Biology Reference
In-Depth Information
nutritional similarities between humans and pigs, as well as the economic
considerations of Ascaris infections in livestock, has prompted use of the
pig as a model for A. lumbricoides infections in humans (see Chapters 14
and 16).
27,28
Morbidity and Mortality due to Ascariasis
The pathology associated with A. lumbricoides infection is predomi-
nantly chronic, mainly related to malnutrition caused by perpetual
infestation with adult worms, but severe acute effects may result from
particularly heavy infections.
13,21
The chronic effects of infection are of
much greater public health significance, as is the case for STHs
in general
29
and indeed for many other helminth parasites of humans.
4
Malnutrition is common to all STH infections
29
although particular
species are associated with specific nutrient deficiencies. For example,
hookworm is strongly associated with anemia caused by iron deficiency
because hookworms suck blood from the intestinal mucosa and secrete
anticoagulants to prevent clotting.
30
By contrast, A. lumbricoides impairs
the absorption of fats, vitamin A and iodine and the digestion of lactose by
reducing the appetite of infected individuals and damaging their intes-
tinal villi.
31
The effects of STH-associated malnutrition are most
pronounced in children and are broadly similar:
impaired growth,
physical ability, work capacity and cognition.
13,29
Larval migration can lead to acute but short-lived pulmonary ascari-
asis or the more severe pneumonitis, both of which are rare, although
their public health significance is not fully understood (Chapter 5).
13
Acute disease caused by adult worms is also uncommon,
32
but tends to be
associated with complications arising from a large number of worms
(worm burden) harbored by the host. The most common causes of such
morbidity are intestinal or bile duct obstruction but many other compli-
cations have been documented.
21
Mortality caused by either larval or
adult A. lumbricoides is exceptional
21,32
and, in general, acute pathology is
of much less public health significance than chronic morbidity.
13
Treatment
A. lumbricoides is extremely susceptible to a single dose of a wide range
of anthelmintic drugs, but particularly to the bezimidazoles, which are
used in mass treatment programs aimed specifically at STHs and also as
part of combination therapy against lymphatic filariasis.
33
A. lumbricoides
is also susceptible to ivermectin, the mainstay of onchocerciasis (and
lymphatic filariasis in Africa) control.
34
The estimated range of efficacies
of the anthelmintic drugs against A. lumbricoides, T. trichiura and hook-
worms are given in
Table 7.1
.
