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 140°F, 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).
Statistics
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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