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 140°F 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).
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.
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 - 45°C (Peters, 1989), with an optimal growth
temperature of 30 - 37°C (USFDA, 1987). The pH range for growth of
Listeria is also tolerant to salt (Peters, 1989).
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.
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 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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