Biology Reference
In-Depth Information
protein nutrition, several studies failed to show consistent changes in
nitrogen absorption, digestibility or retention. One may speculate on the
explanation for this. An obvious reason is the nature of A. suum infection,
characterized by pronounced overdispersion (see Chapter 7), rapid
expulsion of early stages and strong immune responses. One may control
the infection levels, but the resulting worm burdens are highly unpre-
dictable and it is often difficult to get any dose dependency on the
performance parameters, in contrast to, for example, the gastrointestinal
nematode infections of ruminants. For the same reasons, the method of
inoculation, dose level, timing of measurements in relation to infection
stage, stage of immunity, and age of animals become very important
factors that are difficult to align between studies. Further, feed protein
levels seem to affect the disease impact, and only a few proper two-
factorial designs with appropriate statistical analysis have been per-
formed. Also effects of feed components other than protein, e.g. minerals,
cannot be ruled out. A logical consequence of this is that studies exam-
ining effects of A. suum in commercial farms (on-farm studies) are likely to
face even more difficulties. This will be addressed in the next paragraph.
PRODUCTION LOSSES ASSOCIATED WITH
ASCARIASIS IN PIGS
ON-FARM STUDIES
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The liver migratory activity of A. suum leads to a marked focal
inflammatory response and later whitish fibrotic lesions known as milk
spots or white spots, which are easily identified at slaughter. The lesions
are characterized as either superficial granulation tissue-type milk spots
(small or large), which are visible gray
white interlobular septa, or lym-
phonodular milk spots which are elevated and rounded 26 ( Figure 14.1 ). In
practice at slaughter, milk spots are not differentiated. While other
migratory nematodes may cause similar liver lesions in pigs, e.g. Stepha-
nurus dentatus (kidney worm) in warmer regions and occasionally Toxocara
canis , there is a general consensus that milk spots are caused by A. suum
infections in commercial production systems in temperate climates.
The lesions are self-healing and disappear with time and milk spots
thus represent a recent A.
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suum infection,
in most cases within
6 weeks. 26 Despite the disappearance of milk spots, the general texture
of livers after exposure is clearly hardened as a response to migration,
and the livers are less suited for human consumption. 45 It should be noted
that pigs without liver lesions may have adult worms in the intestine and
vice versa. However, other investigators found a good correlation
between herds with high levels of A. suum infection and a high proportion
of livers with milk spots at slaughter. 46 Wagner and Polley 11 calculated
a negative predictive value of 0.90 for milk spots as an indicator of adult
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