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FOODBOR NE ILLNESS
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Foodborne illness almost always
takes one of three forms:
Invasive infection: pathogenic
organisms penetrate and grow in
human tissue and may secrete
toxins. Examples include all
foodborne protists and viruses, the
parasitic worm Trichinella spiralis,
bacteria such as Escherichia coli
O157:H7 and Listeria monocytogenes,
and the bovine spongiform
encephalopathy prion (the agent of
mad cow disease).
Noninvasive infection: pathogens
live in the gut but do not penetrate
it and may secrete toxins there.
Examples include the beef tapeworm
Taenia saginata and bacteria
such as Vibrio cholerae and Yersinia
enterocolitica.
Food poisoning: bacteria release
toxins into food before it is eaten.
Examples include bacterial species
such as Bacillus cereus, which
secretes four different toxins, and
Clostridium botulinum, the organism
that produces the compound in
Botox injections that smoothes
wrinkled skin.
Scientists have found that food can be a conduit
for more than 250 diseases. The more you know
about them, especially the common and severe
ones, the better you can avoid food-related illnesses.
The vast majority of foodborne pathogens
sicken people in one of three ways.
An invasive infection can occur when microorganisms
penetrate a human body and grow
within it. This bacterial presence can directly lead
to inflammation and disease symptoms. Some
microorganisms (primarily bacteria) also secrete
toxins. All forms of microorganisms have at least
one representative capable of invasive infection.
Certain pathogens that multiply in food remnants
in the human gut but do not penetrate gut
tissue can cause a noninvasive infection, the
second primary mode of infection. A noninvasive
infection causes illness mainly through secreted
bacterial toxins. In general, people with invasive
infections suffer more and for longer periods than
those with noninvasive infections, but the scope of
each condition depends upon the specific interaction
between the pathogens and their host.
Food poisoning, the third major mode, results
from bacterial contamination only. Certain
pathogenic species and their subtypes, or strains,
can release very powerful toxins into food long
before it is eaten. Food poisoning usually initiates
symptoms much more rapidly than infections
because the toxins are already in the food when it
is eaten, so no time is needed for the bacteria to
grow inside the body, as happens in an infection.
Although it occurs only rarely, some bacteria
can cause foodborne illnesses by using various
combinations of these three strategies, a further
complication for those who administer treatment
to patients with food-related maladies.
Tracking Foodborne Illness
Health authorities find it difficult to track foodborne
illnesses, in part because many cases resolve
themselves as the symptoms disappear after only
a day or two, so many victims do not seek medical
attention. And even when they do, doctors seldom
report new cases. In 1999, researchers at the U.S.
Centers for Disease Control and Prevention
(CDC) published one of the best large-scale
studies of foodborne illness to date. They relied on
data collected by several medical surveillance
systems and made careful estimates to track
illnesses from 28 foodborne pathogens.
The study’s conclusions may not hold true for
all parts of the world or even for the U.S. in future
years, but the broad patterns it indicates are
mirrored in many other developed countries.
Unsafe drinking water sources are common in
less-developed parts of the world, which makes
both foodborne and waterborne disease much
more prevalent and serious an issue in those areas.
The incidence of common and even endemic (or
always present) pathogens can, in addition, vary
widely because of economic, geographical, climaterelated,
and other factors. Cholera and amebiasis,
for example, are endemic in many poorer sub-
Saharan African countries but are relatively rare in
affluent northern European regions. Even so,
pathogens do not respect national borders, and the
many examples of foodborne illnesses worldwide
represent variations on a common theme.
Clostridium perfringens is an anaerobic bacterium that causes
some types of food poisoning, a condition caused pig bel, and even
gas gangrene. The bacterium (at far left) is rod shaped. Its spores
(shaped like bowling pins) are far more difficult to kill with heat.
For that reason, the spores can cause food safety problems.
The two charts on page 113 summarize some of
the CDC study’s more intriguing findings in this
field. The first chart depicts, by type of causal
microorganism, the distribution of foodborne
illnesses that together caused the roughly 13.8
million annual cases that can be associated with
known sources; another 62 million cases are
attributed to unknown microbes. Viruses accounted
for 9.28 million cases, whereas bacteria
caused another 4.18 million, and protists are held
responsible for most of the rest, or about 357,000
cases (2.6%) per year. The CDC study links
parasitic worms to only 52 cases, and it reports no
prion diseases occurring in the United States.
The relative importance of these pathogens
changes considerably if you look at the most serious
cases of foodborne illness: those that end in death
(second chart). Of the estimated 13.8 million
annual cases from known sources, only 1,809
resulted in fatalities. Rather than viruses, however,
bacteria claimed by far the most victims1,297 in
all (71.7% of the total). Protists rank second with
383 deaths (21.2%)all but eight of those are
blamed on Toxoplasma gondii. The death rate for
protist infection is much higher than that for other
infections, but it is still only about one in every
1,000 cases, so mortality for even the deadliest
foodborne pathogens is quite low.
Rotaviruses cause fever and vomiting.
They are the main cause of severe diarrhea
among children. Although deaths from
rotavirus infection are uncommon in
developed countries, these pathogens kill
nearly one million people worldwide each
year.
110 VOLUME 1 · HISTORY AND FUNDAMENTALS
MICROBIOLOGY FOR COOKS 111