Giardiasis occurs worldwide. In the United States, Giardia is the parasite
most commonly identified in stool specimens submitted to state
laboratories for parasitologic examination. From 1977 through 1979,
approximately 4% of 1 million stool specimens submitted to state
laboratories were positive for Giardia (6). Other surveys have
demonstrated Giardia prevalence rates ranging from 1 to 20% depending on
the location and ages of persons studied. Giardiasis ranks among the top
20 infectious diseases that ca use the greatest morbidity in Africa, Asia,
and Latin America (7); it has been estimated that about 2 million
infections occur per year in these regions (8).
People who are at highest risk for acquiring a Giardia infection in the
United States may be placed into five major categories:
1) People in cities whose drinking water originates from streams or rivers
and whose water treatment process does not include filtration, or
filtration is ineffective because of malfunctioning equipment. 2)
Hikers/campers/outdoorspeople. 3) International travelers 4) Children who
attend day-care centers, day-care center staff, and parents and siblings
of children infected in day-care centers. 5) Homosexual men.
People in categories 1, 2, and 3 have in common the same general source of
infections, i.e., they acquire Giardia from fecally contaminated drinking
water. The city resident usually becomes infected because the municipal
water treatment process does not include a filter that is necessary to
physically remove the parasite from the water. The number of people in the
United States at risk (i.e., the number who receive municipal drinking
water from unfiltered surface water) is estimated to be 20 million. Inte
rnational travelers may also acquire the parasite from improperly treated
municipal waters in cities or villages in other parts of the world,
particularly in developing countries. In Eurasia, only travelers to
Leningrad appear to be at increased risk. In prospective studies, 88% of
U.S. and 35% of Finnish travelers to Leningrad who had negative stool
tests for Giardia on departure to the Soviet Union developed symptoms of
giardiasis and had positive tests for Giardia after they returned home
(10,11). With
the exception of visitors to Leningrad, however, Giardia has not been
implicated as a major cause of traveler's diarrhea. The parasite has been
detected in fewer than 2% of travelers who develop diarrhea. Hikers and
campers risk infection every time they
drink untreated raw water from a stream or river.
Persons in categories 4 and 5 become exposed through more direct contact
with feces of an infected person, e.g., exposure to soiled diapers of an
infected child (day-care center-associated cases), or through direct or
indirect anal-oral sexual practices in the case of homosexual men.
Although community waterborne outbreaks of giardiasis have received the
greatest publicity in the United States during the past decade, about half
of the Giardia cases discussed with staff of the Centers for Disease
Control in the past 2 to 3 years have a
day-care center exposure as the most likely source of infection. Numerous
outbreaks of Giardia in day-care centers have been reported in recent
years. Infection rates for children in day-care center outbreaks range
>from 21 to 44% in the United states and
from 8 to 27% in Canada (12,13,14,15,16,17). The highest infection rates
are usually observed in children who wear diapers (l to 3 years of age).
In one study of 18 randomly selected day care centers in Atlanta (CDC
unpublished data), 10% of diapered children were found infected.
Transmission from this age group to older children, day-care staff, and
household contacts is also common. About 20% of parents caring for an
infected child will come infected.
It is important that local health officials and managers of water utility
companies realize that sources of Giardia infection other than municipal
drinking water exist. Armed with this knowledge, they are less likely to
make a quick (and sometimes wrong) assumption that a cluster of recently
diagnosed cases in a city is related to municipal drinking water. Of
course, drinking water must not be ruled out as a source of infection when
a larger than expected number of cases are recognized in a community, but
the possibility that the cases are associated with a day-care center
outbreak, drinking untreated stream water, or international travel should
also be entertained.
Parasite Biology
To understand the finer aspects of Giardia transmission and the strategies
for control, one must become familiar with several aspects of the
parasite's biology. Two forms of the parasite exist: a trophozoite and a
cyst, both of which are much larger than bacteria (see Figure 1).
Trophozoites live in the upper small intestine where they attach to the
intestinal wall by means of a disc-shaped suction pad on their ventral
surface. Trophozoites actively feed and reproduce at this location. At
some time during
the trophozoite's life, it releases its hold on the bowel wall and floats
in the fecal stream through the intestine. As it makes this journey, it
undergoes a morphologic transformation into an egglike structure called a
cyst. The cyst, which is about 6 t o 9 micrometers in diameter x 8 to 12
micrometers (1/100 millimeter) in length, has a thick exterior wall that
protects the parasite against the harsh elements that it will encounter
outside the body. This cyst form of the parasite is infectious for other
people or animals. Most people become infected either directly by
hand-to-mouth transfer of cysts from the feces of an infected individual,
or indirectly by drinking feces-contaminated water. Less common modes of
transmission included ingestion of fecally contaminated food and
hand-to-mouth transfer of cysts after touching a fecally contaminated
surface. After the cyst is swallowed, the trophozoite is liberated through
the action of stomach acid and digestive enzymes and becomes established
in the small intestine.
Although infection after the ingestion of only one Giardia cyst is
theoretically possible, the minimum number of cysts shown to infect a
human under experimental conditions is ten (18). Trophozoites divide by
binary fission about every 12 hours. What this
means in practical terms that if a person swallowed only a single cyst,
reproduction at this rate would result in more than 1 million parasites 10
days later, and 1 billion parasites by day 15.
The exact mechanism by which Giardia causes illness is not yet well
understood, but is not necessarily related to the number of organisms
present. Nearly all of the symptoms, however, are related to dysfunction
of the gastrointestinal tract. The parasite rarely invades other parts of
the body, such as the gall bladder or pancreatic ducts. Intestinal
infection does not result in permanent damage.
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