Cooked tuna could be hazardous to your health

From Miriam Falco
CNN Senior Medical Producer

(CNN) -- You may think that the seared tuna in your salad or the tuna burger now surfacing on menus is safer to eat than raw tuna consumed by your sushi-loving friends. This could be a logical conclusion, since burgers and seared tuna strips are at least somewhat cooked. But it's not true.

There were 22 cases of histamine poisoning between July 1998 and February 1999 in North Carolina. Researchers for the Centers for Disease Control and Prevention in Raleigh, North Carolina, wanted to know why there was an increase in this type of food poisoning. The experts were trying to find out how the contamination occurred and what could be done to prevent future outbreaks.

The scientists found that 18 people had consumed tuna burgers, a relatively new menu item in restaurants. Two ate salads and two ate tuna filets. None of the patients died, but they did experience symptoms including vomiting, diarrhea, headaches and a metallic taste in the mouth. By tracing shipments of the fish, researchers determined that the tuna was not always handled at near-freezing temperatures. The study is published in this week's Journal of the American Medical Association (JAMA).

"These cases are truly the tip of the iceberg" says Dr. Jeremy Sobel, a medical epidemiologist at the Centers for Disease Control and Prevention (CDC). "The whole question is ... what percentage of cases are actually coming to public attention?"

If there's a suspected case of Ebola, the news spreads fast, but one rarely hears about a case of histamine poisoning, explained Sobel. Histamine poisoning is difficult to diagnose, because it's often mistaken as a food allergy or seafood poisoning, and there's no laboratory test. One has to rely on a doctor to make an accurate diagnosis and then report it.

Histamine poisoning -- also known as scombroid fish poisoning -- is extremely unpleasant, can tie up medical resources and it's scary, Sobel added. Histamine is a toxin that mimics an allergic reaction. When a person has an allergic reaction, the body releases histamine, but in the case of fish poisoning, a person consumes histamine rather than releasing it. If they are properly diagnosed, most patients can be treated with antihistamines.

The CDC reported 145 outbreaks affecting 811 people in 20 states between 1988 and 1997. Forty-two incidents involving 4,122 people were reported in Japan, according to the Food and Drug Administration (FDA). Scombroid poisoning is one of the most common forms of fish poisoning in the United States, according to the FDA.

Symptoms usually appear within a few minutes to a few hours. They include vomiting, diarrhea, a metallic taste in the mouth, headaches, tingling and burning sensations around the mouth, rash and itching on the upper body and heart palpitations. The study's authors noted that some of the symptoms resemble those of coronary heart disease, which increases the chance of invasive medical procedures if misdiagnosed.

Raw fish can be contaminated with fecal bacteria when gutted and if the fish is stored at temperatures above freezing, the bacteria grow. They produce an enzyme that dissolves the tissues of the fish, resulting in the production of histamine. Cooking tuna until well-done might kill the bacteria and cooking it even more may destroy the enzymes, but histamine is not affected by heat.

Raw tuna used for sushi is less likely to be contaminated because it comes from the best cut of the fish, the filet. The tuna for sushi is usually stored in large pieces and at freezing temperatures and sliced right before serving, making it less sucsceptible to warming temperatures.

Tuna is not the only fish implicated in histamine poisoning -- mahi-mahi, bluefish, sardines, mackerel, amberjack and abalone are also mentioned. Researchers explained that tuna is particularly vulnerable to temperature fluctuations because its average body temperature is higher than other types of fish. When tuna is ground for burgers, the friction could raise the temperature of the fish, thus promoting bacterial growth and histamine production. They also noted that tuna used for burgers or slices for salads comes from the tuna's belly, which is can more easily be contaminated in when the fish are gutted.

The easiest way to prevent this type of food poisoning is to store the fish at freezing temperatures (32 degrees Fahrenheit, 0 degrees Celsius). Every time the fish is manipulated, either by slicing or grinding and making patties, the temperature of the fish is likely to go up, so keeping the fish cold is key.

Proper diagnosis and reporting are also important. The JAMA study called for a simple, cost-effective test to detect contamination.

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National Food Processors Association
USDA - Center for Food Safety & Applied Nutrition
Centers for Disease Control and Prevention

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