EATING RAW FISH: THE DANGERS


Published: September 30, 1981

THE reported dangers of illness brought on by eating raw fish and meat often cause fear and confusion in the minds of those who enjoy such foods, either for their esthetic or nutritional benefits.

Among foods p posing such questions are clams and oysters, which can cause hepatitis if taken from polluted waters , and rare or raw meat in the form of steak tartare or beef carpacci o, which can transmit beef tapeworms to humans. Recognizing the possibility of infection, but uncertain of the likelihood, some people forgo the suspected foodentirely, some continue eating it, ignoring t he dangers, and many eatit and worry.

Most recently, such concern has been expressed over the dangers of eating raw saltwater fish in the forms of Japanese sushi and sashimi, Scandinavian gravlax, South American ceviche and raw or green herring, which is popular in the Netherlands.

When raw, most saltwater fish that are eaten can be carriers of anisakis simplex, a parasitic roundworm that may invade the gastrointestinal tract of humans, causing mild to serious complications. But again, the warnings of parasitologists and physicians are difficult to reconcile with the absence of known infection.

''So far we have not found any anisakis,'' said Dr. Edith Newman, medical director of the Jetti-Katz Clinical Laboratory, the leading facility in New York City specializing in testing for gastrointestinal parasites. ''But then we have never specifically looked for it.''

Because raw fish in its various forms is increasing in popularity, the City Health Department will begin a full-scale investigation late next month to determine the incidence of parasite infection in the local fish supply. Jean Cropper, deputy commissioner for environmental services, said last week that fish in the city's wholesale and retail markets and restaurants would be examined for all possible parasite infection.

Attention was focused on the dangers of such parasites last fall, when the Centers for Disease Control in Atlanta reported four cases of infection by the fish tapeworm diphyllobothrium, traced to the eating of raw salmon.

In investigating 10 samples of salmon in Seattle last July, inspectors for the Food and Drug Administration found that although those particular samples contained no tapeworm larvae, they did contain the anisakis larvae. Salmon, an anadromous fish, lives in both fres h an d salt water, and so can contain both types of parasites. Th e dangers of tapeworm from freshwater fish have long been known an d recognized and almost no freshwater fish is used for sushi and sas himi, except on some rare occasions in Japan.

Gravlax, which is only lightly salted, can be a cause of both types of parasite infection. In this country, the incidence of tapeworm infection is restricted primarily to those of Scandinavian descent and to Jewish women who taste raw freshwater fish such as carp, pike and whitefish in preparing gefilte fish. Freshwater fish is generally considered safe if it has been properly salted and smoked, or cured in a heavy salt brine, as for lox.

Anisakis simplex, however, is virtually unknown in this country, and because it is hard to identify, or not even examined for, it may go undiagnosed. Even if it is diagnosed, it is not a disease that must be reported, and, unlike tapeworm, cannot be cured by medication, another reason it is hard to document.

Dr. George J. Jackson, a parasitologist who is the acting branch chief for food and cosmetic microbiology with the F.D.A., said, ''We have seen only a slight increase in the number of anisakis cases here, but I think they are just the tip of the iceberg. Symptoms may be too mild to notice or may be misdiagnosed, most often as appendicitis or stomach cancer.''

The infection can take several forms, as described by Dr. Ronald R. Roberto, deputy chief of the infectious disease section of Department of Health Services of California.

''Man is not a definitive host, so the larvae will not grow or increase in number,'' Dr. Roberto said. ''In the mildest and most frequent cases, the worm lodges in the throat and is coughed up. If it invades the intestinal wall it can cause inflammation, nausea, cramps and diarrhea and will last anywhere from a day to a week, after which it will be over.



''In the most serious cases,'' he continued, ''it penetrates the intestinal wall and enters the peritoneal cavity, where it may cause chronic lesions or prompt white blood cells to gather around to fight it off, thus forming a granuloma, which is often mistaken for a tumor. Then if surgery is performed, the worm can be found within the granuloma. Nevertheless, we do not consider it a major problem, in spite of the potential. But we will probably see more of it as the eating of raw fish increases.''

Symptoms of gastric anisakiasis are said to develop within four to 12 hours of eating contaminated fish (intestinal anisakiasis will develop within seven days).

There is no cumulative effect from eating raw fish. It is possible to eat 100 pieces and not get anisakiasis if it is not present in those pieces, and it is possible to become infected from eating only a single piece that contains larvae, usually about one to one and half inches long.

In a report on anisakiasis written in 1976 for the Armed Forces Institute of Pathology, Ronald C. Neafie and John R. Dooley, both parasitologists, said, ''Infected fish are commonly marketed in this country and elsewhere; for example about 90 percent of cod fillet sold in and around Washington, D.C., contain anisakine larvae.'' They recommended that ''public health education should discourage the eating of raw or inadequately prepared fish or squid.''

At various times, contaminated fish have been found off the waters of Massachusetts, and in 1970, the Beltsville Parasitological Laboratory in Maryland reported that infected fish were found in markets in Connecticut, Maryland and New York, as well as in fish from Canada. Fish from northern waters are generally considered more likely to have anisakis simplex larvae than those from tropical waters, and the proximity to mammals such as whales, seals and certain strains of porpoises also increase possibilities because those animals are important to the life cycle development of the parasitic nematodes.

Fish that have the anisakis larvae do not necessarily transmit it to humans. While the fish are alive, the worms live in their stomachs. Once the fish are dead, the larvae penetrate the tissue that is eaten.

It is safest, therefore, to gut fish as soon as they are caught to minimize chances of the larvae's spreading. It is also possible to remove anisakis larvae from fish as it is being cut, because such larvae are visible to the trained eye, especially those of experienced sushi cutters.

To be safe, however, responsible sushi shop owners buy only the freshest fish and never serve fish in which larvae were discovered. The most susceptible fish, such as mackerel or squid, are not served completely raw.

According to Ryudaburo Kawada, the owner of the Takezushi and Kurumazushi restaurants, all mackerel is salted and then marinated in strong vinegar and squid is purchased frozen, a process that kills the parasite. Mr. Kawada also said that all bonito is infected and so is rarely ordered by non-Japanese and the susceptible yellowtail is also purch ased frozen.

Shrimp, eel and octopus are always cooked for sushi. Nevertheless, fluke, porgy and sea trout are traditionally served raw and all have been found to have anisakis at one time or another. There are no reports citing tuna as a carrier.

Because of an outbreak of the parasite a few years ago in the raw herring favored in the Netherlands, the law there now requires that all herring must be gutted immediately, then salted and frozen at 4 degrees below zero within 12 hours of being caught and for 24 hours thereafter. Such freezing, or heating to 140 degrees, kills the larvae.

But though anisakis simplex is known in Japan, it is not considered dangerous by most experts there and no regulations have been established. Dr. Noboru Kagei, chief of the second division of parasitology at the National Health Institute of Japan in Tokyo, said that the number of cases is considered negligible, although there, too, fish from northern waters are suspect, and mackerel and squid are treated as Mr. Kawada described.


Nevertheless, there has been some increase of infection reported, most notably in the Ohita prefecture, where 42 cases were reported in 1980 as against 12 in 1977, an increase possibly a result of the eating of raw mackerel by a group of men at a party. Stating that local health centers rarely issue warnings because it is not considered one of the most dangerous diseases, Dr. Kagei said that the number of cases of anisakiasis may be much greater than reported.

Acknowledging that he eats raw fish often and that doing so is a custom in Japan, Dr. Kagei said it is difficult to require all fish to be frozen because so many varieties are eaten raw. But he also stated that he believes that some regulations are needed.

Putting the matter in perspective, Dr. Roberto said, ''There's no point in doing fish market surveys. The parasites are there all right and they can't be cleared up. You can't treat fish. People who like to eat raw fish because they enjoy it or feel it is healthful should be aware of the risk, and go only to good clean places and if they feel any gastric symptoms should see a physician.''


 

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