Biology Reference
In-Depth Information
for A. suum. Mature Ascaris thought to be of pig origin have also been
reported in the intestine and bile duct of sheep and cattle,
63,64
despite the
large differences in the alimentary tract between monogastrics and
ruminants. Even though A. lumbricoides and A. suum can be found in
heterologous hosts and cross-infections can occur between them and
despite concerns related to the design of the above-mentioned studies, it
seems safe to assume that Ascaris is most adapted to its “appropriate” host
suggesting that certain aspects of speciation have occurred between them.
INSIGHTS
INTO A. LUMBRICOIDES/SUUM
COMPLEX
The gross morphology of Ascaris worms from pigs and humans is
indistinguishable without exception, but subtle differences in denticle
morphology and lip shape between worms from the two hosts have
been described.
65
e
67
Biochemical studies revealed contrasting profiles
in lysates obtained from human and pig worms (both adults and
larvae).
25,26,68
Differences in the properties of trypsin inhibitors between
A. lumbricoides and A. suum were also identified.
27
However, the worms
compared in these studies were not sympatric, meaning that differences
could be due to geographical and/or intrinsic variability in the Ascaris
populations rather than due to host specificity. In addition, the fact that
worms were generally collected by different methods from humans and
pigs (chemo-expulsion versus collection in abattoirs) may also have
influenced protein profiles. Attempts to apply isoenzyme analysis to
determine variation between pig and human Ascaris from a variety of
locations were not very informative due to these same issues and/or the
low level of polymorphism at the loci examined.
28
e
31
Molecular analysis of Ascaris populations in areas where pigs and
humans live in close proximity (sympatric) has been carried out in
Guatemala, China, and Brazil. Anderson et al. concluded that Ascaris in
humans and pigs in Guatemala represented two different reproductive
populations with little gene flow between them,
31,34
although no single
diagnostic marker was found which could distinguish between pig and
human worms. Researchers in China came to an analogous conclusion,
based on application of similar molecular techniques to human and pig
worms collected from six provinces.
47,48,69
However, a recent reanalysis of
the Chinese mitochondrial data provided evidence for a high level of gene
flow between human and pig-derived Ascaris.
70
In addition, based on
fecal samples collected from humans and pigs in Brazil, shared mito-
chondrial haplotypes were found
71
and human worms from East Africa
had some haplotypes in common with pig worms from China,
42,43
sug-
gesting recent or contemporary gene flow between worms from the two
hosts or retention of ancestral polymorphisms.
