Unusual Outbreak of Salmonella in Imported Ahi Tuna

Daniel M. Keller, PhD

October 29, 2010

October 29, 2010 (Vancouver, British Columbia) — Epidemiologists in Hawaii have traced an outbreak of diarrheal disease to what would be considered an unlikely source — Salmonella from ahi tuna imported from Southeast Asia. They caution that such an outbreak could stretch out for months, and can recur when contaminated frozen fish is defrosted and consumed.

Dr. Sarah Park

Speaking here at the Infectious Diseases Society of America (IDSA) 48th Annual Meeting, Sarah Park, MD, from the Disease Outbreak Control Division of the Hawaii Department of Health (HDOH) in Honolulu, said the organism implicated in a recent outbreak, Salmonella Paratyphi B tartrate-positive (SBP+), was implicated in only 1 previous outbreak of 35 cases in Hawaii and 10 in mainland United States in 2007/08, most of them related to raw fish consumption.

Salmonella is not usually thought of as an etiologic agent in outbreaks involving seafood. On the basis of their findings and the published literature, the investigators speculated that contamination likely occurred before or during packaging.

The current outbreak began with cases appearing on January 10. On March 29, the Centers for Disease Control and Prevention PulseNet reported a laboratory-confirmed cluster of 11 cases in 5 mainland states and 1 in Hawaii linked to SBP+. Over the next 2 weeks, the HDOH identified 6 more cases among state residents. Gel electrophoresis showed that this strain was closely linked to the one that caused the earlier outbreak. Dr. Park said the SBP+ strain is still uncommon in the PulseNet database.

HDOH interviewed each laboratory-confirmed case using a standard questionnaire, with additional questions focusing on fruits, vegetables, fresh salsa, and raw and cooked seafood. Once raw fish consumption was implicated, HDOH modified the questionnaires to focus on that source.

HDOH identified 21 SBP+ infections in state residents with onset between February 28 and May 14. Cases did not appear to cluster but were spread out over this period. The only common exposure in 90% of the cases was raw fish consumption on Oahu, Hawaii, in the 10 days before the onset of diarrhea. Affected individuals ranged from 3 to 43 years (mean, 20 years); 19% were hospitalized, and all recovered. Common symptoms were diarrhea (often bloody), fever, abdominal cramps, fatigue, chills, nausea, and headaches.

A trace-back investigation (involving the US Food and Drug Administration [FDA]) could not identify a single store, chain, restaurant, or distributor linked to all cases. However, in 6 cases, individuals consumed spicy ahi sushi at one restaurant associated with one distributor. That distributor and another one volunteered samples for testing, but they proved negative for Salmonella.

One of those distributors and 2 others imported tuna products from Indonesia. Routine testing of frozen cubed tuna samples isolated Salmonella species but not the outbreak strain, and the product was removed from commerce. The FDA issued an import alert for frozen cubed raw tuna from Indonesia.

The Hawaii investigators reported that, given the large amount of seafood imported and consumed in the United States and the widespread nature of this outbreak across several mainland states and Hawaii, the outbreak was likely the result of low-level contamination, possibly from packing the product in contaminated ice or rinsing it with contaminated water.

Salmonella usually comes from land animals and should not be on deep sea fish. Since Hawaii has a lot of people in a limited geographic area who eat raw fish, even low-level contamination can cause enough cases to raise an alarm. On the mainland where there are "a couple of cases here and there, no one really notices, which is essentially what happened the first time around," Dr. Park told Medscape Medical News.

She said that in the current outbreak, many points in the importation process of the ahi were lacking, including shipping information, receipts, distribution dates, and lot numbers. Reviewing the literature and speaking with colleagues in Asia revealed to her that SBP+ is common in that region, which helped to narrow down the region, if not the specific source, of the implicated product. Because a product passes through several hands (e.g., distributors and wholesalers) before reaching the market or restaurant, better documentation would help in trace-back investigations. Another complicating factor is that distributors might operate under different names in different regions of the country.

"Essentially, this outbreak highlights the critical gap in the security of imported food products," Dr. Park said. "Increased globalization of our food commerce to meet demand means that controlling potential hazards for contamination at the source is a public health imperative. . . . Assuring the US food supply and our food need and demand through global outsourcing, therefore, must be balanced against ensuring our food security."

The percentage of imported food consumed in the United States went from about 6% in 1990 to 15% in 2005, with fish and shellfish accounting for most of that increase. Dr. Park said that more stringent food safety regulations and enforcement are needed, both at the source and at American ports of entry, and that food security is becoming an increasingly important topic at public health meetings.

She warned that an outbreak could go on for months, even after a source has been identified, because of frozen product. "It lasts months [in terms of] shelf life." she said. And fish that is sold in a restaurant as "fresh" might have been flash-frozen. The 2007/08 outbreak lasted from the end of October until April on the mainland.

Mark Dworkin, MD, MPHTM, associate professor in the Division of Epidemiology and Biostatistics at the University of Illinois at the Chicago School of Public Health, and former Illinois state epidemiologist, told Medscape Medical News that Dr. Park's presentation was interesting because of the recurrent nature of the Salmonella problem in ahi. He was not involved in the study.

"There hasn't been a remedy put into place at this point, and they don't have the trace-back information that would help them identify a particular problem location that is the source of this outbreak," he said. "So it won't be surprising, as she indicated, if they see cases again in the future."

Outsourcing of the food supply puts the United States at increased risk because of a lack of control over what goes on outside its borders, Dr. Dworkin warned. "Because we get [the food] here, we just assume that it's safe in way that may not be realistic."

Dr. Dworkin said this study is a good example for an epidemiology course he teaches, because "you don't want to make any assumptions when you're beginning an outbreak investigation." Previous knowledge is not sufficient to be certain about anything at the beginning of an outbreak investigation.

You can do a lot of harm by saying that "fish have never been associated with a Salmonella Paratyphioutbreak before, and therefore that the hypothesis doesn't make sense and we should dismiss it," because you delay the identification of the real source of an outbreak.

Dr. Park has disclosed no relevant financial relationships. Dr. Dworkin reports owning stock in Merck and having spoken for Sanofi Pasteur, but has no commercial research funding.

Infectious Diseases Society of America (IDSA) 48th Annual Meeting: Abstract 1055. Presented October 23, 2010.


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