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SCIENTIFIC
S TAT U S
S U M M A RY
V
iruses have “emerged” as causes of foodborne disease, according to data compiled
by the Centers for Disease Control and Prevention. During 1983–87 in the United
States, Norwalk virus was the fifth leading cause of foodborne disease among
outbreak-associated illnesses; hepatitis A virus was the sixth; and other viruses (principally
rotaviruses) were tenth (Bean et al., 1990). By 1988–1992, the most recent period for which
data have been issued, Hepatitis A virus had risen to the fourth leading cause and Norwalklike viruses first appeared on the top 10 list as the ninth leading cause (Bean et al., 1996).
The numbers of reported foodborne illnesses are fewer than actually occur because the
CDC’s passive data collection system records only illnesses occurring as outbreaks, rather
than those occurring sporadically. Hepatitis A, which is notoriously under reported in the
United States (Cliver, 1985), is the only foodborne viral disease in which official reporting is
mandatory for all diagnosed cases. Thus, records of the incidence of the other viral diseases
are certain to be less accurate. And, just as with Norwalk-like viruses, other new viral agents
are likely to appear among the top 10 causes of foodborne disease in future compilations.
VIRUS
TRANSMISSION
VIA
FOOD
SPECIAL FEATURES OF VIRUSES AMONG FOODBORNE DISEASE AGENTS
Particle as the Transmissible Form. Viruses pass from host to host in the form of inert
particles. The particles are roughly spherical, with diameters of 25 to 35 nm (picornavirus and
calicivirus; for example, hepatitis A and Norwalk-like viruses, respectively) or as large as 75
nm (rotaviruses; Table 1). The smaller foodborne viruses contain single-stranded RNA,
whereas the rotaviruses contain double-stranded RNA. Human enteric viruses containing
DNA are known, but none have been proven to be transmitted via food or water. The outer
surface of the particle is a highly specific protein coat that protects the RNA, interacts with a
susceptible host cell to initiate infection, and acts as the antigen against which the host’s
immune responses are mounted. Because these particles are totally inert, they cannot multiply
in foods or anywhere outside the host. Neither can they carry out any metabolic activity, nor
respond to stresses encountered in the environment.
Viral Infection. The virus particle will enter only a suitable host cell. Specificity depends
on the interaction of the coat protein with receptors on the host cell. Only certain cells in the
bodies of certain species can be infected; essentially all viruses transmitted to humans via foods
are specific for humans and perhaps a few other primates. In practical terms, zoonotic viruses
are not transmitted via foods.
When the viral coat protein reacts with homologous receptors on the cell membrane, the
host cell engulfs and uncoats the viral RNA (Cliver, 1990). The RNA is translated to various
virus-specific proteins and replicated (with the help of virus-specific, RNA-dependent RNA
polymerase) into additional copies of viral RNA. The replicative cycle takes place in the host
cell’s cytoplasm, without participation of DNA. (Reverse transcription of the viral genome
does not occur.) As coat protein and viral RNA accumulate in the host cell, progeny particles
assemble themselves and eventually leave the cell via leakage or in blebs that pinch off the
cell’s surface membrane.
Viral disease may occur when progeny virus spreads and infects enough host cells to
interfere with some normal bodily function. The viral replication may kill or subvert the host
cells. In the case of hepatitis A, however, the host’s infected liver cells apparently are little
affected until the body mounts an immune response and destroys the infected cells by means
of “killer” (cytotoxic) T-cells. Whether the liver or the lining of the small intestine is the site
of viral infection, the hepatitis or gastroenteritis that results is seldom fatal.
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A Publication
of the
Institute of Food
Technologists’
Expert Panel
on Food Safety
and Nutrition
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Author:
Dean O. Cliver
Department of Population Health
and Reproduction
School of Veterinary Medicine
University of California, Davis
Davis, California, USA
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Update of:
Cliver, D. O. 1988. Virus
transmission via foods.
Food Technol. 42:241-248.
EPIDEMIOLOGY OF FOODBORNE VIRUSES
Enteric (fecal-oral) Transmission. Essentially all foodborne viruses are transmitted
enterically; they are shed with feces and infect by being ingested (Cliver, 1990). Like many
other infectious agents that are enterically transmitted, the majority of infections are probably
contracted by person-to-person contact, most probably via fecally soiled hand to mouth. If
vomiting is part of the illness, viral particles may be shed with vomitus. Indirect transmission
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VIRUS TRANSMISSION VIA FOOD
of enteric agents may occur via vectors such as flies,
fomites such as soiled diapers, but most importantly via
the vehicles of food and water. In the United States,
more foodborne than waterborne viral illnesses are
recorded.
Foods as Vehicles for Viruses. A few enteric
viruses of humans have been reported to be indirectly
transmitted via vehicles other than foods; whereas
others are foodborne with some frequency. Because
hepatitis A virus is the only reportable virus in the
United States, it is the only virus for which transmission
via food or water can be compared by using total
recorded incidence figures (Cliver, 1985). The proportion of reported hepatitis A cases attributed to food- and
waterborne transmission, collectively, in the United
States is only 3 to 9% (CDC, 1994). However, it must
be recognized that the numerator (e.g., foodborne
illnesses) and the denominator (total reported illnesses)
are compiled in very different ways. That is, the
foodborne illnesses are only those occurring in outbreaks that happen to have been investigated. Many are
not. Thus, illnesses in unrecorded outbreaks and those
occurring sporadically, though also foodborne, would
not be included. It is also certain that not all diagnosed
cases of hepatitis A are reported through official channels, but the proportion missed in this way is probably
less than for the foodborne category. In any case, it
seems likely that if all of the recorded foodborne
illnesses had been prevented, no statistically significant change in the total rate of recorded hepatitis A in
the United States would be detected. Data from other
countries are very difficult to obtain and are generally
not compiled in ways that permit comparison with
those from the United States.
Viruses among Foodborne Disease Agents.
Various groups of viruses occupied three of the top 10
spots as causes of reported foodborne disease in the
U.S. during 1983–1987 (Bean et al., 1990). This was a
period when the Norwalk virus was regarded as one
agent, rather than a group of variably related viruses.
Therefore, one would have expected that the number of
reported food-associated cases would increase as the
diagnostic “net” broadened. However, Norwalk-like
viruses fell to ninth position (two outbreaks comprising
292 cases) during 1988–1992 (Bean et al., 1996),
which suggests that the many improved diagnostic
methods that have appeared in the literature are being
applied very sparingly. Among reported outbreaks of
foodborne disease in the United States in 1988–1992,
etiologies were determined in 1,001 (41%), comprising 36,890 (48%) of the cases; 4% of these outbreaks
and 6% of the cases were attributed to viruses. Until
methods for detecting viruses in foods are improved
and widely applied, knowledge of foodborne virus
transmission will result from successful diagnoses of
human illnesses. Here, too, it should be noted that much
foodborne illness is endured without consulting a physician and that physicians are unlikely to order diagnostic tests for virus disease because, if the viral diagnosis
is confirmed, the physician has no means of treating the
illness. The hepatitis A virus now appears to be causing
more foodborne illnesses than many of the betterknown bacterial pathogens. Among the 1,422 recorded
foodborne outbreaks during 1988–1992 that were of
undetermined etiology (presumably gastroenteritis in
most instances), it was surmised on the basis of incuba-
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FOOD TECHNOLOGY—VOL. 51, NO. 4, APRIL 1997
tion periods longer than 15 h that 35% might have been
caused by viruses (Bean et al., 1996).
Shellfish as Special Vehicles for Viruses. Bivalve molluscs, such as clams, cockles, mussels, and
oysters, are especially prone to transmit viruses. The
waters in which they grow are increasingly subject to
human fecal contamination, sometimes from sewage
discharges and sometimes from infected shellfish harvesters. The shellfish collect viruses in the course of
their filter feeding activity. Human viruses do not infect
these species, but they are harbored for days or weeks
in the shellfish digestive tract and are apparently more
difficult to remove than bacteria during processes
intended to cleanse the shellfish (e.g., depuration;
Grohman et al., 1981; Power and Collins, 1989).
Unlike many other seafoods, shellfish are usually eaten
with their digestive tracts in place. They are often eaten
raw or lightly cooked. Shellfish, unlike other foods,
may also protect viruses from thermal inactivation
during cooking (DiGirolamo et al., 1970).
The first recorded outbreak of shellfish-associated viral disease resulted from storing clean oysters in
a fecally contaminated harbor while awaiting sale
(Gard, 1957). Over 600 cases of hepatitis A resulted.
More recently, outbreaks of viral gastroenteritis and
hepatitis A have been associated with eating usually
uncooked shellfish. A clam-associated outbreak of
hepatitis A in Shanghai may have been the largest
recorded outbreak of foodborne disease in history
(Halliday et al., 1991). Sporadic viral illnesses associated with shellfish have also been demonstrated (Koff
et al., 1967); it is difficult to avoid bias entirely in such
studies because, at least in coastal states, a diagnosis of
hepatitis A regularly leads to asking the patient about
shellfish consumption, to the exclusion of other foods.
Shellfish-growing waters are typically monitored
for fecal contamination by testing for bacteria of the
fecal coliform group or for Escherichia coli. The
presence of these bacteria, however, has been shown to
be a poor predictor of the presence of human enteric
viruses (Wait et al., 1983). Unfortunately, no more
accurate index of the presence of viruses in shellfish or
their growing waters has yet been identified. Because
it has no other way to guarantee the safety of raw
cockles, the U.K. government allows their sale only if
cooked by an approved method.
Costs and Special Features of Outbreaks. Hepatitis A is one of the more severe of foodborne diseases;
a few weeks of debility are common, and permanent
impairment of some liver functions occurs occasionally. In contrast, viral gastroenteritis typically lasts 1 or
2 days. In the absence of directly applicable data, costs
per case of foodborne hepatitis A have been estimated
at $5000, versus $887 for Norwalk gastroenteritis
(CAST, 1994).
The span of onsets in an “explosive” outbreak —
or one in which many people were infected on the same
occasion — tends to equal the median incubation
period of the disease. Therefore, an explosive outbreak
of hepatitis A may show onsets of illness over a 28- or
30-day period (Cromeans et al., 1994). Under similar
circumstances, outbreaks of Norwalk virus
gastroenteritis would probably show onsets of primary
illness over no more than 2 days (Appleton, 1994).
Secondary cases may occur in both instances, with
transmission of the infectious agent from those who ate
contaminated food, via contact, to others who did not.
Norwalk gastroenteritis is often characterized by shedding of the virus in vomitus as well as in feces, so
opportunities for secondary transmission are enhanced.
Because immunity after Norwalk virus infection is only
briefly protective, and perhaps because infectious doses
are small, attack rates (number of persons ill ÷ number
of persons exposed) are often quite high (e.g., >50%;
Appleton, 1994).
In outbreaks from food contaminated by a single
infected person, or “diffuse” outbreaks, the long duration of shedding (10–14 days) may further spread the
period of onsets of hepatitis A in situations where the
infected person contaminates food on several days.
Shedding of Norwalk-like viruses can continue for a
week, so diffuse outbreaks of viral gastroenteritis are
also possible. Either virus can persist in contaminated
food and, thus, infect people who eat the food on
different days, further “blurring” the outbreak. Given
all these complications, it is remarkable that epidemiologists have been able to solve as many of the mysteries of hepatitis A transmission as they have. As yet,
sporadic transmission resulting in single illnesses has
defied characterization, except in the study concerning
shellfish that was cited earlier (Koff et al., 1967).
Risk Assessment. The process of risk assessment
has generally comprised hazard identification, doseresponse assessment, exposure assessment, and risk
characterization (CAST, 1994). Because foodborne
infectious agents present some special features not
seen with chemical hazards, the risk assessment process may need some modification in this application
(Potter, 1996). Beyond this, viruses transmissible via
foods offer some problems all their own. Hazard
identification, based on the epidemiological record,
focuses attention on hepatitis A virus and the Norwalklike agents of gastroenteritis. Dose-response assessment for these viruses is complicated by the lack of
laboratory hosts that they will infect; when human
volunteer trials have been conducted, precise quantification of the doses administered was impossible (Dolin
et al., 1972; Grohmann et al., 1981). Even when cell
cultured enteric viruses have been administered perorally, the number of viral particles comprising a cell
culture infectious dose (e.g., a plaque-forming unit)
was not determined. Assumptions based on various
models for infectivity yield quite variable predictions.
Exposure assessment is also difficult for foodborne
viruses because there are no standardized methods for
qualitative detection of viruses in foods, and even the
best methods typically are not quantitative. Distribution of the few human viruses present in the food supply
can be expected to be heterogeneous and nonrandom.
The virus detection methods applicable to foods are
unlikely to be used on a routine basis to determine
which foods contain viruses because the cost of performing predominantly negative tests would be huge.
For these reasons, precise risk characterization is not
really an option. All the same, attempts have been
made; results do not conform closely to present knowledge of the incidence of viral infections in the United
States (Rose and Sobsey, 1993).
VIRUSES TRANSMITTED VIA FOODS
The hepatitis A and small gastroenteritis viruses
are more often transmitted via foods than are other
Table 1 — Major groups of nonenveloped1 human enteric viruses
Diameter of nucleic
acid strands
25-35 nm (single)
70-85 nm (double)
Nucleic acid type
RNA
DNA
Astroviruses
Caliciviruses
Picornaviruses
Reoviruses
Rotaviruses
Parvoviruses
Adenoviruses
1
An envelope on a virus is a lipid-containing outer wrap that derives from the
plasma membrane of the host cell in which the virus was produced.
viruses. All known foodborne viruses except the agent
of tick-borne encephalitis are human-specific and transmitted by a fecal-oral cycle. Perhaps incidentally, all of
these “enteric” viruses contain RNA rather than DNA
(parvovirus, if it is really foodborne, is an exception)
and have no lipid envelope around their protein coats
(Table 1).
Hepatitis A Virus. When hepatitis transmission
via food was first recorded, it was not known that
multiple types of viral hepatitis existed (Cliver, 1966).
Now, at least five serological types of hepatitis viruses
are recognized, belonging to various taxonomic groups;
but only hepatitis A is documented as being foodborne
(Cromeans et al., 1994; Table 2). Hepatitis E virus (a
calicivirus) is apparently not present in North America;
it has been implicated in water-associated, but as yet
not food-associated, outbreaks (Cromeans et al., 1994).
Hepatitis A is one of the more severe of foodborne
diseases, especially among those caused by viruses
(Table 3). Both hepatitis A and E viruses are listed as
Severe Hazards in Appendix V of the 1995 FDA Food
Code (USPHS, 1995). Virus produced in the liver is
shed via the common bile duct and occurs at levels
above 106 particles per gram of feces for days or weeks
before the onset of illness. Food becomes contaminated via fecally soiled hands of infected persons or via
fecally contaminated water, as is usual with shellfish.
The virus is more heat resistant than most enteric
viruses and is quite resistant to drying. Now that a
formalin-killed vaccine has been licensed for use in the
United States and Europe, food workers could be
immunized to preclude the possibility that they might
contaminate food with the virus (WHO, 1995). The
vaccine is produced from a mutant strain of the virus
that replicates in cell culture; wild type virus either
does not replicate in cell culture or replicates very
slowly, often without cytopathic effects (Cromeans et
al., 1994).
Small Round Structured Viruses Causing
Gastroenteritis. The Norwalk virus was the first
gastroenteritis virus reported to be foodborne
(Greenberg et al., 1979). Later, other serologically and
genetically related viruses belonging to the calicivirus
group were recognized (Table 4). The term “small
round structured viruses” was applied to the agents
when they were detected by electron microscopy or
immune electron microscopy (Appleton, 1994).
Astroviruses, which also have visible surface structure, are variably included in this group; they are
discussed separately below. Gastroenteritis caused by
the Norwalk-like viruses often includes vomiting, and
virus shed in vomitus may contaminate food. Because
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immunity is transient, persons who have been infected
and ill with the Norwalk virus are subject to reinfection
with it, as well as with other serotypes, after perhaps a
year (Parrino et al., 1977). Susceptibility is common,
and attack rates in outbreaks are often quite high.
Outbreaks have been traced both to ill persons (Kuritsky
et al., 1984) and to food workers who had recovered
from illness days earlier (White et al., 1986). The virus
evidently is shed in large quantities, and small (not yet
measured) quantities are infectious perorally. The individual viral particles, however, do not appear to be
exceptionally resistant to inactivation by heat or chlorine (Appleton, 1994). Members of this group have not
been shown to replicate in cell cultures.
Other Viruses Occasionally Transmitted via
Foods. Many other groups of human enteric viruses are
known, yet these are reported to be transmitted via
foods only infrequently or not at all (Table 5). Factors
such as duration and level of fecal shedding of the virus,
efficiency of peroral infection, or stability of the virus
in the food vehicle may play a role. Given that some of
these viruses are able to replicate in laboratory cell
cultures and cause cytopathic effects, they may be
better characterized than the more important foodborne
viruses discussed above.
• Astroviruses comprise a distinct group of small
round gastroenteritis viruses that have surface projections in patterns often resembling five- or six-pointed
stars (Appleton, 1994). The non-enveloped protein
coat surrounds single-stranded RNA (Table 1). Usual
features of the disease vary slightly from those of the
Norwalk-like viruses; the incubation period is somewhat longer, vomiting is less common, and very young
(<1 yr) children are more often affected. Epidemiologic
evidence of transmission via foods is limited.
• Rotaviruses contain segmented, double-stranded
RNA (Table 1) surrounded by a double protein coat
(Sattar et al., 1994). They, too, infect young children
most often and are occasionally associated with food
and water.
• Picornaviruses other than HAV have single (+)
stranded RNA (Table 1) in a simple protein coat that is
featureless as seen by electron microscopy. The polioviruses were the first viruses shown to be foodborne,
but virulent strains are now rare, and the vaccine strains
are potential indicators of the possible presence of
other, virulent viruses in food and water (Alhajjar et al.,
1988). Although the coxsackieviruses have sometimes
been reported in association with food and water, only
one outbreak of foodborne coxsackievirus illness has
been reported (Osherovich and Chasovnikova, 1967).
Two foodborne outbreaks of echovirus illness have
been recorded (NYDH, 1989; USDHEW, 1979).
• Tick-borne encephalitis virus is the only known
foodborne virus that is not transmitted by a fecal-oral
route (Grešíková, 1994). The agent infects dairy animals in central Europe (principally Slovakia) via the
bites of vector ticks, Ixodes persulcatus and I. ricinus.
Infected animals shed the virus in their milk, which, if
ingested without pasteurization, infects humans. Products made from the unpasteurized milk may also be
vehicles. The virus belongs to the genus Flavivirus,
meaning that the particle contains single (+) stranded
RNA like most foodborne viruses, but has a lipid
envelope around the protein coat. This is the only
enveloped virus known to be foodborne. It is highly
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FOOD TECHNOLOGY—VOL. 51, NO. 4, APRIL 1997
Table 2 — Some viral causes of hepatitis in
humans. Adapted from Cliver, 1997.
Type
Former name
Transmission
Mode
A
Infectious hepatitis
Fecal-oral
B
Serum hepatitis
Parenteral
C
Non-A, non-B
Parenteral
D
Delta agent
Parenteral
E
Non-A, non-B
Fecal-oral
specific to its vectors and has a limited range, so that
outbreaks are now rare. Seven people in Slovakia were
affected during a 1993 incident (WHO, 1994).
• Hepatitis E virus belongs to the calicivirus group,
so it has single (+) stranded RNA coated with protein
that shows characteristic cup-like depressions
(Cromeans et al., 1994). It occurs widely in Asia,
Africa, and Latin America, but rarely elsewhere and
apparently not at all in the United States and Canada
(except for rare imported cases). It is the non-A, nonB hepatitis virus that was known to be transmitted by
the fecal-oral route. Waterborne outbreaks are common; but for some reason, foodborne outbreaks have
not yet been documented.
• Parvoviruses are proposed as causes of human
gastroenteritis. Although documented food-associated
outbreaks are evidently quite rare, Appleton (1994)
describes an outbreak associated with cockle consumption in England that involved at least 800 persons.
DETECTING FOODBORNE VIRUSES
Detection of viruses in food is most likely to be
undertaken when an outbreak has occurred. It would be
helpful, however, if some routine testing method were
available to apply to foods, such as shellfish, that often
serve as vehicles for viruses.
Outbreak investigation and routine monitoring
present very different sets of priorities. When people
have already been made ill, the relatively high costs of
attempting to detect viruses in food samples may be
acceptable, especially if litigation is anticipated. However, as was mentioned earlier, an outbreak of hepatitis
A affecting several people is likely to be recognized
only 4 weeks or more after the contaminated food was
consumed, so pertinent food samples most likely will
be unavailable for testing. With other viral diseases that
have shorter incubation periods, clinical histories and
diagnostic samples of feces and blood serum may
afford some indication of which virus is to be sought in
the food, which could prove very helpful. In contrast,
testing foods for virus contamination in the hope of
preventing human illness presents special problems.
Costs are likely to outweigh demonstrable benefits. In
addition, since almost all foodborne viruses are transmitted by fecal-oral cycles, one virus is as likely as
another to occur in food, subject to fecal contamination. As ingenious as the methods are that have been
devised for detecting foodborne viruses, none could be
considered routine.
A fundamental problem is that the viruses of
greatest concern, hepatitis A viruses and the Norwalklike gastroenteritis viruses, replicate slowly and
Table 3 — Hepatitis A virus and disease
Picornavirus: particles, featureless spheres, 28 nm in diameter, single-stranded RNA coated with protein
Infection via intestine to liver; incubation period 15-50 days (mean 28 days)
Illness from immune destruction of infected liver cells: fever, malaise, anorexia, nausea, abdominal discomfort,
often followed by jaundice; severity tends to increase with age and ranges from inapparent infection to weeks of
debility, occasionally with permanent sequelae
Shedding: peaks during the second half of the incubation period (10-14 days); usually ends by 7 days after onset of
jaundice
Diagnosis: based on detection of IgM-class antibody against hepatitis A virus in the patient’s blood serum (kits
available)
Immunity: durable (possibly lifelong) after infection; active immunity can be produced with a killed-virus vaccine;
passive immunity is imparted by injection of pooled human immune serum globulin
Adapted from Cromeans et al., 1994.
Table 4 — Norwalk-like, small round structured viruses of gastroenteritis
Calicivirus: particles, spheres 25-35 nm in diameter, single-stranded RNA coated with protein that has characteristic cupped surface depressions; Norwalk-like viruses include Cockle, Ditchling, Hawaii, Oklahome,
Parramatta, Snow Mountain, and Taunton agents
Infection of intestinal lining, incubation period usually 24-48 hours
Illness: nausea, vomiting, diarrhea, etc., generally lasting 24-48 hours
Shedding: during illness in vomitus and feces, possibly 7 days after onset
Diagnosis: detection of virus in stool by ELISA or PCR or of antibody against the virus in patient’s blood serum;
no standard methods, reagents not readily available for most agents
Immunity: apparently transient
Adapted from Appleton, 1994.
inapparently or not at all in laboratory cell cultures. If
foodborne viruses cannot be detected on the basis of
their infectivity, alternate bases include their morphology (as seen by electron microscopy), their antigenic
specificity (as demonstrated by reactions with homologous antibody), their genetic specificity (as demonstrated with complementary probes or polymerase
chain reaction [PCR] primers), or combinations of
these. These methods may be less sensitive than tests
based on infectivity, and by their nature all carry some
risk of yielding a positive result with virus that has been
inactivated (no longer infectious). The final detection
method to be used must be considered when the food
sample is being processed for testing. An additional
problem encountered with the hepatitis A virus is that
the incubation period of the illness is so long (average
4 weeks) that pertinent food samples are unlikely to be
available once the disease has been recognized. These
problems will be addressed briefly here, as they have
recently been reviewed fairly extensively elsewhere
(Cliver, 1995; Cliver et al., 1992).
Processing Food Samples for Testing. Most
foods other than milk, water, and a few others require
liquefaction as a first processing step (Cliver et al.,
1992). Addition of liquid for this purpose is usually
kept to a minimum because virus that is present in food
will be diluted, to the detriment of the sensitivity of the
method. Solid food samples are shaken, comminuted,
or otherwise dispersed in a diluent that has been
selected to encourage dissociation of virus from the
food solids, which will be removed by centrifugation,
filtration, or other means. Additives may be used to
encourage separation of the food solids from the liquid
suspension containing the virus. Because foods suspected of viral contamination are likely to contain
bacteria from both the food and feces, a step to remove
or kill bacteria in the suspension is usually included.
And, because the detection methods available can
usually accommodate only very small volumes of
sample, as much water as possible is removed from the
food sample extract before testing begins. Applicable
concentration methods include adsorption-elution, differential precipitation, ultracentrifugation, and ultrafiltration.
Methods for Detecting Viruses Extracted from
Foods. As was mentioned above, infectivity tests are
not applicable to the viruses that are most often
foodborne. Nevertheless, infectivity testing may be
appropriate when astroviruses, rotaviruses, and perhaps a few others are sought (Chung et al., 1996; Wait
et al., 1983). The remaining detection methods are
based on the morphology, antigenic specificity, or
genetic specificity of the viral particle.
• Morphology of viral particles affords an important basis for their classification (Table 1). However,
many viruses not of human origin are indistiguishable
by simple electron microscopy from viruses suspected
as food contaminants. One way to demonstrate that the
viruses seen in the electron micrograph are indeed the
suspected type is to attach them to the grid or to each
other with homologous antibody (“immune electron
microscopy”), which is a combination of serologic and
morphologic criteria. When the serological type of the
virus is unknown, an alternative is to compare immune
electron micrographs done with acute and convalescent serum from a patient, assuming that paired samples
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Table 5 — Diseases caused by less commonly recognized foodborne viruses
Virus group
Illness (major signs)
Incubation
Duration
Remarks
Astroviruses
Diarrhea
3-4 days
2-3 daysa
Picornavirusesb
Rotaviruses
Meningitis, myalgia, etc.
Fever, vomiting, watery diarrhea,
in children, even occasionally severe
dehydration
Fever, headache, nausea, vomiting,
encephalitis or aseptic meningitis
3-5 daysc
1-3 days
Varies
4-6 days
Some replicate in cell
culture
Food vehicle rare
Often severe
death
7-14 days
Varies
Tick-borne encephalitis
(Flavivirus)
a
Sometimes longer
b
Other than hepatitis A virus
c
Variable
are available. If the convalescent phase serum reacts
with the virus and the acute phase serum does not, this
is likely to have been the agent that caused the illness.
Unfortunately, the method will not detect the small
numbers of viral particles that are capable of causing
disease.
• Antigenic specificity of the viral coat protein can
serve directly as a means of detection by enzyme
immunoassay or by radioimmunoassay. These methods have been of considerable use in diagnostic virology, where they are applied to fecal samples containing
levels of virus often exceeding a million particles per
gram. Because contaminated foods are likely to contain onlyimperceptible levels of feces, the quantities of
virus present in food samples are likely to be much
smaller, and immunoassay tests are seldom adequately
sensitive. Therefore, serologic reactions between virus
and antibody are most often applied in combination
with electron microscopy or with nucleic acid-based
tests.
• Nucleic acid tests are generally based on the
specific interactions of portions of the viral genome
with complementary probes, PCR primers, or both.
PCR and other genetic amplification methods afford
highly sensitive means of detecting small quantities of
virus. Because almost all known foodborne viruses are
RNA agents, the viral nucleic acid must be extracted
and reverse-transcribed to complementary DNA before amplification by PCR can begin. Short complementary probes bracketing a selected region of the viral
genome are introduced with a polymerase that functions at high temperature, and many successive cycles
of replication, denaturation, and annealing are carried
out by a programmed thermal cycler. The result is
millions of short copies of the selected region of the
viral genome. The specificity of these copies is demonstrated on the basis of their appropriate length (in terms
of number of nucleotide bases), their reaction with a
complementary probe that is directed at a portion of the
amplified segment, or both. Probes used for this purpose are labeled with a radionuclide, an enzyme, or
some other means of expressing their presence in
association with the PCR-amplified product.
In addition to the challenges of performing the
demanding PCR and probe-specificity tests themselves,
testing food samples has been found to present some
special problems. Certain food components interfere
with reverse transcription or with PCR amplification.
Specialized means of surmounting these problems are
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FOOD TECHNOLOGY—VOL. 51, NO. 4, APRIL 1997
Virus shed in milk of
tick-bitten animals
being reported (Gouvea et al., 1994). The genetic
diversity of the Norwalk-like viruses also causes problems (Ando et al., 1995; Wang et al., 1994). Although
such nucleic acid-based test methods are exquisitely
specific, it is possible to amplify or probe for a segment
of the viral genome that is common to more than one
type, to afford a broader test when needed (Jaykus et al.,
1995).
• Combined methods consist typically of a serologic method followed by another detection procedure,
as with the immune electron microscopy approach
described above. Virus has also been captured from
sample extracts with homologous antibody for later
detection by PCR (Deng et al., 1994). This method
seems to obviate some of the problems with food
inhibitors of reverse transcription and PCR, and it
allows RNA to be released from the viral particle
simply by heating. It seems that detection methods
combining nucleic acid tests and morphologic tests
have not been devised.
Indicators as Alternatives for Monitoring
Foods. Fecal coliform bacteria and Escherichia coli
have long been used for monitoring shellfish and their
growing waters, but bacteria are essentially irrelevant
to the presence of viruses in shellfish and in other foods
(Berg, 1978; Wait et al., 1983). Human enteric viruses
that are capable of replicating in laboratory cell cultures and producing cytopathic effects have been proposed as alternate indicators of virus contamination. At
least in instances of contamination via community
sewage, there may be enough of such viruses, particularly vaccine polioviruses, to afford an indication of
viral contamination. Infectivity tests are inherently
quite sensitive, but are expensive and may take several
days to a few weeks to produce firm results. Other
candidate indicators of viral contamination of food
have been bacteriophages that infect intestinal bacteria, such as E. coli or Bacteroides fragilis. These bacteriophages are relatively easy to detect, with readout
times of as few as 6 h, but they do not seem more
reliably present than are cytopathic human viruses. In
general, any of these indicators might be of value in
identifying foodborne viruses transmitted via sewage
contamination but not via feces-soiled hands.
PREVENTING VIRUS TRANSMISSION VIA FOODS
Clearly, the reason for compiling information
regarding virus transmission via foods is to develop
strategies for preventing viral foodborne diseases. Vi-
ruses present some unique features among foodborne
disease agents. Since they cannot multiply in foods, one
might reason that control would be relatively easy, but
this has not proven to be the case.
Preventing Contamination. With the sole exception of the tick-borne encephalitis virus, foodborne
viruses come from human feces and can be kept out of
foods by preventing human fecal contamination. Obviously, this has proven a difficult task, even in the matter
of pre-harvest monitoring of shellfish and their growing waters. Conditions that had been judged acceptable
can change very rapidly, with highly unforeseen results
(Halliday et al., 1991). Contamination via the hands of
infected food handlers is quite another matter. Infections that are completely inapparent (Eisenstein et al.,
1963) are known, but incubating infections, in the case
of hepatitis A (USDHEW, 1973), and briefly persistent
infections in persons convalescing from viral
gastroenteritis (White et al., 1986) have been greater
problems. With viruses transmitted by the fecal-oral
route, there is a need for good personal hygiene practices and high standards of food protection and sanitation procedures. In these instances, proper hand washing, done frequently and efficiently (using friction
action and a nail brush), is the general preventive
(except for a very few instances in which contamination by vomitus has been suspected). Feces on the
hands of infected persons may contaminate food,
whether the person is a field worker, a kitchen worker,
or a server. Gloves may be of some value in preventing
hand contact, but availability of hand washing facilities
and management’s emphasis on proper hand washing
techniques are the most important precautions to prevent direct human contamination of foods with viruses
(Ansari et al., 1989; Cliver and Kostenbader, 1984).
Vaccination of food workers against hepatitis A is also
of value in locations where natural immunity from
infection is not regularly attained early in life (WHO,
1995).
Inactivation of Viruses in Foods. Assuming that
contamination has not been prevented, viruses in food
cannot multiply, but may be inactivated before someone eats the food. Storage at room temperature may
encourage inactivation of viruses in food, but may lead
to bacterial hazards. Thermal processing is generally
effective, although some further attention to the adequacy of milk pasteurization to inactivate hepatitis A
virus is needed (Parry and Mortimer, 1984). The
British Government recommends heating mollusks to
85–90°C for at least 90 seconds to destroy viruses
(IFST, 1996). Alternately, shellfish may be depurated
(i.e., held in facilities supplied with clean saline water)
or relayed from where they grew into clean water to
purge themselves of contaminants. Although these
practices have served well in removing pathogenic
bacteria from shellfish, success with viruses is not
guaranteed. Longer periods of treatment are required
for removal of viruses than bacteria (Grohman et al.,
1981; Power and Collins, 1989), and individual shellfish may not cleanse themselves. Other food processes
that are not based on heat are less effective: viruses
present a small target for ionizing radiation; enteric
viruses are generally acid-resistant; and the hepatitis A
virus, at least, is quite resistant to drying. Viruses in
water or on surfaces can be inactivated by strong
oxidizing agents, such as chlorine or ozone, and by
ultraviolet light.
HACCP. Food safety, at least in the developed
world, is increasingly predicated on the hazard analysis and critical control point (HACCP) system, along
with appropriate general precautions. Foods subject to
fecal contamination via wastewater or contact by the
feces-soiled hands of infected persons are at risk of
contamination with viruses; this is the essence of
hazard analysis in this case. Critical control points are
points at which contamination can be prevented (as in
disinfecting potentially contaminated water so that
virus is not carried to food) or undone (such as by
cooking an “at-risk” food thoroughly after it is last
handled). Thorough hand washing, or prevention of
hand contact with foods that will not be cooked thereafter before being served, are also very important
measures. Although depuration or relaying of potentially contaminated shellfish may have some preventive value, these probably should not be considered
critical control points in preventing virus transmission
via shellfish eaten raw.
SUMMARY
Viruses are transmitted to humans via foods as a
result of direct or indirect contamination of the foods
with human feces. Viruses transmitted by a fecal-oral
route are not strongly dependent on foods as vehicles
of transmission, but viruses are important among agents
of foodborne disease. The viruses most often foodborne
are the hepatitis A virus and the Norwalk-like
gastroenteritis viruses. Detection methods for these
viruses in foods are very difficult and costly; the
methods are not routine. Indicators that would rapidly
and reliably suggest the presence of viral contamination of foods are still being sought. Contamination can
be prevented by keeping feces out of food or by treating
vehicles such as water to inactivate viruses that might
be carried to a food. Viruses cannot multiply in food,
but can usually be inactivated by adequate heating.
Other methods of inactivating viruses within a food are
relatively unreliable, but viruses in water and on exposed surfaces can be inactivated with ultraviolet light
or with strong oxidizing agents.
REFERENCES
Alhajjar, B.J., Stramer, S.L., Cliver, D.O., and Harkin, J.M. 1988. Transport
modelling of biological tracers from septic systems. Wat. Res. 22: 907-915.
Ando, T., Monroe, S.S., Gentsch, J.R., Jin, Q., Lewis, D.C., and Glass, R.I.
1995. Detection and differentiation of antigenically distinct small roundstructured viruses (Norwalk-like viruses) by reverse transcription PCR and
southern hybridization. J. Clin. Microbiol. 33: 64-71.
Ansari, S.A., Sattar, S.A., Springthorpe, V.S., Wells, G.A., and Tostowaryk,
W. 1989. In vivo protocol for testing efficacy of hand-washing agents
against viruses and bacteria: experiments with rotavirus and Escherichia
coli. Appl. Environ. Microbiol. 55: 3113-3118.
Appleton, H. 1994. Norwalk virus and the small round viruses causing
foodborne gastroenteritis. In “Foodborne Disease Handbook: Vol. 2.
Diseases Caused by Viruses, Parasites, and Fungi,” eds. Y.H. Hui, J.R.
Gorham, K.D. Murrell, and D.O. Cliver, pp. 57-79. Marcel Dekker, New
York.
Bean, N.H., Griffin, P.M., Goulding, J.S., and Ivey, C.B. 1990. Foodborne
disease outbreaks, 5-year summary, 1983-1987. J. Food Protect. 53: 711728.
Bean, N.H., Goulding, J.S., Lao, C., and Angulo, F.J. 1996. Surveillance for
foodborne-disease outbreaks--United States, 1988-1992. CDC Surveillance Summaries, Morbid. Mortal. Weekly Rep. 45 (SS-5): 1-66.
Berg, G., ed. 1978. “Indicators of Viruses in Water and Food.” Ann Arbor
Science, Ann Arbor, MI.
CAST. 1994. Foodborne Pathogens: Risks and Consequences. Task Force
Report No. 122, September 1994. Council for Agricultural Science and
Technology, Ames, IA.
CDC. 1994. Viral hepatitis surveillance program, 1990-1992, pp. 19-34.
Hepatitis Surveillance Report No. 55. Centers for Disease Control and
Prevention, Atlanta, GA.
VOL. 51, NO. 4, APRIL 1997—FOOD TECHNOLOGY 77
SCIENTIFIC
STATUS
SUMMARY
VIRUS TRANSMISSION VIA FOOD
Chung, H., Jaykus, L., and Sobsey, M.D. 1996. Detection of human enteric
viruses in oysters by in vivo and in vitro amplification of nucleic acids. Appl.
Environ. Microbiol. 62: 3772-3778.
Cliver, D.O. 1966. Implications of food-borne infectious hepatitis. Pub.
Health Rep. 81: 159-165.
Cliver, D.O. 1985. Vehicular transmission of hepatitis A. Pub. Health Rev. 13:
235-292.
Cliver, D.O. 1990. Viruses. In “Foodborne Diseases,” ed. D.O. Cliver, pp.
275-292. Academic Press, San Diego, CA.
Cliver, D.O. 1995. Detection and control of foodborne viruses. Trends Food
Sci. Technol. 6: 353-358.
Cliver, D.O. 1997. Foodborne viruses. In “Fundamentals of Food Microbiology,” eds. M.P. Doyle, L.R. Beuchat, and T.J. Montville. American Society
for Microbiology, Washington, D.C., in press.
Cliver, D.O., and Kostenbader, K.D., Jr. 1984. Disinfection of virus on hands
for prevention of food-borne disease. Int. J. Food Microbiol. 1: 75-87.
Cliver, D.O., Ellender, R.D., Fout, G.S., Shields, P.A., and Sobsey, M.D.
1992. Foodborne viruses. In “Compendium of Methods for the Microbiological Examination of Foods, 3rd ed,” eds. C. Vanderzant and D.F.
Splittstoesser, pp. 763-787. American Public Health Association, Washington, D.C.
Cromeans, T., Nainan, O.V., Fields, H.A., Favorov, M.O., and Margolis, H.S.
1994. Hepatitis A and E Viruses. In “Foodborne Disease Handbook: Vol.
2. Diseases caused by viruses, parasites, and fungi,” eds. Y.H. Hui, J.R.
Gorham, K.D. Murrell, and D.O. Cliver, pp. 1-56. Marcel Dekker, New
York.
Deng, M.Y., Day, S.P., and Cliver, D.O. 1994. Detection of hepatitis A virus
in environmental samples by antigen-capture polymerase chain reaction.
Appl. Environ. Microbiol. 60: 1927-1933.
DiGirolamo, R., Liston, J., and Matches, J.R. 1970. Survival of virus in
chilled, frozen, and processed oysters. Appl. Microbiol. 20: 58-63
Dolin, R., Blacklow, N.R., DuPont, H., Buscho, R.F., Wyatt, R.G., Kasel,
J.A., Hornick, R., and Chanock, R.M. 1972. Biological properties of
Norwalk agent of acute infectious nonbacterial gastroenteritis. Proc. Soc.
Exper. Biol. Med. 140: 578-583.
Eisenstein, A.B., Aach, R.D., Jacobson, W., and Goldman, A. 1963. An
epidemic of infectious hepatitis in a general hospital. J. Am. Med. Assoc.
185: 171-174.
Gard, S. 1957. Discussion. In “Hepatitis Frontiers,” ed. F.W. Hartman, pp.
241-243. Little, Brown & Co., Boston.
Gouvea, V., Santos, N., Timenetsky, M. do C., and Estes, M.K. 1994.
Indentification of Norwalk virus in artificially seeded shellfish and selected
foods. J. Virol. Methods 48: 177-187.
Greenberg, H.B., Valdesuso, J., Yolken, R.H., Gangarosa, E., Gary, W.,
Wyatt, R.G., Konno, T., Suzuki, H., Chanock, R.M., and Kapikian, A.Z.
1979. Role of Norwalk virus in outbreaks of nonbacterial gastroenteritis. J.
Infect. Dis. 139: 564-568.
Grešíková, M. 1994. Tickborne encephalitis. In “Foodborne Disease Handbook: Vol. 2. Diseases Caused by Viruses, Parasites, and Fungi,” eds.Y.H.
Hui, J.R. Gorham, K.D. Murrell, and D.O. Cliver, pp. 113-135. Marcel
Dekker, New York.
Grohmann, G.S., Murphy, A.M., Christopher, P.J., Auty, E., and Greenberg,
H.B. 1981. Norwalk virus gastroenteritis in volunteers consuming depurated
oysters. Austral. J. Exper. Biol. Med. 59(Pt. 2): 219-228.
Halliday, M.L., Kang, L.-Y., Zhou, T.-K., Hu, M.-D., Pan, Q.-C., Fu, T.-Y.,
Huang, Y.-S., and Hu, S.-L. 1991. An epidemic of hepatitis A attributable
to the ingestion of raw clams in Shanghai, China. J. Infect. Dis. 164:852859.
IFST. 1996. IFST position statement on foodborne viral infections, September
14, 1996. Institute of Food Science and Technology, London, UK.
Jaykus, L. A., De Leon, R., and Sobsey, M. D. 1995. Development of a
molecular method for the detection of enteric viruses in oysters. J. Food
Protect. 58: 1357-1362.
Koff, R.S., Grady, G.F., Chalmers, T.C., Mosley, J.W., Swartz, B.L., and the
Boston Inter-hospital Liver Group. 1967. Viral hepatitis in a group of
Boston hospitals. III. Importance of exposure to shellfish in a nonepidemic
period. New England J. Med. 276: 703-710.
Kuritsky, J.N., Osterholm, M.T., Greenberg, H.B., Korlath, J.A., Godes, J.R.,
Hodberg, C.W., Forfang, J.C., Kapikian, A.Z., McCullough, J.C., and
White, K.E. 1984. Norwalk gastroenteritis: A community outbreak associated with bakery product consumption. Ann. Intern. Med. 100: 519-521.
NYDH. 1989. A Review of Foodborne Disease Outbreaks in New York State
1988. Bureau of Community Sanitation and Food Protection. New York
Department of Health, Albany, NY.
Osherovich, A.M., and G.S. Chasovnikova. 1967. Study on the isolation of
enteroviruses from environmental objects (in Russian). In “Materialy
Problemnoi Komissii Akad. Med. Nauk SSSR ‘Poliomyelit i Virusnye
Entsefality,’ Vypusk 1, Enterovirusy,” ed. M.P. Chumakov, pp. 89-90.
Akad. Med. Nauk SSSR, Moscow.
Parrino, T.A., Schreiber, D.S., Trier, J.S., Kapikian, A.Z., and Blacklow, N.R.
1977. Clinical immunity in acute gastroenteritis caused by Norwalk virus.
New Engl. J. Med. 297:86-89.
Parry, J.V., and Mortimer, P.P. 1984. The heat sensitivity of hepatitis A virus
determined by a simple tissue culture method. J. Med. Virol. 14: 277-283.
Potter, M.E. 1996. Risk assessment terms and definitions. J. Food Protect.
Supplement: 6-9.
Power, U.F., and Collins, J.K. 1989. The production of microbiologically safe
shellfish - lessons from the classification of shellfish at source. Environ.
Health 97: 124-130.
Rose, J.B., and Sobsey, M.D. 1993. Quantitative risk assessment for viral
contamination of shellfish and coastal waters. J. Food Protect. 56: 10431050.
Sattar, S.A., Springthorpe, V.S., and Ansari, S.A. 1994. Rotavirus. In
“Foodborne Disease Handbook: Vol. 2. Diseases Caused by Viruses,
Parasites, and Fungi,” eds. Y. H. Hui, J. R. Gorham, K. D. Murrell, and D.
O. Cliver, pp. 81-111. Marcel Dekker, New York.
USDHEW. 1973. Common source outbreak of hepatitis A - Ohio. Morbid.
Mortal. Weekly Rep. 27: 86.
USDHEW. 1979. Aseptic meningitis outbreak at a military installation in
Pennsylvania. Aseptic Meningitis Surveillance, annual summary 1976,
HEW-CDC publication number 79-8231, p. 11. U.S. Department of Health,
Education, and Welfare, Atlanta, GA.
USPHS. 1995. Food Code. Food and Drug Administration, U.S. Public
Health Service, Washington, D.C.
Wait, D.A., Hackney, C.R., Carrick, R.J., Lovelace, G., and Sobsey, M.D.
1983. Enteric bacterial and viral pathogens and indicator bacteria in hard
shell clams. J. Food Prot. 46: 493-496.
Wang, J., Jiang, X., Madore, H.P., Gray, J., Desselberger, U., Ando, T., Set,
Y., Oishi, I., Lew, J.F., Green, K.Y., and Estes, M.K. 1994. Sequence
diversity of small, round-structured viruses in the Norwalk virus group. J.
Virol. 68: 5982-5990.
White, K.E., Osterholm, M.T., Mariotti, J.A., Korlath, J.A., Lawrence, D. H.,
Ristinen, T. L., and Greenberg, H. B. 1986. A foodborne outbreak of
Norwalk virus: evidence for post-recovery transmission. Am. J. Epidemiol.
124: 120-126.
WHO. 1994. Outbreak of tick-borne encephalitis, presumably milk-borne.
WHO Weekly Epidemiol. Rec. No. 19, 13 May 1994, pp. 140-141. World
Health Organization, Geneva.
WHO. 1995. Public health control of hepatitis A: Memorandum from a WHO
meeting. Bull. World Health Org. 73: 15-20. World Health Organization,
Geneva.
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