Biology Reference
In-Depth Information
of pre-existing immune response to localized co-infections in the liver (for
example, Schistosoma eggs or hepatitis B virus (HBV)) or lungs (for
example, respiratory syncitial virus (RSV) or Mycobacterium tuberculosis)
on white spot formation in response to migrating larvae. Second, we will
discuss possible effects of diarrheal diseases and co-infecting GI helminth
infections on adult Ascaris nematodes in the gut.
Larval Migration
After migrating through the lungs, Ascaris larve migrate up the trachea
and are re-swallowed allowing re-entry into the GI tract. Migration of
Ascaris larvae may be a beneficial life history trait offering a window
without the harsh environment of the GI tract to maximize growth
leading to adult nematodes with larger body sizes and greater fecundity
than non-migrating counterparts.
67
However, Ascaris larval migration is
not necessarily benign with respect to host health. Mice infected with
Ascaris lose body weight
68
suggesting that this phase of the life-cycle can
be detrimental to the host. White spot formation in the liver of pigs
experimentally infected with A. suum is associated with resistance to the
establishment of Ascaris infection.
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Prevention of Ascaris migration via
white spot formations in turn prevents establishment of infection with
adult nematodes in the gut. Since white spots are generated by an
immune response triggered by larval migration, it is logical to hypothe-
size that pre-existing immune responses generated by co-infecting path-
ogens, particularly in the liver, may alter the formation of white spots, in
turn impacting on the successful migration of Ascaris larvae and the
establishment of Ascaris infection in a co-infected host.
White spot formation in A. suum infections of pigs begins with infil-
tration of eosinophils, as well as neutrophils,
25
suggesting that defense
against migrating Ascaris larvae contains a strong Th2 element. Schisto-
somes are trematode parasites that reside in the portal vein as paired
adult parasites. Some of these eggs, shed upon mating, do not success-
fully exit the body for excretion into the environment, but are swept by the
bloodstream into the liver where they elicit a strong Th2 response
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by
virtue of the omega oils present on the egg surface.
70,71
The infiltration of
immune cells to contain the egg leads to the formation of granulomas
(
Figure 4.4
A) and heavy egg deposition is often correlated with hepato-
megaly. From the perspective of migrating Ascaris larvae, the challenge to
successfully migrate back to the GI tract could be made significantly more
challenging in a host with Schistosome granulomas present in the liver. It
is possible that mature white spots form more rapidly in the context of
a pre-existing Th2 environment.
In the lungs, infection with RSV is common in young children causing
wheezing, bronchiolitis, hypoxia, and airway dysfunction in a subset of
