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Life cycle of Cryptosporidium.
(from: Juranek DD.
Cryptosporidiosis. In: Hunters Tropical Medicine, 8th edition. Strickland
GT,
Editor.)
Sporulated oocysts, containing
4 sporozoites, are excreted by the infected host through feces and possibly other routes
such as respiratory secretions
.
Transmission of Cryptosporidium parvum occurs mainly through contact with
contaminated water (e.g., drinking or recreational water).
Occasionally food sources, such as chicken salad, may serve as vehicles for
transmission. Many outbreaks in the United States have occurred in
waterparks, community swimming pools, and day care centers. Zoonotic
transmission of C. parvum occurs through exposure to infected animals or
exposure to water contaminated by feces of infected animals
.
Following ingestion (and possibly inhalation) by a
suitable host
, excystation
occurs. The sporozoites are released and parasitize
epithelial cells (,
)
of the gastrointestinal tract or other tissues such as the
respiratory tract. In these cells, the parasites undergo asexual multiplication
(schizogony or merogony) (,
,
)
and then sexual multiplication (gametogony) producing microgamonts (male)
and macrogamonts (female)
. Upon fertilization of the macrogamonts
by the microgametes (),
oocysts (,
)
develop that sporulate in the infected host. Two different types of
oocysts are produced, the thick-walled, which is commonly excreted from the
host
,
and the thin-walled oocyst
,
which is primarily involved in autoinfection. Oocysts are infective
upon excretion, thus permitting direct and immediate fecal-oral
transmission. Note that oocysts of Cyclospora cayetanensis, another important
coccidian parasite, are unsporulated at the time of excretion and do not become infective
until sporulation is completed. Refer to the life cycle of Cyclospora
cayentanensis for further details.
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