Biology Reference
In-Depth Information
(B)
(A)
(C)
FIGURE 14.1
Liver white spots or milk spots caused by migrating
Ascaris suum
larvae:
(A) small granulation-tissue type; (B) large granulation-tissue type; (C) lymphonodular
type.
Photos courtesy of N.P.K. Hansen and A. Roepstorff.
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worms, i.e. the absence of milk spots means that it is unlikely that adult
worms will be found in a pig. There is a further caveat of using milk spots
as a diagnostic indicator: in solidly immune pigs there is presumably
a functional pre-hepatic barrier which may result in no (or very limited)
liver migration even if animals are heavily exposed, e.g. 12 weeks'
immunization followed by challenge with 10,000 eggs lead to no liver
pathology,
47
and 14 weeks' continuous infection resulted in very fewmilk
spots.
48
The noticeable decrease in number (or lack) of white spots
suggests that the level of exposure is not reflected in the number of white
spots in immune animals. If adult worms, established before the onset of
the barrier, are eliminated by treatment, such animals are unlikely to
perpetuate the infection, unless the immunological memory wanes.
Abattoirs incur losses due to downgrading or total condemnation of
livers due to milk spots. Data from abattoirs are available from a range of
countries and are in some countries used to monitor prevalence levels.
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However, in many studies it is unclear which case definition has been
applied, whether it be milk spots (present or not; quantified), hepatic
lesions or condemnations, and in the case of condemnations, is there an
upper acceptable limit of number of milk spots which does not lead to
