Biology Reference
In-Depth Information
contamination is high, it will most likely take several fattening rounds to
eventually reduce the infection pressure.
63,64
In a Swedish study it appeared that the pens of sows and fatteners are
the heaviest infected environments.
40
This was shown by the presence of
eggs in old fecal deposits collected from the pens. These findings were
supported by studies from China and Denmark where the highest
prevalence of
A. suum
was also found in breeding sows and
fatteners.
2,65,66
Although the prevalence in breeding boars is usually
lowerthaninsowsorfatteners,
65,67
farmers must not omit them from
treatment. Infected boars could also be an important source of trans-
mission of parasite eggs on the farm since they are often located in more
traditional pens than the sows (e.g. with solid floor and/or bedding).
68
Moreover, boars are regularly used for contact stimulation of the sows,
which allows them to spread the infection by defecating in areas other
than their own pen.
Strong evidence exists that under indoor conditions, massive infection
of piglets with
A. suum
usually occurs shortly after arrival in the highly
infectious fattening units and not in the farrowing pens.
2,40,69
Several
studies have shown that
A. suum
egg excretion was nearly absent in
weaners despite the, sometimes substantial, presence of eggs in the far-
rowing pens of intensive herds.
2,3,65,66
Here, the increased hygienic
standards in combination with the low humidity in the farrowing pens
may significantly reduce the presence of favorable microenvironments for
egg survival. On the other hand, in piglets raised under outdoor condi-
tions, significant transmission is thought to occur soon after birth and pigs
are infected before fattening.
2,14
Anthelmintic treatment has limited to no residual activity, depending
on which anthelmintic is used. As a consequence, pigs are immediately
exposed to new infections and continually host migrating larvae in their
bodies. This larval migration has a significant impact on pig performance.
Increasing doses of infective eggs in experimentally infected pigs has
shown to be correlated with decreasing productivity,
18
thereby ques-
tioning the traditional dogma that attributes the reduced productivity to
the presence of adult worms in the intestine. Although the presence of
adult worms in the intestine is important for the dispersal and epidemi-
ology of the parasite, their effect on economical profitability of a herd does
not seem to be as important as the migrating larval stages. This is
underpinned by the fact that within an infected pig herd, the distribution
of worms is highly aggregated, with the majority of adult worms being
harbored in a minority of the host population.
23
Moreover, most of the
pigs that do end up having patent infections only harbor a fewworms
25,70
and it is hard to imagine that this would have a noticeable impact on the
pig's production parameters. In contrast, we can assume that all pigs in
the same pen are exposed to roughly an equal amount of infectious larvae
