Giardiasis and Balantidiasis

Etiology

The life cycle of G. duodenalis (also known as Giardia lamblia or Giardia intestinalis ) is composed of two stages: trophozoites and cysts. Giardia infects humans after ingestion of as few as 10-100 cysts, which measure 8-10 µm in diameter. Each ingested cyst produces two 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 µm in length and 5-15 µm in width. Giardia trophozoites contain two oval nuclei anteriorly, a large ventral disk, a curved median body posteriorly, and four pairs of flagella. As detached trophozoites pass down the intestinal tract, they encyst to form oval cysts that contain four 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 United States, where it is estimated that up to 2 million cases of giardiasis occur annually. Giardia infection usually occurs sporadically, but Giardia 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 United States is as high as 20–30% in children younger than 36 mo of age attending childcare centers. Asymptomatic carriage may persist for several months.

Transmission of Giardia is common in certain high-risk groups, including children and employees in childcare 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 foodborne transmission occurs. The seasonal peak in age-specific case reports coincides with the summer recreational water season and may 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 childcare centers. Childcare centers play an important role in transmission of urban giardiasis, with secondary attack rates in families as high as 17–30%. Children in childcare centers may pass cysts for several months. Campers who drink untreated stream or river water, particularly in the western United States, and residents of institutions for the developmentally delayed are also at increased risk for infection.

Humoral immunodeficiencies, including common variable immunodeficiency and X-linked agammaglobulinemia, predispose humans to chronic symptomatic Giardia infection, suggesting the importance of humoral immunity in controlling giardiasis. Selective immunoglobulin A deficiency is also associated with Giardia infection. Although many individuals with AIDS have relatively mild Giardia infections, Giardia infection refractory to treatment may occur in a subset of individuals with AIDS. A higher incidence of Giardia infection in patients with cystic fibrosis was reported in 1988, particularly in older children and adults, but there have been no subsequent confirmations of this risk. Human milk contains glycoconjugates and secretory immunoglobulin A antibodies that may provide protection to nursing infants against Giardia .