Seafood Safety

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Detection and Prevention

Hepatitis A appears to be more resistant to heat than other viruses. A laboratory study by Peterson et al. (1978, as cited in Gerba et al., 1985) showed that hepatitis A viruses in infected oysters were inactivated after heating at 140F for 19 minutes. Therefore, mollusks which are steamed only until the shells open (a common cooking practice) are not exposed to heat long enough to inactivate hepatitis A viruses.
Norwalk virus was first recognized as a pathogen during an outbreak of gastroenteritis in Norwalk, Ohio in 1968 (Adler and Zicki, 1969, as cited in Gerba et al., 1985). It is now considered a major cause of non-bacterial gastroenteritis. From 1976 to 1980, the CDC reported that 42% of the outbreaks of non-bacterial gastroenteritis were caused by Norwalk virus (Kaplan et al., 1982, as cited in Gerba et al., 1985).

Contaminated Species

Illness from norwalk virus has been associated with eating clams (both raw and steamed) (Morse et al., 1986; Porter et al., 1987), oysters (Gunn et al., 1982; Eyles et al., 1981) and cockles (Appleton and Pereira, 1977, as cited in Gunn et al.,1981).

Symptoms & Treatment

Norwalk virus causes nausea, vomiting, diarrhea, abdominal cramps and occasionally fever in humans. Symptoms of gastroenteritis usually begin within 40 hours (range 12 - 72 hours) of consuming contaminated food. Gastroenteritis coused by norwalk virus is a self-limiting illness which usually persists < 48 hours, but can last a long as 1 week.


Some of the more frequently recovered viruses from shellfish are the polioviruses because of the common practice of immunizing American children against polio (Larkin and Hunt, 1982). The vaccine consists of live attenuated viruses that replicate in the intestine but produce few or no clinicval symptoms. Children who have been immunized excrete viruses (from 1000 to 1,000,000 viruses/gram feces) for several days after the vaccine is administered. An examination of 20% of the polioviruses isolated from the Texas Gulf showed that all were of vaccinal origin. Since the viruses in the vaccine are modified, they present no health hazard if consumed by humans.

Bacterial Contaminants

Listeria Monocytogenes

In the early 1900's Listeria monocytogenes was recognized as a bacterium which caused illness in farm animals. More recently it has been identified as the causative agent of listeriosis in humans.

Listeria is ubiquitous in nature and has been isolated from soil, vegetation, marine sediments and water (Peters, 1989). It is a gram-positive, non-spore forming, motile rod (Gellin and Broome, 1989). This facultative anaerobe, can grow between 1 - 45C (Peters, 1989), with an optimal growth temperature of 30 - 37C (USFDA, 1987). The pH range for growth of Listeria is also tolerant to salt (Peters, 1989).

Contaminated Species

The greatest threat of listeriosis is from ready-to-eat products which do not require further cooking at home. Listeria in raw food is less of a concern to the food industry since the bacteria are killed when cooked thoroughly. Listeria has been isolated from dairy products (MMWR, 1989), vegetables (Hughey and JOhnson, 1987), seafood (Lennon et al., 1984), beef and poultry (Peters, 1989). Seafood that have tested positive for Listeria include: raw fish (NFI, 1989), cooked crabs (Anonymous, 1987), raw and cooked shrimp (Anonymous, 1987), raw lobster, surimi and smoked fish (NFI, 1989).

Although the USFDA has isolated Listeria from seafood, listeriosis has not been directly associated with the consumption of finfish or shellfish. It is not understood why Listeria has been recovered from seafood but has not caused illness. The USFDA has proposed a number of theories to explain this phenomenon (USFDA, 1987). It is posible that cases of listeriosis have occurred from seafood but have been unreported or misdiagnosed. It has also been suggested that Listeria may not be virulent in all foods. Seafood may contain components that reduce the virulence of Listeria; or conversely milk and vegetables may contain components that enhance the virulence.

Geographical Area

Listeria is a contaminant introduced to foods during processing. Therefore, no particular geographic areas are especially susceptible to contamination. Listeriosis is reported to occur most commonly in the summer months, but a consistent seasonality has not been observed in systematically collected data (Gellin and Broome, 1989).

Symptoms & Treatment

The incubation period of Listeria is estimated to be between 4 days and 3 weeks (Gellin and Broome, 1989). Exposure to the bacteria does not constitue disease. Pathogenic strains of Listeria have been recovered from the gastrointestinal tract of asymptomatic individuals (Lamont and Postleth-Waite, 1986, as cited in Gellin and Broome, 1989). Most healthy individuals are either unaffected by Listeria, or experience only mild flu-like symptoms (Peters, 1989).

Victims of severe listeriosis are usually immunocompromised. Those at highest risk of contracting listeriosis include: cancer patients, individuals taking immunosuppressive drugs, alcoholics, pregnant women, patients with diminished gastric acidity (Ho et al., 1986, as cited in Gellin and Broome, 1989) and individuals with AIDS (Mascola et al., 1988, as cited in Gellin and Broome, 1989). Severe listeriosis can cause meningitis, abortions, septicemia, encephalitis, endocarditis, abscesses and local purulent lesions, malaise, fever, vomiting, violent or bursting headache and convulsions (Lennon et al., 1984; Gellin and Broome, 1989).


Since 1981 there have been three major outbreaks of listeriosis in North America (Gellin and Broome, 1989). The three outbreaks were traced to contaminated coleslaw (occurred in Nova Scotia, Canada), milk and Mexican-style cheese. Aside from these major outbreaks, listeriosis is generally a sporadic illness (Gillin and Broome, 1989). Recently there was an isolated case (a female cancer patient in Oklahoma) of listeriosis which was traced to contaminated turkey hot dogs (Anonymous, 1989). A recent epidemic of perinatal listeriosis in New Zealand was loosely linked to the consumption of shellfish and raw fish, but a definitive connection to seafood could not be drawn (Lennon et al., 1984).

Detection & Protection

Sterile-site cultures can be used to detect Listeria monocytogenes. This method requires 10 days for negative results and 14 days for presumptive positive results (USFDA, 1988). More rapid results are now possible with commercial DNA hybridization kits (Gene-Trak) (King et al., 1989; Klinger et al., 1988; USFDA, 1989), or ELISA kits (Organon Teknika) (Mattingly et al., 1988; USFDA, 1988).
Listeriosis can be prevented by thoroughly cooking food, and by preventing cross contamination once the food is cooked.

Vibrio cholerae

Vibrio cholerae is an autochthonous bacteria of brackish water, estuaries, and salt marshes of temperate zone coastal areas (Hood and Ness, 1982; Colwell et al., 1981). Unlike the other vibrios, Vibrio cholerae (and Vibrio mimicus) does not require salt for growth (Blake, 1983). It is a Gram negative, curved, rod-shaped bacterium which is actively motile (Morris and Black, 1985). It has been suggested that V. cholerae exists in association with copepods since the bacteria produce chitinase and exhibit similar seasonal fluctuations (Shandera et al., 1983). Higher densities of V. cholerae are recovered during warmer months (Blake, 1983).

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