Clinical Manifestations

Clinical symptoms of Giardia infection range from asymptomatic cyst passage to acute, self-limited diarrhea, to prolonged diarrheal disease.^1–3 Asymptomatic infection is common. Symptoms of acute illness include watery diarrhea with abdominal pain; stools may be foul-smelling with accompanying flatulence, abdominal distension, and poor appetite. A small proportion of patients will develop a prolonged diarrheal syndrome with intermittent diarrhea, abdominal and epigastric discomfort exacerbated by eating, malaise, fatigue, and occasional headache, often leading to weight loss (a distinguishing feature) and anemia. Lactose intolerance is common (20–40% of cases), may persist for weeks after infection, and may be mistaken for relapse or reinfection.⁴ Giardia has been implicated in growth failure in children in the developing world who are chronically infected or frequently reinfected. Catch-up growth has been shown to occur when Giardia is eliminated.

Manifestations of Giardia infection in children may be non-specific and include anorexia, irritability, flatulence, chronic abdominal pain, behavioral changes, or poor weight gain. Eosinophilia is rare and should prompt a search for other diagnoses.

Fever and vomiting occur in a minority of patients. Gross blood or polymorphonuclear cells in the stool and tenesmus do not occur. Rarely, there may be an association with urticaria, and reactive arthritis has been reported.

The disease is transmitted by ingestion of cysts. Onset of symptoms occurs 1–2 weeks after cyst ingestion, though the period of time to detection of cysts in the stool may be longer than the incubation period. The disease is communicable for as long as cyst passage occurs (may be as long as 6 months).¹

Individuals with humoral immunodeficiencies are at increased risk of chronic disease if infected.

Etiology

Giardia is a flagellate protozoan. The two stages in the life cycle are the free living trophozoite form, and the infectious form, the cyst. Acquisition of disease is by ingestion of as few as 10–25 cysts, occurring most frequently by drinking contaminated water, but also by person-to-person transmission. Under the influence of gastric acid, excystation releases trophozoites which colonize and multiply in the upper small bowel and biliary tract (Fig. 29.1).²

Fig. 29.1 Life cycle of Giardia lamblia. Cysts are resistant forms and are responsible for transmission of giardiasis. Both cysts and trophozoites can be found in the feces (diagnostic stages)(1). The cysts are hardy and can survive several months in cold water. Infection occurs by the ingestion of cysts in contaminated water, food, or by the fecal–oral route (hands or fomites)(2). In the small intestine, excystation releases trophozoites (each cyst produces two trophozoites) (3). Trophozoites multiply by longitudinal binary fission, remaining in the lumen of the proximal small bowel where they can be free or attached to the mucosa by a ventral sucking disk (4). Encystation occurs as the parasites transit toward the colon. The cyst is the stage found most commonly in nondiarrheal feces (5). Because the cysts are infectious when passed in the stool or shortly afterward, person-to-person transmission is possible. While animals are infected with Giardia, their importance as a reservoir is unclear.

(Adapted from Centers for Disease Control and Prevention DPDx.)

Epidemiology

Giardiasis occurs worldwide, in both temperate and tropical climates. Giardia is one of the first parasites to infect children in the developing world; prevalence rates may be as high as 15–20% in children under 10 years of age. Over 60% of children may be infected at some point during childhood.¹ In the US, it is the most commonly identified intestinal parasite, with prevalence rates as high as 16% in some areas. Outbreaks have been associated with contamination of water supplies, exposure to water contaminated by animal feces containing cysts infectious for humans, and person-to-person transmission in settings such as childcare centers. Food-borne transmission also occurs. Exposure to infected feces may also occur during sexual contact.