Biology Reference
In-Depth Information
INTRODUCTION
Infections with the parasitic large roundworm give rise to ascariasis,
which is a particularly common disease throughout the world.
1,2
Initially
described by Linnaeus in 1758, Ascaris lumbricoides has a cosmopolitan
distribution being found in both temperate and tropical regions, espe-
cially where there is adequate moisture matched with inadequate sani-
tation and hygiene. The closely-related A. suum is present in pigs
worldwide, though its distribution is strongly influenced by local farming
practices and animal husbandry.
3
As Ascaris worms do not directly
multiply within their host, to complete its life-cycle the parasite requires
the ingestion of fertile eggs produced by adult females within the intes-
tinal lumen and passed out in the host feces. In the environment the
parasite eggs mature and become infective. Upon entry into the host by
the oral route, eggs hatch within the duodenum releasing the third stage
rhabditiform larvae which penetrate across the intestinal mucosa even-
tually reaching the liver and then the lungs. Larvae then ascend the
trachea to be swallowed and pass down the esophagus into the ileum
where they molt twice and further mature and mate, typically resid-
ing there for 1
2 years unless expelled by host immune response or
de-worming medications.
1
e
EVOLUTIONARY HISTORY OF ASCARIDOIDEA
AND HOST POTENTIAL
With more than 50 described genera, nematode worms within the
superfamily Ascaridoidea are ubiquitous helminth parasites being found
in a variety of mammals inclusive of farmed livestock (e.g. Ascaris suum in
pigs and Toxocara vitulorum in cattle), a variety of domesticated and wild
mammals (e.g. Parascaris equorum in horses and Baylisascaris procyonis in
raccoons), as well as in man (e.g. Ascaris lumbricoides), notwithstanding
occasional infections in non-human primates such as chimpanzees (see
Table 10.1
). Attempts to bring taxonomic and evolutionary systematic
order to classification of these nematodes has not always been successful,
as morphologically distinctive characters are often lacking and those used
are frequently judged to be homoplastic (i.e. misleading) by character
congruence testing, in which the distributions of shared character states
are compared among taxa.
4
The use of molecular phylogenetics, targeting
nuclear ribosomal 18S and 28S regions augmented with the mitochondrial
ribosomal 12S or cox2, has attempted to provide systematic order
5
and is
summarized in schematic form in
Figure 10.1
.
4,6,7
The use of a combina-
tion of nuclear and mitochondrial markers yielded trees with higher
