For most people, sushi, sashimi and other raw fish dishes are safe if made with fresh, commercially frozen fish.
Fish, like all other living things, can contain parasites. However, freezing and cooking kills the parasites.
Most home freezers are not cold enough to kill parasites. If you freeze fish for raw dishes, be sure that your freezer can reach -4 F or lower. The fish must then be kept at this temperature for 168 hours (seven days).
Though freezing kills parasites, it does not kill bacteria. Thus people at risk should not eat raw finfish.
Norwalk and Similar Viruses
Norwalk viral infection is caused by the Norwalk virus. It and similar viruses are recognized as leading causes of food-borne illness in the United States. They have been linked to intestinal illness in communities, camps, schools, institutions and families. Contaminated water is the most common source of outbreaks. Shellfish and salad ingredients are the foods most often implicated. Eating raw or partially cooked clams and oysters poses a high risk for infection.
The symptoms of infection by the Norwalk virus appear one to two days after eating contaminated food. They include nausea, vomiting, diarrhea and stomach cramps. The illness lasts two to three days. Severe illness is uncommon.
Some viruses are heat-resistant and are not destroyed by cooking.
Other seafood safety concerns include scombroid poisoning and ciguatera poisoning, the two most common diseases caused by marine toxins.
Scombroid poisoning, also known as histamine fish poisoning, is caused by consuming scombroid and mackerel-like marine fish that have not been kept at low enough temperatures after being caught. Fish most commonly involved include tunas, mackerels, bluefish, dolphin (mahi-mahi) and amberjacks. When these fish are not quickly and properly iced after capture, bacteria on the surface begin to convert the amino acid histidine to histamine. Large amounts of histamine can cause an allergic response such as rash, diarrhea, facial flushing, sweating, headache and vomiting. There may also be burning or swelling of the mouth, abdominal pain or a metallic taste. The symptoms appear within a few minutes to two hours after eating the fish. Symptoms usually last four to six hours and rarely exceed two days.
Histamine in fish cannot be destroyed by freezing or cooking. Although scombroid poisoning can be a problem in the commercial seafood industry, it can also pose a significant problem for recreational fishers who do not handle their catch properly. Illnesses due to these natural toxins do pose serious safety concerns, but seem to be improving as people, especially anglers, ice seafood more and properly.
Ciguatera occurs most often from eating reef fish that harbor the toxin ciguatera. Ciguatoxic fish are not inherently toxic. The poisonous substance originates in microscopic sea plants. Small fish feeding on these organisms become toxic. They in turn get eaten by larger fish, and in time, the larger fish become toxic, too. Thus the toxin moves up the food chain into the large predatory fish often favored by consumers. While barracuda are commonly associated with ciguatera, some other fish, including grouper, snapper and sea bass that live in tropical reef waters, can be toxic.
Toxic fish cannot be detected by appearance, taste or smell, and cooking does not inactivate the toxin. Vacationers and inexperienced recreational fishers should use care in waters where ciguatera thrives.
Symptoms of ciguatera begin from 30 minutes to six hours after eating contaminated fish. They include nausea, diarrhea, vomiting, headache and muscle aches. These symptoms can be followed by neurological discomforts such as headache, flushing, a tingling or numb sensation on the lips, tongue and mouth, nightmares or hallucinations. In more severe cases, the most definitive symptom is a cold-to-hot sensory reversal so that cold objects feel hot and hot objects feel cold. Symptoms usually last from one to four weeks.
Mercury in Fish
While seafood makes part of a healthy diet, some fish contain high levels of a form of mercury, methylmercury, that can harm an unborn child’s developing nervous system if eaten regularly. Nearly all fish contain trace amounts of methylmercury that are not harmful to humans. But large, long-lived fish that feed on other fish accumulate the highest levels and pose the greatest threat to people who eat them regularly.
The FDA advises pregnant women, and women who may become pregnant, not to eat four fish: shark, swordfish, king mackerel and tilefish. It also recommends that nursing mothers and young children not eat these fish.
In 2001, the FDA advised these women to select a variety of other seafood, including shellfish, canned fish, smaller ocean fish and farm-raised fish. The FDA states that they can safely eat 12 ounces of cooked fish per week.
Current FDA advisories on seafood safety can be found online at www.fda.gov.
Contributed by Joyce Taylor