Seafood Safety

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Seafood Safety:

Food-borne illness is not limited to seafood, but is a common concern of all food industries. The recent media attention to seafood has led to an increase in public awareness and a number of misconceptions about the safety of eating seafood.

Between 1973 and 1987, shellfish accounted for 2.8% of the cases of food-borne illness reported to the Centers for Disease Control (CDC), and finfish accounted for 2.2% of the cases. These statistics may seem high at first glance, but they are somewhat misleading. For example, 37% of the cases of seafood-borne illness in the U.S. between 1977 and 1981 were attributed to ciguatera, a toxin found only in tropical and subtropical fish. An additional 37% of the cases during the same time period were attributed to scombroid poisoning, a toxin produced in the flesh of some species of fish when improperly stored at high temperatures. Therefore, the statistics reported by the CDC are skewed by illnesses which either affect only a small geographical area, or only occur with mishandling of fish.

The incidence of illness attributed to seafood can be reduced if the public is better informed, understands the risks, and most importantly, learns to prevent seafood-borne illness. When handled properly, finfish and shellfish are as safe to eat as any other source of protein. For healthy individuals, the nutritional benefits of seafood far outweigh the safety concerns. Persons with compromised immune systems, such as those with liver disease, can also benefit from eating seafood but should follow a few precautionary measures when preparing seafood.

There are more than 110 different viruses known to be excreted in human feces, collectively called the "enteric viruses" (Goyal, 1984). Viruses survive better at low temperatures and are inactivated at high temperatures (Lo et al., 1976, as cited in Goyal et al., 1984). As a result, most outbreaks of hepatitis occur during winter and early spring. Viruses can remain viable for long periods of time in seawater and have been shown to survive as long as 17 months in marine sediment (Goyal et al., 1984). Viruses associated with sediment are as infectious to animals as those that are freely suspended. Marine sediment acts as a reservoir of viruses, which may be resuspended by any kind of turbulence, such as boating, storms and dredging (LaBelle et al., 1980). Rainstorms can also increase viral concentration in the water by increasing land runoff (Gerba et al., 1979) and by release of sewage from overburdened treatment plants (Goyal, 1984).

Virus Uptake & Elimination by Shellfish

Viruses have been isolated from hard clams, oysters, mussels, soft clams, crabs, cockles, lobster and conch. In filter-feeding mollusks, the viruses can become concentrated at a level higher than the surrounding water. The viruses do not multiply in bivalves, but accumulate in the liver-like digestive gland.

Carnivorous shellfish, such as, crabs and lobster can accumulate viruses by contact with contaminated seawater and/or by consuming contaminated bivalves (Hejkal and Gerba, 1981) Viruses are generally present in crabs at a level below that of the water. The highest concentrations of viruses are found in the inedible portions of crabs (Goyal et al., 1984). However, the potential health hazard should not be overlooked since tissue contamination could occur when crabs are prepared for consumption.

A number of experiments on the efficiency of viral depuration have been conducted and have resulted in a range of conclusions, although the more recent studies generally do not support the use of depuration for viruses. One of the earlier studies, using artificially infected soft shell clams, reported that most viruses are purged within a 24-48 hour period, and low levels of viruses are depurated more rapidly than high levels (Metcalf et al., 1979). A more recent study (Hay and Scotti, 1986) using insect picornavirus and Crassostrea gigas, showed that viruses were present in the oyster tissue even after 64 hours of depuration. In a related experiment (Scotti, et al., 1983), both uptake and elimination of viruses were shown to be variable even when bacterial depuration appeared to be normal. These researchers concluded that bacterial depuration rates can not accurately predict viral contamination levels. Finally, an Australian study (Grohmann et al., 1981) using naturally infected oysters, indicated that norwalk virus is not completely depurated after 48 hours. In this study, some of the volunteers, who were fed depurated oysters (which met bacteriological standards), become ill with viral gastroenteritis (60% of illnesses occurred during periods of heavy winter rain).

General Viruses - Detection & Prevention

Fecal coliforms are used as indicator bacteria to predict the possible presence of viruses and other pathogens in shellfish. The water standard for harvesting mollusks is 14 fecal coliforms or less per 100 ml of water (NSSP 1989). However, it is generally accepted that coliforms do not accurately indicate the presence or absence of viruses (Goyal and Gerba, 1978; Gerba et al., 1979; LaBelle et al., 1980; Goyal et al., 1984). Generally, bacteria do not live as long as viruses in the marine environment (LaBelle et al., 1980). Therefore, it is possible for viruses to be present in water which is free of bacteria. In a Texas Gulf coast survey, enteroviruses were detected 35% of the time in waters which met acceptable standards for shellfish harvesting (Gerba et al., 1979). A Similar study of shellfish beds open to harvesting in the Great South Bay, Long Island, NY, resulted in enterovirus recovery in 37.5% of the water and shellfish samples (Vaughn and Landry, 1977, as cited in Gerba and Goyal, 1985). Also, outbreaks of hepatitis A have been associated with oysters harvested from certified grounds (Mackowiak et al., 1976; Portnoy et al., 1975).

Fecal coliform standards only apply to filter-feeding mollusks. The regulations do not apply to commercial harvesting of crabs and lobsters. Although viruses accumulate in the nonedible portions of crabs and lobsters, they have caused viral illness due to contamination of edible tissues while cooking (Goyal et al., 1984). Mobile shellfish, such as crabs and lobsters, also present a problem since they can accumulate viruses in polluted waters and move to cleaner areas and act as vectors of viral disease.

Some cases of illness have been linked to insufficiently cooked shellfish (Feingold, 1973). Most viruses (excluding Hepatitis A) are inactivated when the internal temperature of the mollusk reaches 140F, which requires 4 to 6 minutes of steaming (Koff and Sear, 1967; Giusti and Gaeta, 1981). A common cooking practice is to steam mollusks only until the shell opens. It has been demonstrated that shells open after only about 1 minute of steaming, which is not sufficient time to inactivate all of the viruses.

Hepatitis A is 27 nm in diameter and has single-stranded RNA (Gerba et al., 1985). The first outbreak of seafood-borne (oysters) hepatitis A occurred in Sweden in 1955 (Lindberg-Braman, 1956). Hepatitis B has never been associated with shellfish consumption, although hepatitis B antigen was recovered from clams near a hospital sewage outlet along the coast of Maine (Mahoney et al., 1974, as cited in Portney et al., 1975). In temperate climates, peaks in hepatitis outbreaks occur in the late fall and early winter (Gerba et al., 1985).

Contaminated Species

Both raw and steamed hard clams (Feingold, 1973), oysters (Mackowiak et al., 1976; Portnoy et al., 1975), mussels (Dienstag et al., 1976, as cited in Gerba and Goyal, 1a978) and soft cvlams (Grady et al., 1965, as cited in Gerba and Goyal, 1978), have been implicated in outbreaks of hepatitis A.

Symptoms & Treatment

Symptoms of hepatitis A infection usually begin within 4 weeks (range: 2 - 6 weeks) of exposure to the virus. The initial symptoms are usually weakness, fever, malaise and abdominal epigastric pain. As the illness progresses, the individual uauslly becomes jaundice, and may have dark urine. The severity of the illness ranges from very mild (young children are often asymptomatic), to severe, requiring hospitalization. The fatality rate is low (<0.1%), and deaths primarily occur among the elderly and individuals with underlying diseases (Anonymous, 1989; Bryan, 1986; Feingold, 1973).


Residence of coastal states have a higher incidence of infection than inland states (Goyal et al., 1979; Goyal, 1984). The CDC reported 4 outbreaks of hepatitis A traced to seafood consumption between 1977 and 1981 (USFDA, 1984).

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