Biology Reference
In-Depth Information
human and pig hosts in sympatry.
22
These results were in accordance
with previous mtDNA-based studies.
16
e
18
Moreover, the multilocus
genotype data enabled the identification of hybrid worms (4% in
Guatemala and 7% in China).
22
The finding of hybrids necessarily implies
that there was cross-transmission between human and pig hosts because
a worm of pig origin and of human origin had to meet in the same host in
order to mate. This cross-transmission and interbreeding had to be recent
as the methods employed can only detect hybrids going back two
generations.
26
Zhou and colleagues
28
used the same methods to ascertain
the frequency of cross-transmission across six provinces in China. They
observed similar results and identified both first generation migrants
(~7% of sampled worms) and hybrid worms (also ~7%), both of which
were predominantly collected from human hosts. Notably, the authors
state “The results strongly suggest pig
Ascaris
as an important source of
human ascariasis in endemic area where both human and pig
Ascaris
exist. In consideration of current control measures for human ascariasis
targeting only infected people, it is urgently needed to revise current
control measures by adding a simultaneous treatment to infected pigs in
the sympatric endemics”.
28
With these newmolecular tools at hand, it will
be prudent to perform additional studies from sympatric populations to
determine if limited cross-transmission is a global theme especially in
relation to different pig-raising, cultural, or economic conditions. It will
also be of interest to see if cross-transmission continues to show a largely
pig to human pattern and to explore the mechanisms that generate the
genetic differentiation between the host-associated populations despite
the high frequencies of cross transmission.
7
Aside from the direct inference of cross-transmission, what is the
epidemiological significance of limited cross-transmission and intro-
gression? Criscione and colleagues
22
highlight two critical aspects. First,
while there is significant genetic differentiation between
Ascaris
pop-
ulations in humans and pigs, the long-term ability to cross-transmit
between host species remains possible. Thus, even in non-endemic
sites, human infection via a pig source remains possible (as evidenced
by several studies
19
e
21
). Also, this ability may have led to a complex
evolutionary history of multiple host colonization events.
7
Second,
hybridization can lead to introgression of adaptive genes
29
and
hybridization itself may produce new combinations of parasite geno-
types that increase parasite virulence or host range via host immune
evasion.
30
Little attention has been given to these aspects of
Ascaris
epidemiology. Because parasite hybridization is of long-term epidemi-
ological significance in terms of the evolution of novel host infectivity
genes or drug-resistant genes, it will be critical to begin mapping regions
of genomic introgression in relation to host species infectivity patterns
in
Ascaris
.
