Biology Reference
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more efficient utilization of feed. It has to be noted that the uninfected
group unintentionally became infected (mean of 3.2 vs. 18.5 worms). Pigs
each infected with 20,000 eggs and uninfected controls were compared in
metabolic cages. Infected pigs had significantly reduced digestibility of
dry matter (5.1%) and crude protein (6.8%) as well as lowered nitrogen
retention (11.0%) 33
37 days p.i. 39 However, these authors failed to
demonstrate any effect at earlier stages of infection. Urban et al. 40
reported 21% lower weight gain in untreated pigs in the finishing phase
(days 53
e
110) after natural exposure to A. suum in a contaminated outside
yard (days 0
e
52) as compared to pigs reared under similar conditions but
treated twice with either fenbendazole or ivermectin during the exposure
phase. The yard was also contaminated with T. suis (low numbers) which
may have added to the poorer performance. To support this, a group
trickle infected with (only) A. suum for the same period but kept in a clean,
concrete pen did not exhibit reduced performance in the finishing phase.
The study concluded that stringent control of environmental contami-
nation, which is difficult to achieve in outdoor production systems, and
strategic application of anthelmintics is needed to avoid long-term effects
of infection. A replicated study 41 failed to find any beneficial effects of
intensive anthelmintic treatment (fenbendazole or pyrantel) applied early
in life on weight gains or feed utilization in growing pigs naturally
exposed/infected with A. suum on a dirtlot. Bernardo et al. 42
e
reported
a very limited but significant effect (
1%) of A. suum life-time burden (a
composite measure based on repeated fecal egg counts during fattening)
on average daily weight gain of 352 pigs, produced in 15 Canadian farms.
Two studies from the southern USA have investigated the combined
effects of experimental A. suum , Oesophagostomum spp., and S. ransomi
infections on performance of growing pigs. 43,44 The first three-factorial
study examined the combined effects of pyrantel given continuously for
6 weeks, two protein levels, and order of infection and reported no
significant effect of anthelmintic treatment on overall weight gains. 43 The
pyrantel treatment resulted in only moderately reduced worm burdens.
The other study compared uninfected controls, infected controls, and
infected pigs treated with ivermectin after 34 days (exp. 1) or 37 days
(exp. 2) 44 . In exp. 1, infected pigs had 12
<
14% reduced weight gains and
10% reduced carcass weight as compared to uninfected controls but no
differences were found in exp. 2.
Based on this range of controlled, mostly experimental studies reported
here, it is evident that it has been difficult to demonstrate
e
e
in a statisti-
cally meaningful way
consistent and major changes in parameters
commonly associated with subclinical gastrointestinal parasitism (e.g.
reduced weight gain, feed intake, digestibility, nitrogen retention), even
when larger groups are used. In most cases, any observed differences
were found in the late intestinal phase. At the commonly used levels of
e
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