Fate and Transport of Zoonotic Bacterial, Viral, and - The Pork Store ...

4. Fate and Transport of Zoonotic Parasitic Pathogens
enteric pathogens; some pigs apparently do not seem to
show any signs of infection. Typically, cryptosporidiosis
is an infection of young animals. Published reports
document that Cryptosporidium sp. commonly infect
nursing and weaned pigs more than sows (Xiao, Herd,
and Bowman 1994). In Ontario, Canada, 5.3 % of 3,491
pigs primarily 1–12 wk of age were infected (Sanford
1987). Based on these data it is possible that farrowing
houses are more likely to be found as sources of
Cryptosporidia oocysts compared with sites housing
feeder pigs or mature breeding stock.
Epidemiology
The current state of knowledge of Cryptosporidium
is incomplete. This is because of the large number of
mammalian species that can become infected with
Cryptosporidium sp. and the evolving understanding
of host specificity. New molecular methods are
elucidating relationships between animal hosts and
species of Cryptosporidium (Caccio et al. 2005), but there
is no question that some species of Cryptosporidium
in domesticated animals are associated with human
infection. For example, young dairy calves are well
documented as a source of C. parvum, which can cause
human sickness. The incidence of infection is high
in young dairy calves, and the potential for human
exposure is significant among dairy calf handlers
and veterinary students working with dairy calves.
But the zoonotic potential with most other species of
Cryptosporidium, including those of swine origin, is
much less clear.
Two species of Cryptosporidium are reported to
occur in humans, C. hominis and C. parvum. Humans
are the major reservoir for C. hominis, and this species
tends to account for most human outbreaks worldwide.
C. parvum is clearly a zoonotic species and usually is
associated with cattle. Increasing reports of C. parvum
in certain regions of the world may result, in part, from
the intensive husbandry practiced for ruminants and
the associated high concentrations of young animals at
these feeding operations (Xiao et al. 2004).
In a molecular comparison of Cryptosporidium
isolates from a variety of hosts, there was evidence of
a geographically widespread but swine-specific strain
of C. parvum (Morgan et al. 1998). Nonetheless, two
genotypes (Type 1 and 2) of Cryptosporidium have been
reported in swine and also have been found in humans.
But pig genotype 1 has a very low prevalence in humans
and is unlikely to emerge as a major human pathogen
(Xiao et al. 2004). More recently, Xiao and colleagues
(2006) studied Cryptosporidium genotypes obtained from
unseparated pig slurry in storage tanks on pig farms
in Northern Ireland. Polymerase chain reaction (PCR)
and sequencing of PCR products revealed 3 genotypes of
Cryptosporidium in the slurries: C. muris, pig genotype II,
and C. suis. Only C. muris and C. suis have been isolated
from human cases. It is noteworthy that piglets have
been infected with C. hominis, but caution should be
used in interpreting the significance of finding parasites
traditionally associated with animals in humans and
vice versa. Although an animal source of C. parvum
often is suspected to be the source of C. parvum or other
zoonotic Cryptosporidium spp. found in humans or the
environment, many human infections are traced back to
human sources (Xiao et al. 2004).
Fate and Transport
In a Canadian study, Cryptosporidium was found
on four of six swine farms, with an overall prevalence
of 11% among the evaluated animals (Olson et al.
1997). No information about the management systems
used by the studied farms was provided. Although no
published studies have examined the concentration of
Cryptosporidum oocysts in swine manure, Thurston-
Enriquez, Gilley, and Eghball (2005) referenced
unpublished data reporting 20 to 90 oocysts found per
gram of swine lagoon wastewater. The Cryptosporidium
oocyst is a resistant parasitic stage and can retain
its viability in typical environmental conditions.
Compared with Giardia, Cryptosporidium is much more
resistant to decay over a wide range of temperatures.
Cryptosporidium can remain viable for more than 6 mo
at 20°C (Fayer 2000; Gajadhar and Allen 2004), and
at 25–30°C, infectivity can be retained for up to 3 mo
(Fayer 2000). Even at -20°C, Cryptosporidium oocysts
can remain viable for up to 8 hr, and oocysts held at
-5°C were infectious for up to 2 mo (Fayer 2000). When
Cryptosporidium oocysts were inoculated into pig
slurries and persistence measured in unstirred tanks,
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