Biology Reference
In-Depth Information
Detailed genetic analysis based on microsatellite loci identified 4 and
7% of worms (n
129) from China and Guatemala, respectively, as being
hybrids.
52
A similar study of 258 worms from China found evidence of
hybrids and also 19 examples of “pig” worms infecting humans and one
example of a “human” worm infecting a pig.
46
Together these results
suggest that cross-infections can occur between the pig and human worm
populations and that fertile offspring are produced (hybrids). In addition,
cross-infection and cases of hybrids seem to be a more frequent event in
humans than in pigs. A possible explanation for the difference between
these data and previous findings is that earlier studies may have lacked
the power to detect hybridization and transmission between hosts due to
the use of only a few markers.
In countries where Ascaris infections are rare among humans,
molecular evidence (together with epidemiological findings) suggests
that the vast majority of infections result from cross-infections from
pigs. This has been demonstrated for 10 worms from nine patients in
North America, for 32 worms found in humans in Denmark and for 11
worms from patients in the UK.
32,35,72
Interestingly, in Japan it was
found that humans were infected with both “pig” and “human”
worms.
41,45
Two out of nine patients infected with “human-like” worms
had a history of travel to Ascaris-endemic areas but the others did not,
suggesting that a human
¼
human transmission cycle may still exist for
Ascaris in Japan, or that ingestion of contaminated imported foods is
taking place.
e
GLOB
AL AND LOCAL GENETIC DIVE
RSITY
Increased sampling of Ascaris worms from a variety of global locations
has uncovered greater levels of genetic diversity in this parasite. For
example, at least 50 cox1 and 35 nad1 haplotypes have been identified to
date.
42,43,45,47,71
Recent work from Brazil has shown that ITS-1, which
is present in around 40 copies in the Ascaris nuclear genome,
73
shows
intra-individual variability and 21 genotypes/haplotypes have been
identified.
44
As well as partitioning between host species, a number of studies have
investigated whether there is evidence for geographical genetic struc-
turing on a macro- and micro-scale. Anderson et al. compared samples
from pigs in Guatemala, the Philippines, Switzerland, Scotland, and Peru
and humans in Guatemala, Bangladesh, Madagascar, and Peru and found
that geographical differences explained 17% of genetic variation
observed.
34
Differences in mitochondrial haplotype frequencies were
observed in Ascaris fromChina and worms fromGuatemala,
69
and Ascaris
from humans in Bangladesh and Nepal fell into separate genetic clusters
