbreaks were going on before; they were just not detected. Consumers view news of all these outbreaks now and get concerned, but the truth is they were always occurring. We’re just finding them, and finding them earlier.

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