Biology Reference
In-Depth Information
have since been applied in a number of studies investigating population
genetics.
42,43,46,52,53
At the present time, PCR amplification of nuclear and
mitochondrial markers followed by direct sequencing and microsatellite
analysis are the most common methods used for detecting genetic vari-
ation in Ascaris. In addition, complete mitochondrial genome sequencing
is now becoming a realistic means of comparing the genetic diversity of
different Ascaris isolates.
54,55
EXPERIMENTAL STUDIES
A classical way to explore host affiliation and define species is to
conduct experimental infection studies and these have been used to
illuminate the relationship between A. suum and A. lumbricoides and their
host specificity. In this way Takata infected 17 volunteers with A. suum
and found egg excretion among one-third of the participants suggesting
that pig Ascaris can establish and mature in the human host.
56
However
egg excretion was found in all of seven other humans infected with
A. lumbricoides suggesting differences in host preference. Differences
related to quality and infectivity of eggs can, however, not be ruled out.
Likewise, Galvin infected pigs with A. suum and A. lumbricoides and was
also able to demonstrate that cross-infection can occur,
57
but there was
also clear evidence of host preference both with respect to number of egg-
excreting individuals and number of worms in the intestine of the pigs. In
this study, more larvae of A. lumbricoides were isolated from the lungs and
small intestine of rabbits whereas the opposite was the case for A. suum in
pigs suggesting that differences in establishment rate in pigs cannot
purely be attributed to egg quality.
57
Numbers of larvae isolated from the
lungs of mice 7 days post-infection were also higher when infected with
genotype G1 (predominantly found in humans) compared with genotype
G3 (predominantly found in pigs).
38
In contrast, while genotype G3
established well in pigs, none of the pigs infected with eggs of genotype
G1 were found to excrete eggs and only one immature worm was found
among 47 infected pigs. However, one caveat to this study is that as eggs
from only two female worms of each genotype were included in the study,
the differences may purely reflect worm to worm variations in estab-
lishment rate.
58
Several case reports on accidental infection of humans after handling
infective A. suum eggs or after contact with pig manure also suggest that
cross-infections can take place.
59
e
62
The broader host spectrum is further
supported by the finding of zoo chimpanzees infected with A. suum
36
and
the evidence would seem to suggest that a permanent transmission cycle
has been established.
40
However, the fact that the viability of excreted
eggs seems to be reduced suggests that chimpanzees are not an ideal host
