680 Modern Food Microbiology
Order Amoebida
Family Endamoebidae
Genus Entamoeba
Phylum Apicomplexa (= Sporozoa)
Class Sporozoea
Class Lobosea
Order Eucoccidiida
Family Sarcocystidae
Genus Toxoplasma
Genus Sarcocystis
Family Cryptosporidiidae
Genus Cryptosporidium
Genus Cyclospora
Giardiasis
Giardia lamblia is a flagellate protozoan that exists in environmental waters at a higher level than
Entamoeba histolytica. The protozoal cells (trophozoites) produce cysts, which are the primary forms
in water and foods. The cysts are pear shaped, with a size range of 8–20 µm in length and 5–12 µm
in width. The trophozoites have eight flagella that arise on the ventral surface near the paired nuclei
and give rise to “falling-leaf” motility.
Upon ingestion, Giardia cysts excyst in the gastrointestinal tract with the aid of stomach acidity and
proteases and give rise to clinical giardiasis in some individuals. Excystation of the trophozoites occurs
somewhere in the upper small intestine, and this step is regarded as being equivalent to a virulence
factor.
9 The trophozoites are not actively phagocytic, and they obtain their nutrients by absorption.
Occasionally, bile ducts are invaded, leading to cholecystitis. Compared to some of the other intestinal
protozoal parasites, Giardia trophozoites do not penetrate deeply in parenteral tissues.
Environmental Distribution
Water is the second most common source of giardiasis. The first recorded outbreak occurred at a ski
resort in Aspen, Colorado, in 1965 with 123 cases.
22 Between 1965 and 1977, 23 waterborne outbreaks
were recorded that affected over 7000 persons.
23 Between 1971 and 1985, 92 outbreaks were reported
in the United States.
22 Giardia cysts are generally resistant to the levels of chlorine used in the water
supply. Beavers and muskrats have been shown to be the major sources of this organism in bodies of
water. In a study of 220 muskrat fecal specimens collected from natural waters in southwestern New
Jersey, 70% contained Giardia cysts.
59 It is estimated that up to 15% of the U.S. population is infected
with this organism.
Syndrome, Diagnosis, and Treatment
The incubation period for clinical giardiasis is 1–4 weeks, and cysts appear in stools after 3–4 weeks.
Asymptomatic cyst passage is the most benign manifestation of G. lamblia infection in humans, but
when clinical giardiasis occurs, symptoms may last from several months to a year or more. Up to 9.0 ×
10
8 cysts are shed each day by patients, and they may survive as long as 3 months in sewage sludge.
3
Order Amoebida
Family Endamoebidae
Genus Entamoeba
Phylum Apicomplexa (= Sporozoa)
Class Sporozoea
Class Lobosea
Order Eucoccidiida
Family Sarcocystidae
Genus Toxoplasma
Genus Sarcocystis
Family Cryptosporidiidae
Genus Cryptosporidium
Genus Cyclospora
Giardiasis
Giardia lamblia is a flagellate protozoan that exists in environmental waters at a higher level than
Entamoeba histolytica. The protozoal cells (trophozoites) produce cysts, which are the primary forms
in water and foods. The cysts are pear shaped, with a size range of 8–20 µm in length and 5–12 µm
in width. The trophozoites have eight flagella that arise on the ventral surface near the paired nuclei
and give rise to “falling-leaf” motility.
Upon ingestion, Giardia cysts excyst in the gastrointestinal tract with the aid of stomach acidity and
proteases and give rise to clinical giardiasis in some individuals. Excystation of the trophozoites occurs
somewhere in the upper small intestine, and this step is regarded as being equivalent to a virulence
factor.
9 The trophozoites are not actively phagocytic, and they obtain their nutrients by absorption.
Occasionally, bile ducts are invaded, leading to cholecystitis. Compared to some of the other intestinal
protozoal parasites, Giardia trophozoites do not penetrate deeply in parenteral tissues.
Environmental Distribution
Water is the second most common source of giardiasis. The first recorded outbreak occurred at a ski
resort in Aspen, Colorado, in 1965 with 123 cases.
22 Between 1965 and 1977, 23 waterborne outbreaks
were recorded that affected over 7000 persons.
23 Between 1971 and 1985, 92 outbreaks were reported
in the United States.
22 Giardia cysts are generally resistant to the levels of chlorine used in the water
supply. Beavers and muskrats have been shown to be the major sources of this organism in bodies of
water. In a study of 220 muskrat fecal specimens collected from natural waters in southwestern New
Jersey, 70% contained Giardia cysts.
59 It is estimated that up to 15% of the U.S. population is infected
with this organism.
Syndrome, Diagnosis, and Treatment
The incubation period for clinical giardiasis is 1–4 weeks, and cysts appear in stools after 3–4 weeks.
Asymptomatic cyst passage is the most benign manifestation of G. lamblia infection in humans, but
when clinical giardiasis occurs, symptoms may last from several months to a year or more. Up to 9.0 ×
10
8 cysts are shed each day by patients, and they may survive as long as 3 months in sewage sludge.
3