Etiology

The life cycle of G. lamblia (also known as Giardia intestinalis or Giardia duodenalis) is composed of 2 stages: trophozoites and cysts. Giardia infects humans after ingestion of as few as 10-100 cysts (which measure 8-10 mm in diameter). Each ingested cyst produces 2 trophozoites in the duodenum. After excystation, trophozoites colonize the lumen of the duodenum and proximal jejunum, where they attach to the brush border of the intestinal epithelial cells and multiply by binary fission. The body of the trophozoite is teardrop shaped, measuring 10-20 mm in length and 5-15 mm in width. Giardia trophozoites contain 2 oval nuclei anteriorly, a large ventral disk, a curved median body posteriorly, and 4 pairs of flagella. As detached trophozoites pass down the intestinal tract, they encyst to form oval cysts that contain 4 nuclei. Cysts are passed in stools of infected individuals and may remain viable in water for as long as 2 mo. Their viability often is not affected by the usual concentrations of chlorine used to purify water for drinking.

Giardia strains that infect humans are diverse biologically, as shown by differences in antigens, restriction endonuclease patterns, DNA fingerprinting, isoenzyme patterns, and pulsed-field gel electrophoresis. Studies suggest that different Giardia genotypes may cause unique clinical manifestations, but these findings appear to vary according to the geographic region tested.

Epidemiology

Giardia occurs worldwide and is the most common intestinal parasite identified in public health laboratories in the USA, where it is estimated that up to 2 million cases of giardiasis occur annually. Giardia infection usually occurs sporadically but is a frequently identified etiologic agent of outbreaks associated with drinking water. The age-specific prevalence of giardiasis is high during childhood and begins to decline after adolescence. The asymptomatic carrier rate of G. lamblia in the USA is as high as 20-30% in children younger than 36 mo of age attending child-care centers. Asymptomatic carriage may persist for several months.

Transmission of Giardia is common in certain high-risk groups, including children and employees in child-care centers, consumers of contaminated water, travelers to certain areas of the world, men who have sex with men, and persons exposed to certain animals. The major reservoir and vehicle for spread of Giardia appears to be water contaminated with Giardia cysts, but food-borne transmission occurs. The seasonal peak in age-specific case reports coincides with the summer recreational water season and might be a result of the extensive use of communal swimming venues by young children, the low infectious dose, and the extended periods of cyst shedding that can occur. In addition, Giardia cysts are relatively resistant to chlorination and to ultraviolet light irradiation. Boiling is effective for inactivating cysts.

Person-to-person spread also occurs, particularly in areas of low hygiene standards, frequent fecal-oral contact, and crowding. Individual susceptibility, lack of toilet training, crowding, and fecal contamination of the environment all predispose to transmission of enteropathogens, including Giardia, in child-care centers. Child-care centers play an important role in transmission of urban giardiasis, with secondary attack rates in families as high as 17-30%. Children in child-care centers may pass cysts for several months. Campers who drink untreated stream or river water, particularly in the western USA, and residents of institutions for the developmentally delayed are also at increased risk for infection.

Humoral immunodeficiencies, including common variable hypogammaglobulinemia and X-linked agammaglobulinemia, predispose humans to chronic symptomatic Giardia infection, suggesting the importance of humoral immunity in controlling giardiasis. Selective immunoglobulin A (IgA) deficiency is also associated with Giardia infection. Although many individuals with AIDS have relatively mild Giardia infections, some reports suggest that severe Giardia infection, often refractory to treatment, may occur in a subset of individuals with AIDS. There is a higher incidence of Giardia infection in patients with cystic fibrosis, probably owing to local factors such as the increased amount of mucus, which may protect the organism against host factors in the duodenum. Human milk contains glycoconjugates and secretory IgA antibodies that may provide protection to nursing infants.

Clinical Manifestations

The incubation period of Giardia infection usually is 1-2 wk but may be longer. A broad spectrum of clinical manifestations occurs, depending on the interaction between G. lamblia and the host. Children who are exposed to G. lamblia may experience asymptomatic excretion of the organism, acute infectious diarrhea, or chronic diarrhea with persistent gastrointestinal tract signs and symptoms, including failure to thrive and abdominal pain or cramping. Giardia was the cause of 15% of nondysenteric diarrhea in children examined in U.S. outpatient clinics in 1 study. Most infections in both children and adults are asymptomatic. There usually is no extraintestinal spread, but occasionally trophozoites may migrate into bile or pancreatic ducts.

Symptomatic infections occur more frequently in children than in adults. Most symptomatic patients usually have a limited period of acute diarrheal disease with or without low-grade fever, nausea, and anorexia; in a small proportion of patients, an intermittent or more protracted course characterized by diarrhea, abdominal distention and cramps, bloating, malaise, flatulence, nausea, anorexia, and weight loss develops (Table 274-1). Stools initially may be profuse and watery and later become greasy and foul smelling and may float. Stools do not contain blood, mucus, or fecal leukocytes. Varying degrees of malabsorption may occur. Abnormal stool patterns may alternate with periods of constipation and normal bowel movements. Malabsorption of sugars, fats, and fat-soluble vitamins has been well documented and may be responsible for substantial weight loss. Giardia has been associated with iron deficiency in internationally adopted children. Giardiasis has been associated with growth stunting and repeated Giardia infections with a decrease in cognitive function in children in endemic areas.

Table 274-1 CLINICAL SIGNS AND SYMPTOMS OF GIARDIASIS