Although fish that is unsafe to eat sometimes announces itself with an unpleasantly fishy, ammonia-heavy aroma and limp appearance, some harmful species containing dangerous toxins can appear perfectly fresh and appetizing. Knowledge of a fish’s origin, and proper storage and serving, are the best guarantees against a dose of food poisoning.
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Ciguatera poisoning is the scourge of tropical waters in the Caribbean, Florida and even Hawaii. The poisoning comes from a toxin produced by algae on coral reefs, which then travels up the food chain until it is concentrated at dangerously high levels in around 400 species of larger predatory fish, such as barracuda, jack, grouper and snapper. Transmitting fish display no visible signs, and cooking or freezing does not destroy the toxin, which is concentrated in the fish’s liver and kidneys. Symptoms appear as early as two hours after having eaten a ciguatera-infected fish, but typically take 12 to develop. At first, the poisoning resembles conventional food poisoning with cramps, vomiting and diarrhea, but ciguatera is the likely culprit once the victim complains of confusion between hot and cold temperatures, a metallic taste and the feeling that the teeth are falling out. Worse still, symptoms can last for weeks or return sporadically years later.
Numerous species of dark-meat fish, such as tuna, mackerel and mahi mahi, start to produce histamine in significant quantities once the fish dies. The risk of poisoning is most elevated in catch that has been exposed to air or water temperatures above 83 degrees Fahrenheit. As a result, most commercial fisheries refrigerate or freeze their tuna immediately. The danger with scombroid food poisoning, which is also referred to as histamine poisoning, is that the bacteria are heat-stable, so cooking will not eliminate the danger. Symptoms occur very quickly after eating the fish, but do not usually last longer than 24 hours. Telltale signs include hives, red skin on the face and body, flushes, nausea, vomiting and difficulty breathing. Poisoning does not usually last for longer than a few hours, but could lead to hospital treatment in the elderly or those with low immune systems. Canned fish are not exempt, especially if not kept below 40 degrees Fahrenheit once open.
The Food and Drug Administration warns that botulism is a risk in poorly canned fish. Symptoms usually appear within 12 hours and include muscle weakness, blurred vision and, at worst, breathing failure. Vibrio, on the other hand, can thrive in raw fish and oysters. Spread by poor sanitation practices, the bacteria can survive in salt or fresh water, and cause cramps, fever and vomiting within four hours, with the danger particularly elevated for infants, the elderly and those with a weakened immune system. Most harmful bacteria on fish grow quickly at temperatures above 40 F, so fish should be refrigerated immediately before and after cooking. Cooking to at least 145 F, or up to 165 for stuffed fish, will also neutralize bacteria, but not toxins. Bacteria also spread easily when cross-contamination between raw and cooked fish and meats is allowed to occur, and pregnant women are advised against eating refrigerated smoked fish, such as salmon, whitefish and mackerel pates.
The most fearsome form of food poisoning from fish comes from the puffer fish, also known as the fugu or blowfish in Japan, where it is a delicacy. The fish contains a lethal tetrodotoxin in the liver and skin that first causes numbness or tingling around the mouth, but soon leads to twitching, paralysis and suffocation, with death coming in as little as 20 minutes. As a result, puffer fish must be prepared by a specially licensed chef in Japan and is banned in the U.S. In Florida, however, several native species of puffer fish that feed on toxic algae can cause poisoning in cases where the fish is mistaken for a safer species. The FDA also warns that some species of fish can contain trace elements of mercury, a possible risk to pregnant women, who should avoid shark, swordfish and king mackerel accordingly.