Biology Reference
In-Depth Information
INTRODUCTION
The large roundworms, Ascaris lumbricoides and A. suum, are ubiqui-
tous pathogens of humans and pigs respectively with worldwide distri-
butions. A. suum and A. lumbricoides are highly related genetically and
are able to cross-infect pigs and humans. Although it has been suggested
that they represent the same species,
1
this remains an area of controversy
2
(see Chapter 10).
Both A. suum in pigs and A. lumbricoides in humans have similar life-
cycles in which the mammalian host is infected orally with infective ova.
Larvae hatch in the small intestine and migrate to the caecum and
proximal colon where they penetrate the mucosa,
3
and migrate to the liver
through the portal system. The larvae reach the lungs via the systemic
circulation where they embolize in the pulmonary capillaries, penetrate
the alveoli, and are coughed up and then swallowed reaching the small
intestine about 2 weeks following infection.
4
Roundworms reach sexual
maturity in the small intestine where female and male adults mate.
Fecund females release hundreds of thousands of eggs daily into the host
feces.
In pigs, the majority of A. suum larvae are expelled between 2 and 3
weeks after infection
5
and among those that survive and develop into
adults, most are expelled from the pig intestine within 6 months.
6
In the
case of A. lumbricoides in humans there are no published data to our
knowledge on the survival of larvae and adults although adult A. lum-
bricoides may survive 1 to 2 years in the human gut.
7
Host susceptibility and genetics play an important role in the distri-
bution of parasites within human and pig populations characterized
by an aggregated distribution of parasites in which few individuals
harbor the majority of parasites while most have few or no parasites
6,8
(see Chapter 7). Resistance to infection and the acquisition of resistance
upon exposure are determined largely by the host innate and adaptive
immune response. Recent research, particularly of A. suum in the pig, has
identified specific immune parameters associated with the development
of protective immunity that may be relevant also to our understanding of
immunity to A. lumbricoides in humans. The long-term survival of para-
sites in the host depends on the modulation of the host's immune
response by the parasite. Understanding the mechanisms by which
chronic infections modulate the immune response will allow us to
understand better immune modulation and homeostasis in mucosal and
other tissues.
9,10
It has been suggested that the relative absence of chronic
parasitic infections such as A. lumbricoides and their immune modulatory
effects may have a role in the emerging epidemic of inflammatory
diseases such as autoimmune and allergic diseases in populations living
