Science of Water : Concepts and Applications

142 The Science of Water: Concepts and Applications
Persons in categories 4 and 5 become exposed through more direct contact with feces or an
infected person; exposure to soiled 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 the staff
of the Centers for Disease Control (CDC) over a 3-year period had a day-care 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 (Black et al., 1981; Pickering et al., 1984). The highest
infection rates are usually observed in children who wear diapers (1–3 years of age).
Local health offi cials and managers or water utility companies need to 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 is 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.
To understand the fi ner aspects of Giardia transmission and strategies for control, the drinking
water practitioner 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 5.9). 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 here. At some time during the trophozoite’s life, it releases its hold on the bowel wall and
Cyst Excystation in duodenum
(infective stage)
External Man
environment
FIGURE 5.9 Life cycle of Giardia lamblia.
Multiplication by
longitudinal
binary fission