Thorough heating is inadequate protection against toxins that may be present in certain tropical seafood species, according to a study by the Centers for Disease Control in Atlanta.
An article in the agency's Morbidity & Mortality Weekly Report found that cooking did not destroy elevated levels of scombroid toxin--a naturally occurring contaminant--in fish identified as the source of two outbreaks last year.
In each of the incidents, which occurred in Chicago and Charleston, S.C., victims were diagnosed as suffering from scombroid poisoning.
This particular toxin is fast-acting and is capable of producing symptoms in otherwise healthy adults in as little as five minutes after a meal.
The attack, which can last up to 10 hours and may require emergency medical care, includes nausea, dizziness, facial swelling, diarrhea and fever.
There have been 192 scombroid fish poisoning outbreaks in this country between 1973 and 1987 resulting in more than 1,000 illnesses, according to the CDC. Hawaii, with 51, and California, with 29, topped all other states in the number of such incidents. None of the cases involved fatalities.
Those species most often implicated in these poisonings are mahi-mahi, tuna and bluefish.
The scombroid toxin is produced in dark-fleshed fresh fish with high levels of the compound histidine, an amino acid. Under poor storage conditions--whether on fishing boats, docks or in warehouses--the histidine can be converted into histimine by surface bacteria on the fish.
"This is a situation where the bacteria that are normally present on the fish break down the other normal surface materials to form abnormally high levels of toxic products," said Stephen Ostroff M.D., CDC medical epidemiologist familiar with the report.
As little as 20 milligrams of histimine per 100 grams of fresh fish can cause symptoms, according to the CDC. Hazardous levels of the compound in tuna are considered to be 50 milligrams of histimine per 100 grams.
In the Chicago outbreak, which produced eight illnesses, mahi-mahi was implicated as the source of the contamination. Investigators found that the fish showed signs of freezer burn or had been thawed and improperly refrozen. Laboratory tests found levels of histimine in the remaining fish to be three times that considered hazardous by the U.S. Food and Drug Administration.
As for the Charleston case, contaminated yellow-fin tuna was identified as the cause of nine illnesses. However, local health officials who reviewed the incident found that the suspect fish was kept thoroughly iced throughout storage and handling.
Even so, investigators found that histimine present in uneaten tuna was 10 to 15 times above the level considered hazardous.
"As these outbreaks demonstrate, cooking toxic fish is not protective," the article stated.
The authors recommend continuous icing or refrigeration for all potentially scombroid-toxic fish from the point at which the catch leaves the water until cooking. This recommendation, however, is standard advice for all fish species--not just the dark-flesh varieties.
"There is nothing else the consumer can do if they get a bad piece of fish. You can't smell the toxin, feel it or see it," said Ostroff. "But people have to realize that, while this is a problem, it is not a frequent occurrence."
Tropical Toxins--Often mentioned in tandem with scombroid poisoning is ciguatera, another toxin found in tropical reef fish. The number of poisoning incidents involving ciguatera are estimated to be 25% higher than that from scombroid, Ostroff said.
From 1973 to 1987 there were a total of 233 ciguatera outbreaks with a total of 1,045 illnesses and three fatalities. Most of these incidents, or 167, occurred in Hawaii. Another 35 were in Florida and only three in California.
Species linked to ciguatera, which is formed when fish eat a certain poisonous reef algae, include mahi-mahi, barracuda, amber jack and some red snapper varieties.
The CDC's statistics for seafood-related poisonings, including bacteria such as Vibrio cholera and Vibrio vulnificus, are considered fairly well documented because the contaminations are so distinctive.
"Seafood-related illnesses tend to be more severe (than those caused by other foods)," said Ostroff. "Cholera, ciguatoxin, scombroid are often easily recognizable entities. Another one is paralytic shellfish poisoning. They usually involve a small number of people when noticed (by health officials). Cholera cases sometimes only involve one person."
In terms of outbreaks, it appears that there are more poisonings and illnesses related to seafood than any other individual protein source. That's because illnesses caused by meats, such as salmonellosis and camplylobacteriosis, are harder to detect and are often not as identifiable. Symptoms in these cases can also be more variable.
"A substantial portion of the outbreaks (documented by CDC) where there's a known contaminant and a specific food are seafood related," said Ostroff.