Biology Reference
In-Depth Information
being infected on free-range and organic farms, while none were found to
be positive on conventional farms. A marked difference between
production systems was also confirmed with regard to prevalence of milk
spots: 8% vs. 1% in pigs slaughtered from Danish organic and conven-
tional farms, respectively, based on standard recordings of condemna-
tions from the abattoir.
24
In Denmark, all pigs from the organic farms
would have stayed outside until at least weaning. A recent English study
confirmed a higher prevalence of milk spot livers in outdoor as compared
to indoor reared pigs.
25
Although it is tempting, based on these findings,
to conclude that endoparasites are among the main disease problems in
organic/free-range pig production, it may not be the case. Most studies
measure prevalence or occurrence of parasites (see Chapter 9) while
neglecting intensity of infections. Intensity of infections in a herd is
probably a major determinant of impact in affected pigs, as will be clear
from later paragraphs.
IMP
ACT ON CLINICAL HEALTH IN
PIGS
The majority of
A. suum
infections in pigs are subclinical. The pene-
tration of cecum and proximal colon by newly hatched third-stage larvae
(L3) results in petechial bleedings of the mucosa only, whereas the
following liver migration is accompanied by severe pathology, thoroughly
described by Roepstorff.
26
We have found no clinical reports of this early
intestinal and liver migratory phase (days 0
6 post infection (p.i.)). During
e
8 days p.i.
27
pigs are subject to respiratory
stress (pneumonitis) reflected in increased breathing rate, dyspnea
and dry coughing.
28
e
30
The condition resembles Loeffler's syndrome in
humans infected with
A. lumbricoides
26
and is associated with an increased
number of blood eosinophils. Considering the large number of experi-
mental infections performed in our and other laboratories, it is remarkable
that barely any of these have reported such symptoms, whichmay indicate
low incidence or only vague symptoms.
30
From a herd health viewpoint, it
is difficult to define this pathological stage in an endemic situation, and
presumably in most infected herds it will remain unconfirmed, although
cases have been reported.
31
Earlier studies have also commented on this,
e.g. Spindler
32
stating “the clinical manifestations of an invasion of the
lungs by large numbers of ascarid larvae are sometimes spectacular and
easily recognized” (see also Chapter 5).
The third-stage larvae (L3) molt to L4 when the nematodes return to the
small intestine around day 10 p.i.,
26
and a further molt takes place 2 weeks
later to reach L5, the final stage that will ultimately become sexually
mature by days 42
the lung penetration phase 6
e
50% of a single infection dose can be
isolated from the small intestine at day 12 p.i.
26
but the majority will
49 p.i. About 40
e
e
