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THE DI A G N OSIS OF
Which Bug Is to Blame?
Definitive diagnoses of most foodborne illnesses require
the expertise of physicians and advanced tests. In fact,
doctors working without test data commonly misdiagnose
foodborne illnesses. Determining the true causes can
require specialized techniques, such as tests that identify
pathogenic DNA and the timing of a patient’s exposure to
the contaminant. Most physicians do not, in practice,
diagnose a foodborne illness unless it is severe or part of an
outbreak that affects many people.
CLASS OF SYMPTOMS
Gastroenteritis
(primarily vomiting but also possibly fever and diarrhea)
Common culprits: rotavirus in an infant; norovirus or related viruses in
adults; food poisoning from ingested toxins of Staphylococcus aureus or
Bacillus cereus. Symptoms can also indicate heavy-metal poisoning.
Noninflammatory diarrhea
(usually no fever)
Common culprits: nearly all foodborne pathogens, including bacteria,
protists, and viruses. Noninflammatory diarrhea is a classic symptom of
Escherichia coli toxin in the small intestine.
Inflammatory diarrhea
(often bloody stools and fever)
Common culprits: invasive bacteria such as Shigella spp., Campylobacter
spp., Salmonella spp., and E. coli; the protist Entamoeba histolytica.
Inflammatory diarrhea can be a sign of invasive gastroenteritis in the large
intestine.
The type, timing, and severity of symptoms can, however,
point physicians and health professionals toward the offending
pathogen. The sudden onset of vomiting, for example, can
often be linked to food poisoning from bacteria such as
Bacillus cereus.
The list below gives a simplified version of the guidelines
that health professionals use to determine whether an illness
is related to the consumption of food. Confirmation of that
link requires biomedical test results.
Persistent diarrhea
(lasting days to two or more weeks)
Common culprits: parasitic protists, including Cryptosporidium spp.,
Cyclospora cayetanensis, E. histolytica, and Giardia lamblia.
Neurologic symptoms
(tingling or numbness, impaired vision, breathing difficulties)
Common culprits: botulism caused by the bacteria Clostridium botulinum;
poisoning by pesticides, thallium, or mushrooms; toxins from fish or
shellfish.
General malaise
(weakness, headaches, muscle and joint pain, fever, jaundice)
Common culprits: bacteria such as Listeria monocytogenes, Salmonella
typhi, and Brucella spp.; worms, including Trichinella spiralis; viruses such
as hepatitis A; protists, including Toxoplasma gondii.
or their eggs, which are otherwise known as
oocysts. Each pathogenic species has a characteristic
source of contamination, as well as a distinct
infectious dose, which refers to the number of
organisms an average person would need to
consume before contracting a foodborne illness.
Many of the pathogens that most commonly
cause foodborne illness spread predominantly
through the secretions (such as saliva) and
excretions (such as feces and vomit) of animals,
including humans. It is therefore not much of an
exaggeration, if any, to say that just two basic rules
would prevent 99% of foodborne illnesses, if only
people could follow them scrupulously:
1. Do not consume the feces, vomit, or spittle
of other humans.
2. Do not consume the feces, vomit, or spittle
of animals.
Sounds simple, right? The unfortunate fact is
that this is much harder to do than it would
naively seem. The surprising situation is that food
safety problems are not, to first order, intrinsic to
the food supply, as many people seem to believe
(see Common Misconceptions About Microbes,
page 117). In truth, the problem is us: the cooks
and consumers of food, who typically buy that
food clean and then too often contaminate it
ourselves as we handle it.
To discern why this is true, it helps to understand
a few technical concepts. In most cases, just
one microbe is not enough to cause illness. The
exact number that does lead to symptoms, the
so-called infectious dose, varies according to the
individualsome people can tolerate more
pathogens because of differences in their digestive
tract or immune system. Scientists thus often
speak of the number of microorganisms that gives
the disease to 50% of the individuals exposed to it.
They call that average infectious dose the ID50.
Similarly, the lethal dose corresponds to the
number of organisms required to kill an individual,
and the LD50 refers to the dose that kills half of
those exposed to it.
During, and immediately after, an illness,
infected hosts can shed pathogens through their
feces. The fecal load refers to the number of
disease-causing organisms in one gram (four
hundredths of an ounce) of contaminated feces
that an infected person or animal releases.
Although the numbers can vary considerably
based on the characteristics of both the invading
microbe and host, the fecal load for many foodborne
pathogens is around 100 million organisms
a gram.
As a theoretical exercise, consider what that
statistic means for a pathogen that has an ID 50
of
one, meaning that half the people consuming
a single microbe would become infected. Then
a single gram of feces harboring 100 million of
the microbes could, in principle, infect 50 million
people. The total feces shed (usually as diarrhea)
during the course of an illness contain enough
A Dose of Pathogen-Related
Terms
Infectious dose: the number of
organisms (viral particles or
bacterial cells, for example)
required to cause an infection in
a particular individual
ID 50
: the number of pathogens per
individual required to cause
infection in 50% of test subjects
Lethal dose: the number of
pathogens required to cause fatal
disease in a particular individual
LD 50
: the number of pathogens per
individual that causes fatal disease
in 50% of test subjects
Fecal load: the number of pathogens
per gram in a sample of human
or animal feces
THE SCIENC E O F
Determining the Infectious Dose
Public health professionals determine infectious dose numbers
in a two-stage process. In the first phase, specialists who
investigate food-related outbreaks measure the amount of
contamination in the pathogen source. They also estimate
the size of the portion of contaminated food its victims
consumed, then calculate the average infectious dose (ID 50
).
After that, other researchers attempt to confirm the projected
infectious dose level through volunteer studies in which
healthy people consume measured amounts of a specific
pathogen. For ethical reasons, these studies are not done for
serious or potentially fatal diseases, but they yield solid numbers
for less severe diseases.
Different strains of the same pathogenic species can have
vastly different infectious doses. Some strains of E. coli, for
example, require as many as 100 million microorganisms for an
infection, whereas other strains can be infectious with as few as
50. Noroviruses have ID 50
values estimated at fewer than 20
viral particles. For a few foodborne pathogens, the ID 50
is as
low as one—meaning that for half a population, ingesting
a single microbe is enough to cause an infection.
TIME TO ONSET OF SYMPTOMS
AFTER EXPOSURE
½–8 h: Staphylococcus aureus
1 h–1 d: Bacillus cereus
2 h–8 d: Clostridium botulinum and its toxins
6 h–1 d: Clostridium perfringens
6 h–10d: Salmonella spp. (nontyphoidal)
9–48 h: Listeria monocytogenes (initial gastrointestinal symptoms)
12–48 h: norovirus
12 h–6 d: Shigella spp.
1–3 d: rotavirus
1–10 d: E. coli O157:H7, Yersinia enterocolitica
2–10 d: Campylobacter jejuni
3 d–3 wk: Giardia lamblia
10–13 d: Toxoplasma gondii
15 d–7 wk: hepatitis A
114 VOLUME 1 · HISTORY AND FUNDAMENTALS
MICROBIOLOGY FOR COOKS 115