Biology Reference
In-Depth Information
INTRODUCTION
The life-cycles of Ascaris lumbricoides,thehumanroundwormand
Ascaris suum, its counterpart in the pig, consist of three very important
elements. The adult male, female and immature worms that live in the
small intestine, the resistant egg stages that are produced by adult
worms and pass out into the environment via the host feces and the
larval parasites that emerge from the hatched eggs into the host
intestine and undergo a migratory pathway within the tissues of the
human or porcine host. Each of these phases of infection contributes to
the unique biology of the parasite, to its public health impact and to
a myriad of questions that arise in the mind of the researcher interested
in Ascaris.
However, it is clear that our understanding of the properties of the
adult worms and the resistant free-living egg stages of the parasite far
outweigh that of larval infection. The reasons for this are mainly due to
the fact that adult worms can be recovered from humans and pigs after
the administration of anthelmintic drugs that result in the expulsion of
worms from the intestine and eggs can be dissected from the uteri of adult
worms, detected in the feces using diagnostic procedures and recovered
from soil and pasture in the environment. Tissue-resident and migrating
larvae, however, undergo a more cryptic manifestation within the
parenteral tissues of humans and pigs and can only be recovered at
postmortem. This has resulted in a paucity of information on the public
health significance of larval ascariasis in humans due to ethical and
logistic constraints. Data on larval ascariasis have mainly been accrued
from animal model studies including the natural host of A. suum, the pig,
and a range of abnormal hosts including cattle, rabbits, and small rodents
such as guinea-pigs and mice (
Table 5.1
). This chapter will focus upon our
existing knowledge of larval ascariasis derived from model organisms
and the potential advantages of the use of a mouse model of resistance
and susceptibility to ascariasis.
THE HEPATO-TRACHEAL MIGRATION
IN ASCARIASIS
Ascaris is a parasite that exists not only as an adult worm in the
intestine but also has a migratory pathway, known as a hepato-trachael
migration,
1
that progresses from an infective egg (containing an L3
larva covered by an L2 cuticle)
2,3
hatching in the intestine, to a larva
migrating via the portal blood vessels to the liver. After migration within
the liver, and some growth, the larvae advance to the lungs, penetrate the
alveolar spaces and move to the pharynx where they are coughed up and
