Cryptosporidium parvum is a very
successful and lethal parasite
By Garry Palmateer, M.Sc.
Director of Technical Services
GAP EnviroMicrobial Services
Figure 1. Developmental stages of C. parvum.
The water-borne protozoan
parasite, Cryptosporidium
parvum has gained worldwide
recognition during the
past ten years as it has caused more human
illness and mortality than any other
fecal-associated microorganism. Clarke,
in 1895, may have been the first to view
species of Cryptosporidium which were
described 15 years later and named as
Cryptosporidium muris in mice by
Tyzzer. Eighteen additional species have
been defined since that time.
Although it was originally thought
that each species was host-specific,
Cryptosporidium parvum, which is commonly
found infecting calves and lambs,
also resides in rodents, puppies and kittens,
using them as reservoir hosts. The
ability of C. parvum to cross species
barriers is a major reason why there are
so many human infections. This mode
of transmission, however, cannot explain
the large number of infections in a
human population whose exposure to
animal feces is limited.
The success of the survival and transmission
of C. parvum can also be attributed
to a specific aspect of its life cycle.
There are six developmental stages as
outlined in Figure 1. The oocyst is the
environmentally-resistant form of the
parasite life cycle. The resistance of an
oocyst is demonstrated well by its survival
in the natural environment.
Stressors, such as temperature, including
freezing, pH, osmolarity, and chemicals,
such as common disinfectants that
kill many other microorganisms, including
other parasites, have limited effect
on the viability of the oocyst of
Cryptosporidium parvum.
The oocyst develops from the microgamete
stage in the intestine of the host
into a spherical double-layered wall that
covers three unit membranes. Within the
membranes are four viable sporozoites,
which are released from the oocyst
through a suture in the inner oocyst wall.
Once the ingested oocyst passes through
the digestive tract to the small intestine
and reaches the epithelial tissues in the
intestine of the host (whether it be a human
or a specific animal), then the
sporozoites released from the suture on
the oocyst wall penetrate
the epithelial
cells and initiate the
life cycle of the parasite.
Figure 2. Thick-walled oocyst of C. parvum (Scanning
Electron Micrograph)
The thick-walled
oocyst, as shown in
Figure 2, comprises
approximately 80 percent
of the oocysts
formed. The remaining
20 percent of the
oocysts develop only a
single unit membrane
covering. These thin-walled
oocysts are susceptible
to any environmental
stressors, which
means that the viable
sporozoites within the membrane are not
sufficiently protected once they are outside
of the intestine.
The obvious strategy of this parasite,
to ensure its ultimate success in surviving,
is to produce four times as many of
the resistant thick-walled oocysts as the
thin-walled variety.
The function of the thin-walled
oocysts, which also contain four
sporozoites, is to immediately initiate
infection within the epithelial cells of the
animal intestine and not pass into the
animal feces.
Cryptosporidium parvum can infect
humans by causing diarrhea that can last
from three to twelve days and, in some
cases, the diarrhea may last up to three
weeks.
As there is no cure for human infection
as yet, Cryptosporidiosis can be
fatal for immuno-compromised persons,
which makes this parasite a serious human
health concern.
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