Giardia lamblia or Giardia
intestinalis is a protozoan flagellate that is among the
most common disease-causing parasites in the United States and the
most frequently identified agent of waterborne diarrhea.1 Cases
are especially common in areas with inadequate water and sanitation
facilities. Humans are the major reservoir of infection, although
other mammals, such as dogs, cats, and beavers, may be colonized
and excrete cysts. Massive epidemics have occurred after the contamination
of reservoirs, lakes, and streams, especially when community water
supplies are not adequately filtered.
Although giardiasis affects persons of all age groups, the populations
at highest risk are children ages 0 to 5 years and adults ages 31
to 40. In developing countries, Giardia infection
has been reported in almost 100% of children who were followed
prospectively from birth until age 2 years. In the United States
and other developed countries, Giardia is prevalent
in childcare centers and custodial institutions, among backpackers
and others spending time in wooded areas, and among travelers to
disease-endemic areas. Among children in daycare centers, Giardia cyst
carriage has been documented to be as high as 50%, and
many of these are asymptomatic carriers who can spread infection
to household contacts. Person-to-person spread via fecal-oral route
and ingestion of contaminated water are the most common modes of
transmission; infection through food is less common.
Giardia cysts, present in the stool of infected
persons, are the infective form. After ingestion, they excyst in
the small intestine, yielding trophozoites that subsequently multiply.
The trophozoites remain limited to the mucosa, mucus, or lumen of
the intestine and are rarely, if ever, invasive (Fig.
349-1). Encystation normally occurs prior to expulsion in the
feces. Contamination of oneself and the environment with cysts is
common. The number of cysts excreted varies, but it may reach as
many as 10 million per gram. Infections are relatively frequent,
because as few as 10 cysts can infect 30% of inoculated
Giardia lamblia trophozoite seen by
scanning electron microscopy.
The exact pathophysiology of the diarrhea is not known. The most severe
cases are characterized by malabsorption and lactose intolerance,
with varying degrees of inflammation and villous blunting. Host
factors are also determinants of disease outcome; only 40% of humans
infected with the same inoculum develop diarrhea. Patients with
hypogammaglobulinemia frequently suffer from particularly severe
cases of giardiasis. The observation that asymptomatic infections
are more common in persons previously infected with Giardia also
suggests partial immunity.
Clinical manifestations and duration of symptoms vary. Infections
range from asymptomatic cases to severe, life-threatening diarrhea
accompanied by malabsorption and dehydration. Infections may last
from a few days to years. In naturally occurring infections, symptoms
usually appear approximately 12 to 14 days after presumed exposure.
Passage of cysts usually begins 7 to 10 days after inoculation ...