Microcheck 247

Giardiasis, caused by the flagellated protozoan Giardia lamblia, is a common waterborne disease, but it can also be transmitted person-to-person. Cryptosporidiosis is another waterborne disease that is also transmissible person-to-person, but it is caused by a coccidian member of the Apicomplexa that multiplies within intestinal epithelial cells. There is no satisfactory treatment. Cyclosporiasis is a similar disease, but it is not transmissible person-to-person and it can be effectively treated. Amebiasis is caused by an ameba that ulcerates the large intestinal epithelium, resulting in dysentery.

■ Explain why person-to-person spread does not occur in cyclosporiasis.

■ Explain why individuals with severe giardiasis often have bulky feces containing fat.

■ Would you expect an individual with giardiasis who has diarrhea to be more likely to transmit the disease than an individual with giardiasis who does not have diarrhea? Explain.

Table 24.16 Amebiasis

Symptoms

Diarrhea, abdominal pain, blood in feces

Incubation period

2 days to several months

Causative agent

Entamoeba histolytica; a protozoan of the group Sarcodina

Pathogenesis

Ingested cysts liberate trophozoites in the small intestine; upon reaching the large intestine, trophozoites feed on mucus and cells lining the intestine; enzymes are produced that allow penetration of the intestinal epithelium, sometimes the intestinal wall and blood vessels, and thence to the liver and other organs, resulting in abscesses

Epidemiology

Ingestion of fecally contaminated food or water; disease associated with poverty, homosexual men, and migrant workers

Prevention and treatment

Good sanitation and personal hygiene.Treatment: metronidazole, paromomycin

Nester-Anderson-Roberts: I IV. Infectious Diseases I 24. Alimentary System I I © The McGraw-Hill

Microbiology, A Human Infections Companies, 2003

Perspective, Fourth Edition

(b) Four daughter protozoa are released and develop into trophozoites

Mature cysts contaminate soil, water, hands, and food

(d) Immature cyst with single nucleus

(e) Quadrinucleate cyst with chromatoid bodies

Life cycle

(b) Four daughter protozoa are released and develop into trophozoites

Mature cysts contaminate soil, water, hands, and food

(d) Immature cyst with single nucleus

(e) Quadrinucleate cyst with chromatoid bodies

Life cycle

Future Challenges

Figure 24.24 Life Cycle of Entamoeba histolytica Quadrinucleate cysts enter the mouth (a) and pass through the stomach to the lower small intestine (b). Four daughter protozoa are released from each cyst and develop into trophozoites (c), the feeding form (d). Dehydration in the large intestine stimulates progressive stages of cyst development (e). Mature cysts are passed in the feces to contaminate soil, water, hands, and food.Trophozoites that burrow into intestinal blood vessels can be carried to the liver or other organs, causing abscesses.The electron micrograph shows amebic cysts and trophozoites on bowel mucosa.

is repeated

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