Parasite Hazards of Public Health Significance to U.S. Consumers of Raw Fish: How Safe is Sashimi?
J. John Ranako, MS, DVM
In recent years there has been a steady increase in seafood consumption in this country. This increase has occurred for several reasons. Improved seafood quality, handling, distribution and marketing efforts have all contributed. Perhaps the most important reason has been the popularization of the health benefits of eating fish (omega-3 fatty acids and a low fat protein source. During the same period, consumption of raw fish preparations has increased significantly in the U.S. The American seafood consumer is becoming more adventurous as evidenced by the growing popularity of sashimi, sushi, seviche and Poisson cru. There has also been a trend for undercooking fish in the "grill it rare" styles of preparation.
Along with this increase in consumption of raw or rare cooked fish has been an increase in reported cases of parasite problems of public health significance. Periodically, reports of the dangers of eating raw fish appear in the popular press, causing great distress to consumers and to people in the seafood industry. Fortunately for the consumer, there are relatively few species of fish that pose potential health hazards when eaten raw. These species can be avoided in raw fish preparations or rendered harmless by freezing prior to consumption. Conversely, many species of fish are virtually free of harmful parasites and can be enjoyed fresh and raw.
There are three general categories of parasites in fish that pose potential public health hazards when consumed raw. These include the cestodes (ex. Diphyllobothrium spp.), trematodes (ex. Nanophyetus salmonicola) and the nematode. (ex. Anisakis simplex).
Several species of the genus Diphyllobothrlum are well known to cause parasitism in man, the most common being D. latum, the broadfish tapeworm. This parasite is common in freshwater fish such as the pike (Esox spp.), perch (Perca spp.), and burbot (Lota ssp.). The pacific salmon (Ochorhynchus ssp.) are the most important sources for infection in man in North America (Ache and Szyfres, 1987). Although most cases have been within Eskimo populations, a recent outbreak in California occurred as a result of improperly prepared salmon sushi at a party of physicians (MMWR, 1981). The salmon was airflown fresh from Alaska and had not been frozen as a preventative measure.
The trematode, Nanophyetus salmonicola is the vector of the rickettsial agent Neorickettsia helminthoeca in dogs and other canids. This parasite is common in salmonid fish and accounts for the common term "salmon poisoning" in dogs (Millemann and Knapp, 1970). This trematode is known to infect man on occasion, however the rickettsial agent has not been associated with disease in man. Raw, incompletely cooked or smoked salmon and steelhead have been implemented in the 10 cases reported in the Pacific Northwest between 1974 and 1985 (Eastburn et al,1987).
The Anisakid nematodes have received the most attention with the press in recent years although the total number of cases of human anisakiasis reported in the U.S. is very low (less than 100 ever reported). The definitive hosts of the anisakids are marine mammals. They utilize copepods as a first intermediate host and a variety of marine fishes as a second intermediate host. Anisakis simplex is commonly known as the "herring worm" and Pseudoterranova decipiens is known as the "codworm". Both types can cause infection in man varying from noninvasive to invasive infections requiring endoscopic or surgical removal of the worm. It is generally accepted that the primary source of infections in the U.S. has been the Pacific salmon (Onchorhynchus spp) and the Pacific rockfish (Sebastes spp) commonly sold as the "Pacific red snapper" (Higashi,1985; Oshima,1987: McKerrow,et al,1988).
As discussed, there are relatively few species of fish that have been implemented as the cause of significant parasite problems in the U.S. The Pacific salmon are known to harbor parasites of D. latum, N. salmonicola and the Anisakis simplex and therefore should never be eaten raw unless previously frozen to kill the larvae of these parasites. The many species of Pacific rockfish are very common in the market and are known to harbor Anlsakid larvae. Again, these fish should never be used in raw fish preparations unless previously frozen as a preventative measure.
The U.S. Food and Drug Administration recommends that all fish intended for raw or rare consumption be previously frozen to eliminate the potential health risk from parasites (FDA, 1987). This position has keen taken because various fishes contain harmful parasites and consumers are eating a variety of fishes raw, therefore all species of fish should be frozen prior to raw consumption. The problem with this approach is that the list of fish traditionally eaten raw are not necessarily the same fish that contain the parasites of concern. Freezing is not necessary for all types of fish prior to raw consumption.
In particular, the large tuna (Thunnus spp.) do not appear to pose a significant public health risk from parasites when eaten raw. The large tuna species (blue fin, big eye and yellow fin) are the staple of the sushi bar and are the most common fish used for sashimi in the U.S Equally important in the consumption of fresh tuna is the "grill it rare" style of cooking which has become popular in recent years. It is generally accepted that the large tuna species are safe to eat raw or rare.
A survey conducted in Hawaii on 134 species of marine fishes found that infections with larval anisakids were primarily visceral with very few species having larvae in the edible musculature. This survey did not find any anisakid larvae in the muscles of the yellow fin and the big eye tuna evaluated (Deardorff et al, 1982) Similarly, in an extensive review of parasitological studies in Japan, neither the yellow fin or the blue fin tuna were found to harbor anisakid larvae in the muscles (Oshima,1972). Dr. Noboru Kagei, parasitologist at the Japanese National Institute of Health reports that of 5757 cases of human anisakiasis in Japan between 1964 and 1988, none could be directly connected with the consumption of the large tuna species (personal communication). In Hawaii, thousands of pounds of fresh tuna are eaten raw daily. To date, no cases of human anisakiasis have positively implemented the large tuna species.
The consumer deserves to have adequate protection from food borne illness whether that food is a product of agriculture, aquaculture or captured from natural stocks as in the case of our fisheries. Parasite problems related to the consumption of raw fish can be alleviated through consumer education and appropriate preventative measures (freezing) applied to fish species known to be hazardous. The following recommendations are suggested as an approach to protect the consumer from fish borne parasites of public health concern.
1. Identify the "high risk" species of fish and the parasites that they harbor. Compile a comprehensive list of fish that should not be eaten raw or undercooked unless previously frozen.
2. Identify the fish species that do not harbor harmful parasites and can be considered safe to eat fresh and raw.
3. Concentrate efforts on education programs for consumers and food service personnel to reduce the risk of parasitism from raw fish consumption.
4. Apply appropriate preventative measures (ex. freezing) to those species that pose significant public health hazards.
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