Excerpt from Giardiasis

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Background

Giardia lamblia is a ubiquitous gastrointestinal protozoon that may be identified in individuals with asymptomatic colonization or acute or chronic diarrheal illness. Infection is recognized more commonly in children than in adults.

Giardia species are endemic in areas of the world that have poor sanitation. In developing countries, the disease is an important cause of morbidity, and water- and food-borne outbreaks are common. Because of the small inoculum of organisms necessary to cause infection, giardiasis is common in daycare center attendees and institutionalized patients in developed countries. G lamblia is a particularly significant pathogen for people with malnutrition, immunodeficiencies, or cystic fibrosis. Beavers may be an important reservoir host for G lamblia.

G lamblia is genetically heterogeneous, with 2 major genotypes (A and B). Some strains appear more biologically fit than other strains, a factor that is potentially important in disease pathogenesis. Studies indicate that G lamblia undergoes surface antigenic variation that is most likely stimulated by the host immune response or the intestinal mucosa.

High-risk groups for giardiasis include travelers to highly endemic areas, immunocompromised individuals, and certain sexually active homosexual men. Cyst passage rates as high as 20% have been reported among certain groups of sexually active homosexual men. These individuals were frequently symptomatic.

Pathophysiology

The mechanisms by which G lamblia causes diarrhea and intestinal malabsorption are probably multifactorial and not yet fully elucidated. Although the parasite appears to alter epithelial structure and function, leading to malabsorption, diarrhea can occur in individuals in the absence of obvious light microscopic changes in small intestinal structure. Also, marked or moderate partial villous atrophy in the jejunum can be observed in histologic sections from asymptomatic individuals who are infected. In addition to disrupting the mucosal epithelium, effects in the luminal may contribute to malabsorption and the production of diarrhea.

G lamblia trophozoites attach to the epithelium and distort microvilli at the site of attachment. The trophozoite has a convex dorsal surface and a flat ventral surface containing a disk, which is often referred to as the sucking or adhesive disk. The parasite has powerful adhesion, catching, and holding abilities with its disk. In the murine model of giardiasis, the ventral disk adhesion imprints are marked but less impressive than in the human small intestine. However, this direct injury is an unlikely cause of the more extensive reduction in microvillus surface area, the reduction in disaccharidase activities, and the more pronounced abnormalities of villous architecture.

Varying degrees of malabsorption of sugars (eg, xylose, disaccharides), fats, and fat-soluble vitamins (eg, vitamins A and E) may contribute to substantial weight loss. The histopathologic response to giardiasis varies and imperfectly correlates with the clinical symptoms.

G lamblia may release cytopathic substances that damage the intestinal epithelium. Giardia species contain thiol-dependent and thiol-independent proteinases, which may find substrates in the microvillus membrane. In addition, the surface mannose-binding lectin of G lamblia may contribute to epithelial damage. Whatever the mechanism by which G lamblia damages villous epithelial cells, the result consistently appears to be an increase in crypt length and crypt cell proliferation.

Genotypically diverse isolates of Giardia species may vary in their ability to produce morphologic changes in the small intestine epithelium and to impair fluid, electrolyte, and solute transport.

Frequency

United States

G lamblia is the parasite most commonly identified in stool specimens. The age-specific prevalence of giardiasis is highest in childhood and adolescence and begins to decline thereafter. Overall, the asymptomatic carriage rate of G lamblia is estimated to be 3-7%. The asymptomatic carriage rate in children may be as high as 20% in southern regions and in children younger than 36 months who attend daycare centers. Asymptomatic carriage may persist for several months. Many children with giardiasis are symptomatic, have been shown to spread the disease within their homes, and may contribute to high endemic rates in their communities.

• The incidence is high among individuals who camp and backpack in mountainous Western states. Incidences of 30-40% have been reported in Europeans and North Americans traveling to certain parts of the former Soviet Union (1974).

International

Giardiasis occurs in temperate and tropical regions worldwide, and it continues to be the most frequently identified human protozoal enteropathogen. In the industrialized world, overall prevalence rates are 2-5%. In the developing world, G lamblia infects infants early in life. Prevalence rates of 15-20% in children younger than 10 years are common.

Mortality/Morbidity

Most infected individuals are asymptomatic, and most infections are self-limited. However, chronic infections marked by chronic diarrhea or steatorrhea and malabsorption can occur and may last from weeks to months.

• Death is rare and usually occurs in malnourished children.
• G lamblia has been implicated as the chief cause of growth retardation in infected children, even after other diarrhea-causing agents are controlled.
• Giardiasis is not associated with mortality except in cases of extreme dehydration, primarily in infants. Morbidity is moderate and involves primary GI symptoms.

Race

No racial predilection exists.

Sex

Giardiasis is slightly more common in males than in females.

Age

All ages are affected. Infants and young children have an increased susceptibility to giardiasis, although infection is rare during the first 6 months of life in breastfed infants. Age-specific prevalence of giardiasis continues to rise through infancy and childhood and begins to decline only in adolescence.

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