Biology Reference
In-Depth Information
this phase of the life-cycle.
9,10
Considerable evidence from both human
and porcine studies points to the presence of adult worms in the intestine
contributing to nutritional impairment, including growth retardation.
11
In
contrast, there is a paucity of data on the impact of larval ascariasis on
host fitness, including nutritional status and body weight. Hale et al.
12
concluded that the effect of migrating larvae on pigs was less pronounced
than that of adult worms and Stephenson et al.
13
infected pigs with
fourth-stage larvae and demonstrated that adult worms depressed the
growth rate in the absence of larval migration. Nevertheless, fenbenda-
zole treatment during the liver migration phase in experimentally infec-
ted pigs improved feed conversion rates by 22%, and treatment of pigs
during the pulmonary migration phase improved these rates by 8% in
comparison to controls.
14
The initial stages of the migratory pathway involve newly hatched L3
larvae penetrating the walls of the large intestine and migrating via the
bloodstream to the hepatic circulation. Despite the interest in protective
intestinal immunity and a pre-hepatic barrier originally proposed by
Urban et al.,
15
there is a lack of published work on the local response to
this short-lived stage of the early migratory pathway. Trickle infections of
A. suum in pigs revealed a reduction in liver white spots and larvae
getting through to the lungs over time, suggesting a slow build-up of the
efficiency of the pre-hepatic barrier in blocking access of larvae to the
liver.
16
The liver phase in humans remains particularly subterranean for
obvious reasons. In an extensive prospective study from India, Javid and
colleagues
17
described 510 patients over a 10-year period that were
admitted to hospital with liver abscess. Of these, 74 (14.5%) had biliary
Ascaris as the cause and 11 patients had intact Ascaris within the liver
abscess. Both Sakakibara et al.
18
and Kakihara et al.
19
also reported liver
abscesses in patients infected with A. suum. Increased levels of hepatic-
originating acute phase reactants, such as C-reactive protein, ferritin,
and eosinophilic cationic protein, found in putatively immune Nigerian
children, indicate that an ongoing inflammatory response may represent
an anti-parasite effector mechanism targeting migrating and developing
liver-stage and possibly lung-stage larvae.
20
In pigs, the consequences of Ascaris infection in the liver are well
documented as white spots (WS), pathological lesions, composed of
leucocyte infiltrations, formed as a consequence of the damage inflicted
by migratory larvae and the associated inflammatory response.
21
e
24
There are three types of WS observed in A. suum-infected pigs (see
Figure 14.1, Chapter 14). The granulation type WS (GT-WS) have been
suggested to form along larval migration routes (small GT-WS) or
encapsulate trapped larvae (large GT-WS)
23,25,26
and have been proposed
to play a role in immunity to A. suum in pigs.
26,27
GT-WS have been
