CHIKUNGUNYA (39): MOSQUITO VECTORS
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A ProMED-mail post
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http://www.promedmail.org>
ProMED-mail is a program of the
International Society for Infectious Diseases
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Date: Wed 10 Sep 2008
From: Ernest Gould <
eag@ceh.ac.uk> [Edited]
There have been many reports during the past months of the continuous
spread of chikungunya virus (CHIKV). Compared with past experience,
this is totally atypical and has recently been largely explained in
the publication of 2 papers: De Lamballerie (2008) Virology Journal,
5. 33 doi: 10.1186/1743-422X-5-33. and Tsetsarkin et al PLoS Pathog.
2007;12:e201. doi: 10.1371/journal.ppat.0030201, which have not been
cited by ProMED moderators [Genetic adaptation of CHIKV to _Aedes
albopictus_ was reported in the ProMED post of 9 Dec 2007 archive no.
20071209.3973, cited below. - Mod.TY].
One feature of the current CHIKV is that a single mutation in the
envelope protein has adapted this virus to _Aedes albopictus_, the
so-called tiger mosquito. As is well known, this mosquito has not
only been extremely successful in its spread throughout the tropics
and sub-tropics, but it has a wider distribution (i.e. urban,
semi-urban, rural) than _Aedes aegypti_, which appears to favor urban
environments. Thus, the selection of the variant of CHIKV by _Aedes
albopictus_ is almost certainly the major factor that has determined
why this virus has been so much more successful in causing epidemics
than previous CHIKV strains. Almost certainly, this success of CHIKV
is the result of a combination of additional factors that include
increased movement of _Aedes albopictus_ via ships carrying scrap
tires and other commercial products that provide a suitable
environment for these mosquitoes, increased travel by humans
incubating CHIKV, increased population densities of the tiger
mosquito, increased opportunities for _Aedes albopictus_ to transmit
the virus in urban regions as the result of the increased number of
cases of CHIKV due to _Aedes albopictus_, and other factors, possibly
including climate change.
One of the obvious implications of this adaptation to _Aedes
albopictus_ is that CHIKV could continue to disperse and eventually
reach the New World, where the appropriate mosquitoes will be waiting
(the virus has already proven it can establish in northern Italy)!
Thinking more laterally, should a similar adaptation (possibly by
selection of variants from quasi-species populations) take place in
dengue virus, yellow fever virus (or other arboviruses), the
consequences could be even more serious.
Clearly, the need for effective vaccines and antivirals has never
been greater, but I have never really understood why we cannot
reproduce the mosquito control activities that worked so well to
reduce yellow fever virus and presumably dengue virus in the Americas
all those years ago. Countries such as Singapore, Cuba, New Zealand,
and Australia effectively reduce the risk to their human populations
by concerted mosquito control strategies. We do not have to develop
new concepts to control mosquito populations; we need to organize our
infrastructures and possibly look for a safe alternative to DDT.
--
Professor E A Gould
Oxford
England
<
eag@ceh.ac.uk>
[ProMED is grateful to Professor Gould for emphasizing the importance
of adaptation of chikungunya virus to _Aedes albopictus_, with the
risk of wide geographic spread to areas where this mosquito has
become established. Unfortunately, many CHIKV outbreak reports that
ProMED receives do not indicate whether the mosquito vectors involved
are _Aedes albopictus_ or _Ae. aegypti_. Control of these mosquitoes
is extremely difficult and currently requires reduction of the water
catchments that are the breeding sites for these vectors. That
requires active public participation, as the task is too vast for
government agency staff alone. Even well organized efforts, such as
those in Singapore, have been unable to halt completely current CHIKV
transmission. Vector control relying mainly on insecticide
application has limitations as well. Fogging to kill adults provides
only short-term vector control. Larvicides can work well but must be
applied at the household level, and there may be public resistance to
their use in drinking and bathing water containers. Larvicide control
will be more difficult for _Ae. albopictus_, which can breed outside
houses and other buildings in discarded containers and also in more
natural sites, such as rock holes and bamboo stumps, making breeding
sites more difficult to find.
Professor Gould points out the importance of _Ae. albopictus_ as a
vector of other important viruses such as dengue. As an example of
the threat that _Ae. albopictus_ poses as a vector of other viruses,
the USA CDC reports that: "Since the discovery of _Ae. albopictus_ in
the United States, 5 arboviruses (eastern equine encephalomyelitis,
keystone, Tensaw, Cache Valley, and Potosi) have been isolated from
this mosquito. Of these 5 viruses, only eastern equine
encephalomyelitis and Cache Valley viruses are known to cause disease
in humans (see
<
http://www.cdc.gov/ncidod/dvbid/Arbor/albopic_new.htm>).
More recently, LaCrosse virus, an additional virus of public health
importance in North America, has been isolated from _Ae. albopictus_
in Tennessee. - Mod.TY]
