Biology Reference
In-Depth Information
worldwide (source: FAO). Since the early days of pig farming, farmers
have seen the productivity of their animals compromised by the many
parasitic infections affecting pigs. Fortunately, in the last couple of
decades, the impact of these parasites on pigs and their productivity has
been significantly reduced. The main reasons for this are the switch from
smaller, open air, rural farming to more industrial-sized, high-intensity,
indoor farming and the advent of pharmaceuticals directed against the
parasites that affect pigs. Infections with
Oesophagostomum
spp.,
Ascaris
suum
and to a much lesser extent
Trichuris suis
have successfully survived
the transition to conventional
indoor conditions and are currently
thriving in some farms.
2
Infections with
A. suum
, however, show the
highest prevalence.
3
e
5
In order to understand the problems associated with the methods of
control and diagnosis discussed in the second and third part of this
chapter, a proper understanding of the life-cycle of this parasite is
indispensable. Pigs become infected with
A. suum
when they take up
eggs from the environment in which an infective stage larva has devel-
oped. The larva emerging from the egg is not a second-stage larva (L2) as
was previously presumed but rather a third-stage larva (L3) covered by
a loosened second-stage cuticle.
6
The larvae of
Ascaris
complete two
molts within the egg. After oral intake, hatching is induced by the
altering chemical and physical factors of the new environment. The
larvae inside the egg are stimulated to secrete proteinases and chitinases,
which presumably help them degrade the different layers of the eggshell
from the inside out.
7,8
When the larvae emerge from the egg, they start
their hepatotracheal migration by penetrating the wall of the cecum and
upper part of the colon.
9
The L3s are then transported by the mesenterial
blood veins and can reach the liver as fast as 6 hours post-infection.
9,10
The larvae are stuck in the capillaries and destroy liver tissue in order to
reach the efferent blood vessels.
The inflammatory reaction to this damage is manifested as the so-
called “white spots” that are visible on the surface of the liver. These
white spots appear as early as 3 days post-infection and start to resolve
after about 2
e
3 weeks post-infection.
11,12
The bloodstream carries the
larvae to the next capillary system, which is the lung, where they pene-
trate the alveoli, move up the respiratory tree, and are eventually swal-
lowed again. From 8 days onwards, the L3s finally return to the small
intestine where they start their first ecdysis inside the host, to reach the
L4 stage. Between days 17 and 21 post-infection, most of the larvae are
eliminated.
11,13
The mechanisms responsible for this “self-cure” reaction
are currently still unknown.
The result of this elimination is a small, overdispersed population of
adult worms in the intestine, characteristic of
Ascaris
infections.
11,14
After
about 6 weeks, the worms have reached maturity and egg shedding by the
