Biology Reference
In-Depth Information
control and intervention strategies (see Chapter 15).
8
Research priorities
include: fundamental biology and transmission dynamics,
9
diagnostics
10
and mathematical modeling.
11
Biology and Life-Cycle
A. lumbricoides is the largest intestinal nematode to infect humans.
Females typically weigh 4
e
7 grams and measure 20
e
30 cm long. Males
are smaller, weighing 2
e
3 grams. Adult worms tend to inhabit the
jejunum
feeding on ingesta
and swimming against the flow to maintain their position. Reproduc-
tively mature females mate with mature males enabling production of
fertile eggs. Unfertilized mature females release infertile eggs which can
be distinguished from their fertile counterparts under microscopic
examination. Eggs are egested into the environment with the host's
feces.
12
Shaded moist soil and temperatures of 28
e
the middle section of the small intestine
e
32
C provide optimum
conditions for embryonation, under which L1 larvae will develop within
the egg approximately 10 days after release from the host.
12
This process
takes around 50 days at a less favorable 17
C.
13
Embryonation is followed
by the first molt and formation of an L2 larva a few days later, a process
that again occurs within the egg, giving rise to the infectious stage; an egg
containing an L3 larva covered by an L2 cuticle.
14,15
In this state, and
under favorable conditions, eggs are thought to be able to survive for up
to 15 years,
12,13
although their life expectancy is stated as 28
e
84 days
16
e
reflecting high variability among survival times.
Humans ingest infectious eggs through fecal contamination of soil,
foodstuffs, vegetable crops (for instance, fertilized with “night soil”) and
water supplies. Eggs hatch within the duodenum and larvae penetrate the
intestinal mucosa, migrate towards the liver, where the L2 cuticle is shed,
and from there pass to the lungs. This takes approximately 10 days during
which L3 larvae molt to L4 before returning to the small intestine via the
bronchi, trachea and esophagus. Here the final molt occurs and immature
adults are formed.
12,17
12 weeks for worms to reach repro-
ductive maturity, females maturing slightly faster than males.
18
The life
expectancy of an adult worm is thought to be 1
It takes 7
e
2 years.
19,20
There are debatably 13 or 16 known species of Ascaris which
exclusively infect mammalian hosts (see Chapter 10).
21
Ascaris suum
preferentially parasitizes the domestic pig but is very closely related to
A. lumbricoides. Indeed the two possibly represent strains of the same
species;
12,22
both cross-infection
22,23
and hybridization
24
have been
demonstrated in sympatric populations, although the former not consis-
tently across geographical locations.
25,26
This genetic closeness, combined
with the anatomical, physiological,
e
immunological, metabolic and
