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

4. Fate and Transport of Zoonotic Parasitic Pathogens
A-II—and A-II
has been isolated
only from humans,
suggesting a human
reservoir for this
group (Eligio-García
and Cortes-Campos
2005).
Because
molecular
characterization of
G. intestinalis strains
has identified some
host specificity
among genetically
similar groups,
the role of animals
(including swine)
in transmitting
G. intestinalis to
humans remains
unclear. The infective
dose is small, perhaps as few as 1 in 10 cysts (Caccio et
al. 2005). No reports have confirmed transmission to
humans from swine or their manure, but G. intestinalis
assemblage A has been isolated from pigs (Ey et al. 1997;
van Keulen et al. 2002), including at least one isolate that
was molecularly indistinguishable from human isolates
(Ey et al. 1997), suggesting a potential for human illness
after exposure to swine manure or waste-contaminated
media.
Fate and Transport
The concentration of Giardia cysts in swine lagoon
wastewaters can be as high as 1,075 cysts/g (Thurston-
Enriquez, Gilley, and Eghball 2005), but survival of
Giardia cysts seems to be highly temperature dependent
(Olson et al. 1999). For 90% degradation of cysts
inoculated into mixed human and swine manure at 5°C,
129 d were required, but only 4 d were required at 25°C
(Deng and Cliver 1992). The researchers also noted that
swine manure seemed much more toxic to cysts than
human sewage effluent—cysts exhibited 90% die-off in
8.5 d in swine manure compared with 28.6 d in human
effluent—and speculated that this difference might be
attributable to increased bacterial degradation in swine
manure relative to human effluent. Research in cattle
manure has shown Giardia cysts to be noninfective
within 1 wk after freezing at -4°C and infective for only 1
wk at 4° and 25°C. Giardia cysts are sensitive to freezing
of soil, becoming noninfective after only 7 d at -4°C, but
Giardia cysts were recoverable from soils maintained at
4°C for up to 8 wk. Soils maintained at 25°C inactivated
Giardia cysts within 1 wk, but Giardia seems to be
effectively retained in soil columns: sandy soils reduced
cysts over 7 logs, and gravel soil rarely resulted in
breakthrough recovery of cysts (Hijnen et al. 2005).
In a study of transport of Giardia cysts during runoff
events (Thurston-Enriquez, Gilley, and Eghball 2005), up
to 3.58 x 10 6 cysts could be recovered from 0.75-m x 2-m
field plots after simulated rainfall. In water, cysts survive
less than 14 d at 25°C but up to 77 d at 4–8°C (Olson et al.
1999). Research suggests, however, that human sewage
effluents may represent the most significant source of
infectious Giardia cysts in water (Thompson 2004).
45