Biology Reference
In-Depth Information
mosquitoes incapable of transmitting dengue (
Blagrove et al. 2012
).
Lu et al. (2012)
compared the density
of three types of
Wolbachia
in
Ae. aegypti
and
Ae. albopictus
and found the
Wolbachia
titer is too low in
Ae.
albopictus
to induce resistance to dengue, perhaps because
albopictus
is naturally infected with two strains of
Wolbachia
and has developed mechanisms to suppress replication of
Wolbachia
.
The use of novel
Wolbachia
strains could potentially be useful in control of malaria transmission.
Jin
et al. (2009)
demonstrated that the
w
MelPop strain of
Wolbachia
establishes in the malaria vector
Anopheles
gambiae
, although it was not virulent to the mosquito.
Hughes et al. (2011)
injected
An. gambiae
with the
w
MelPop and
w
AlbB strains of
Wolbachia
and found both infections to be virulent and to inhibit
Plasmodium
infections. The
w
MelPop strain is unusual in that it is virulent to blood-fed females.
Kambris et al. (2009)
found that
w
MelPop up-regulates the immune system of
Ae. aegypti
and inhibits the development of filarial
nematodes, so could be of potential use in the effort to control lymphatic filariasis.
Is the use of
Wolbachia
infection to control disease transmission likely to succeed? It is not clear, and
Sabesan and Jambulingam (2012)
discussed some of the questions and possible limitations to this
approach. They noted, “it is not known for how long and to what extent these exercises are required to be
implemented to achieve the desired target (infection in wild population) because the vertically inherited
parasites such as
Wolbachia
are predicted to evolve towards reduced virulence over time.” They also were
concerned that maintaining the
Wolbachia
infection in field populations, “where the environment governs
the balance and is beyond human control” could be difficult, especially because, “With the increases
in temperature that have been widely experienced in many areas, viral pathogens such as dengue could
easily reach its infective titre in a shorter duration in the field by simple multiplication.” They conclude,
“At the moment, we do not have much to claim to success in our war against mosquito vectors, except in
situations where there has been vector habitat destruction
…
The mosquito vector host(s) and parasites
and/or endosymbionts will try to adapt themselves to maintain a balance in nature.”
Koehncke et al. (2009)
note that
Wolbachia
rarely cospeciates with their hosts, indicating that infections
are lost in host species over time. They modeled the spread of potential mutants that would repress
Wolbachia
action either by affecting bacterial transmission or the level of cytoplasmic incompatibility and show that
host mutations spread, even at a cost to host males, ultimately leading to loss of
Wolbachia
infections.
They concluded that
Wolbachia
must move horizontally to survive.
Vavre and Charlat (2012)
suggest future
modeling and research is necessary to resolve some of these important issues. There is a possibility that
mosquitoes, or the pathogens, will evolve adaptations so that the
Wolbachia
no longer functions as expected.
Enserink (2010a)
quoted O'Neill as responding to this concern, “With some luck, that will take at least a
couple of decades ... during which time humans may have developed vaccines or another way to thwart their
enemies.” Thus, this method of pest control, like the use of chemical pesticides, is perceived to be potentially
subject to selection for resistance in the hosts (or to reduced effects on the host by the
Wolbachia
).
What if something goes wrong? Pesticide applications could reduce the mosquito populations. In
addition,
Dobson (2003)
evaluated whether it would be possible to reverse
Wolbachia
-based population
replacement in case of unexpected consequences. He concluded, based on models, that it would be
possible to slow, stop, or reverse the spread of a released strain of
Wolbachia
if a strain was released that
contained a
Wolbachia
type that was bidirectionally incompatible with the
Wolbachia
driver strain. It would
also be possible to reverse spread of an undesirable
Wolbachia
strain by unidirectional incompatibility,
if a superinfected strain were released. The effectiveness of these approaches depends on the degree of
cytoplasmic incompatibility, the rate of transmission of the symbiont to progeny (it is not always 100%),
and the release rates. The effect of the
Wolbachia
strain on fecundity of the host females, and the migration
rate of the host population are relevant.
Rasgon (2008)
also evaluated models to optimize the use of
Wolbachia
in managing vector-borne diseases.
This case study demonstrates that regulatory protocols are limited with regard to permanent releases
of genetically modified arthropods into the environment.
Marshall (2011)
discusses the issues related to
