Epidemiology

Transmission of G. lamblia is usually by the fecal-oral route, especially via fecally contaminated water. Known or suspected sources of G. lamblia include beavers, raccoons, muskrats, dogs, cats, and humans. A single human stool can contain 300 million G. lam-blia cysts; only 10 cysts are required to establish infection. The cysts can remain viable in cold water for more than 2 months. Usual levels of chlorination of municipal water supplies are ineffective against the cysts, and water filtration is necessary to remove them. Hikers who drink from streams, even in remote areas thought to contain safe water, are at risk of contracting giardiasis.

Although waterborne outbreaks are the most common, person-to-person contact can also transmit the disease. This mode of transmission is especially likely in day care centers where hands become contaminated in the process of diaper changing. People who promiscuously engage in anal intercourse and fellatio are also prone to contracting the disease. Transmission by fecally contaminated food has also been reported. Good personal hygiene, especially handwashing, decreases the chance of passing on the infection. ■ day care centers, p. 498

626 Chapter 24 Alimentary System Infections

Table 24.13 Giardiasis

Symptoms

Mild illness: indigestion, flatulence, nausea; severe: vomiting, diarrhea, abdominal cramps, weight loss

Incubation period

6 to 20 days

Causative agent

Giardia lamblia, a flagellated pear-shaped protozoan with two nuclei

Pathogenesis

Ingested cysts survive stomach passage; trophozoites emerge from the cysts in the small intestine, where some attach to epithelium and others move freely; mucosal function is impaired by adherent protozoa and host immune response

Epidemiology

Ingestion of fecally contaminated water; person-to-person, in day-care centers

Prevention and treatment

Boiling or disinfecting drinking water; filtration of community water supplies.Treatment: quinacrine hydrochloride (Atabrine) or metronidazole (Flagyl)

10 mm

Figure 24.23 Oocysts of Cryptosporidium parvum in the Feces of an Infected Individual (Acid-fast Stain) Person-to-person and waterborne transmission are common.

10 mm

Figure 24.23 Oocysts of Cryptosporidium parvum in the Feces of an Infected Individual (Acid-fast Stain) Person-to-person and waterborne transmission are common.

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