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How Toxoplasma Infects Humans
Toxoplasma gondii usually infects rodents and the cats that eat them.
Because most people do not eat cats or mice, the primary risk to humans
comes by way of cat feces, which contain the oocysts of the parasite. In
principle, a human could also become infected by eating beef, lamb, or
1
Life stages
Oocysts are shed in cat feces.
Oocysts shed into food, water, or soil
are eaten by a secondary host (such
as a mouse or a cow) and become
tissue cysts.
Tissue cysts are eaten by a cat and
reproduce sexually in the
intestinal tract.
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Low High
Contamination
Cats are allowed in the kitchen,
raising the risk of fecal oocysts
contaminating food surfaces.
Contamination risk
Water or vegetable or meat
surfaces are contaminated by
oocysts.
Contamination risk
Low High
Livestock eat contaminated meat
(containing tissue cysts) or drink
contaminated water (containing
oocysts). Their tissue becomes
contaminated as well.
Contamination risk
Low High
3a
3b
The G. intestinalis protist was the first pathogen
ever diagnosed by microscopic examination when
Dutch merchant and microscopy pioneer Antony
van Leeuwenhoek spied it in 1681 while examining
his own diarrhea. Ever since, the protist has
been deemed primarily a waterborne rather than
a foodborne pathogen.
The CDC estimates, however, that 10% of
infections with this pathogen begin with food and
other meat from animals that consumed the oocysts, but such routes of
infection are very rare. Once in food, however, the oocysts are difficult
to destroy with heat. The best precaution is to keep cat feces well away
from food and the kitchen.
Unsafe storage
Refrigeration and most freezing
does not kill the parasite’s cysts
in either meat or vegetables;
insufficient washing of vegetables
and kitchen surfaces allow them
to remain.
Contamination risk
Low High
Safe storage
Vegetables are peeled or
properly washed.
Contamination risk
Low High
4a
4b
Unsafe cooking
Meat is undercooked, leaving
risk of contaminated surface or
interior, and vegetables are
served raw after washing.
Contamination risk
Low High
Meat
Vegetables
Safe cooking
Meat and vegetables are
properly cooked.
Contamination risk
Low High
lead to 200,000 giardiasis illnesses in the United
States every yearnearly double the number of
foodborne toxoplasmosis cases. FDA officials have
traced outbreaks of giardiasis to food handlers.
Fortunately, foodborne giardiasis is far less severe
than toxoplasmosis and on average results in one
death each year in the U.S.
The usual explanation for the high prevalence of
giardiasis is that many animals carry it and foul
water sources with their feces. Giardiasis is called
“beaver fever” because these animals were thought
to be a primary reservoir of the protist.
Recent research, however, shows that the
Giardia pathogen that infects humans is genetically
distinct from the Giardia pathogen that
infects beavers; it is probably a separate species or
subspecies. This finding suggests that most, if not
all, giardiasis in humans results from contamination
by human waste rather than by animal waste.
Hiking and backpacking guides often warn
people to avoid drinking from streams that may
support Giardia, and an entire industry has
sprung up around protective filter units that sift
the oocysts out of contaminated watera necessary
approach because the eggs are extremely
resistant to heat or chemical treatments. Water
that has been treated with enough chlorine or
iodine can kill Giardia, but the chemicals leave
a very strong taste and odor, and they take time
to work. Iodine disinfection, for example, takes
eight hours.
Swimming pools can easily get contaminated
and remain so because insufficient chlorine levels
or other sanitation measures fail to kill the oocysts.
Boiling water also kills Giardia oocysts, and
good hygiene practices in the kitchen and bathroom
should help minimize the risk of infection.
Two other important protist parasites,
Cyclospora cayetanensis and Cryptosporidium
parvum, have a similar life cycle to that of Giardia.
They too can produce hard oocysts that can
abound in feces, which may then infect humans
via the fecal-oral route.
C. cayetanensis is a good example of a so-called
“new” foodborne pathogen. Although it has
undoubtedly been infecting humans for ages, the
protist was virtually unknown before scientists
began studying it extensively in the 1990s. Health
authorities consider the formerly obscure organism
to be a chief cause of “traveler’s diarrhea.”
Investigators have also traced two cyclosporosis
outbreaks in the 1990s in the U.S. and Canada to
fresh raspberries imported from Guatemala and to
a salad mix of baby lettuce and basil.
Cryptosporidium, on the other hand, is a close
relative of both Plasmodium and Toxoplasma. It has
the dubious distinction of producing the most
resilient oocysts of any genus of pathogenic protist.
As a result, most water treatment plants and
swimming pool sanitation systems are unable to
eliminate it. Large outbreaks of cryptosporidiosis
have occurred in Oslo and Sydney. A 1993 outbreak
in Milwaukee, Wisconsin, ranks as the
biggest outbreak of waterborne disease in the U.S.;
it resulted in more than 400,000 cases. Smaller
outbreaks that were linked to apple juice and green
onions highlight the protist’s potential as a foodborne
pathogen. So far, shortwave ultraviolet
(UVC) light and very heavy concentrations of
ozone are about the only practical methods found
to eliminate the oocysts.
At least six species of an amoeba genus known
as Entamoeba can colonize the human gut, but
only one, E. histolytica, causes disease. Like
Giardia, Cyclospora, and Cryptosporidium infections,
E. histolytica infections develop after
humans ingest oocysts, generally in contaminated
water or food. Unlike most other pathogenic
protists, E. histolytica can lead to a serious illness
known as amebiasis, which can result in dysentery
and liver abscesses as the organisms lodge in
the gut wall or liver and destroy tissue. Amebiasis
can be fatal if not treated. Cases are rare in most
developed countries, although the disease can
become chronic, and infected food workers can
contaminate kitchens.
Protists such as Giardia (above) and
Entamoeba parasites may be considered
waterborne pathogens, but beware: they
can also contaminate food. Good kitchen
and bathroom hygiene should greatly
reduce the risk of contamination.
128 VOLUME 1 · HISTORY AND FUNDAMENTALS
MICROBIOLOGY FOR COOKS 129