Wednesday, August 22, 2018

PRO/AH/EDR> Vector mosquito control methods

VECTOR MOSQUITO CONTROL METHODS
*******************************
A ProMED-mail post
<http://www.promedmail.org>
ProMED-mail is a program of the
International Society for Infectious Diseases
<http://www.isid.org>

Date: Mon 20 Aug 2018 11:30 BST
Source: The Conversation [edited]
<http://theconversation.com/genetically-modified-mosquitoes-may-be-best-weapon-for-curbing-disease-transmission-100719>


Mosquitoes are some of the most deadly creatures on the planet. They
carry viruses, bacteria, and parasites, which they transmit through
bites, infecting some 700 million people and killing more than 1
million each year.

With international travel, migration, and climate change, these
infections are no longer confined to tropical and subtropical
developing countries. Pathogens such as West Nile virus and Zika virus
have caused significant outbreaks in the United States and its
territories that are likely to continue, with new invasive pathogens
being discovered all the time. Currently, control of these diseases is
mostly limited to broad-spectrum insecticide sprays, which can harm
both humans and non-target animals and insects. What if there was a
way to control these devastating diseases without the environmental
problems of widespread insecticide use?

Genetically modifying mosquitoes to prevent disease may sound like
science fiction, but the technology has advanced in recent years to
the point where this is no longer a scenario relegated to late-night
movies. In fact, it's not even a new idea; scientists were talking
about modifying insect populations to control diseases as early as the
1940s. Today, genetically modified (GM) mosquitoes, developed during
the past several decades of research in university laboratories, are
being used to combat mosquito-borne pathogens -- including viruses
such as dengue and Zika -- in many locations around the globe,
including the United States. Progress is also being made to use GM
mosquitoes to combat malaria, the most devastating mosquito-borne
disease, although field releases for malaria control have not yet
taken place.

I have been working on GM mosquitoes, both as a lab tool and to combat
disease, for over 20 years. During that time, I have personally
witnessed the technology go from theoretical, to seeing it used in the
field. I've seen older techniques that were inefficient, random, and
slow pave the way for new methods like CRISPR [clustered regularly
interspaced short palindromic repeats], which enable efficient, rapid,
and precise editing of mosquito genomes, and ReMOT Control which
eliminates the requirement for injecting materials into mosquito
embryos. These new technologies make GM mosquitoes for disease control
not a question of "if," but rather a question of "where" and "when."

Don't worry, these genetic changes only affect the mosquitoes -- they
are not transmitted to people when the mosquito bites them.

There are 2 alternative methods currently used to control
mosquito-borne diseases using GM mosquitoes. The 1st is "population
replacement" in which a mosquito population biologically able to
transmit pathogens is "replaced" by one that is unable to transmit
pathogens. This approach generally relies on a concept known as "gene
drive" to spread the anti-pathogen genes. In gene drive, a genetic
trait -- a gene or group of genes -- relies on a quirk on inheritance
to spread to more than half of a mosquito's offspring, boosting the
frequency of the trait in the population.

The 2nd approach is called "population suppression." This strategy
reduces mosquito populations so that there are fewer mosquitoes to
pass on the pathogen.

While the concept of gene drive in mosquitoes is many decades old, the
gene-editing technique CRISPR has finally made it possible to easily
engineer it in the laboratory. However, CRISPR-based gene drives have
not yet been deployed in nature, mostly because they are still a new
technology that lacks a firm international regulatory framework, but
also due to problems related to the evolution of resistance in
mosquito populations that will stop the gene from spreading.

It may not be immediately obvious, but the gene in "gene drive" need
not be a gene at all -- it can be a microbe. All organisms exist not
just with their own genomes, but also with the genomes of all their
associated microbes -- the "hologenome." Spread of a microbial genome
through a population by inheritance can also be thought of as gene
drive. By this definition, the 1st gene drive that has been deployed
in mosquito populations for disease control is a bacterial symbiont
known as _Wolbachia_. _Wolbachia_ is [the genus of] a bacterium that
infects up to 70 percent of all known insect species, where it hijacks
the insect reproduction to spread itself through the population.

Thus, the _Wolbachia_ itself (with its genome of approximately 1500
genes) acts as the genetic trait that is driven into the population.
When _Wolbachia_ is transferred into a previously uninfected mosquito,
it often makes the mosquito more resistant to infection with a
pathogen that can cause disease in humans, such as multiple viruses
(including dengue and Zika viruses) and malaria parasites.

In the last 8 years, researchers have taken _Wolbachia_ present in
fruit flies and transferred that bacteria into mosquitoes that
transmit dengue virus. Those modified insects were then released in a
dozen countries to control the disease. Although marketed as a "non-GM
strategy," artificially infecting mosquitoes with _Wolbachia_ clearly
falls under the GM umbrella, as over 1500 genes (the entire bacterial
genome) have been transferred from the original fruit fly host into
the mosquitoes.

Preliminary dengue control results from these releases in Australia
have been promising. However, control of the disease in other release
areas with higher disease risk, such as South America and Asia, still
needs to be determined, particularly as some studies have demonstrated
that _Wolbachia_ can sometimes increase pathogen infection in
mosquitoes rather than suppress it.

[Map -- available at the source URL above]: Estimated range of the
dengue and Zika virus carrying mosquito species in the United States,
_Aedes aegypti_, blue, and _Aedes albopictus, red. States and
territories where both species have been collected are purple. All US
states and territories except Alaska are at risk for West Nile
virus.]

The best current example of population suppression is the release of
genetically modified sterile mosquitoes. This is a modern spin on the
decades-old sterile insect technique (SIT), where sterile male insects
are released into natural populations to mate with the wild females,
reducing the mosquito population. But, rather than crudely sterilizing
mosquitoes with radiation or chemicals, clever genetic engineering is
now used to sterilize them instead. The company Oxitec has engineered
mosquitoes with a gene that is lethal to females but not to males,
which do not bite or transmit disease. Thousands of these transgenic
males are released into nature, where they mate with the wild females
in the population. The genetic modification is inherited by the
offspring of these matings; female offspring die, while male
offspring, which carry the gene, survive and continue passing the
trait to further generations. With fewer and fewer females the
mosquito population is drastically suppressed. Oxitec has conducted
releases in the Grand Caymans, Malaysia, Brazil, and Florida.

There has been some opposition to these sterile mosquito releases,
particularly in Florida. For example, in 2016, an Oxitec trial in the
Florida Keys was met with some local resistance. However, unlike gene
drive strategies, release of sterile mosquitoes (genetically modified
or not) has about the smallest environmental footprint and highest
safety of any disease control strategy; certainly safer than
broad-spectrum insecticide sprays. It is highly targeted, and thus if
it works, will only result in elimination of the target mosquito
species, which in this case (_Aedes aegypti_) is a highly invasive and
non-native mosquito in Florida.

In addition to gene drive, _Wolbachia_ bacteria have also been used
for population suppression. Males infected with the bacteria are
released into a mosquito population that is either not infected, or
infected with a different _Wolbachia_ strain, which leads to
"incompatible" or sterile matings. This strategy again has a long
history, and was first used to suppress mosquito populations in the
1960s before people even knew that _Wolbachia_ was causing certain
populations of mosquitoes to be sterile when mated with one another.
In current times, _Wolbachia_-sterilized males have been released in
multiple countries including Australia and the US, in California and
Florida, to control dengue virus.

In an increasingly interconnected world, and with the added problems
of global climate change, pathogens are not likely to stay confined to
the developing world, but will be an increasing issue for the US as
well. With the evolution of insecticide resistance in mosquitoes a
certainty, GM technology has the potential to reduce the burden of
mosquito-borne diseases across the globe, without the environmental
and health risks associated with harmful pesticide use.

Don't be afraid if it sounds like science fiction; it may just save
your life.

[Byline: Jason Rasgon]

--
Communicated by:
ProMED-mail from HealthMap Alerts
<promed@promedmail.org>

[This is a clearly written overview of contemporary approaches to
vector mosquito control. They are being tested in the field in various
localities and some of these trials look very promising. The future of
vector mosquito control looks bright. - Mod.TY]

[See Also:
Dengue/DHF update (15): Asia, Europe, Australia, vector, vaccine
http://promedmail.org/post/20180805.5947027
Zika virus (03): Americas, Asia, research, observations
http://promedmail.org/post/20180201.5600535]
.................................................lxl/ty/lxl/mj
*##########################################################*
************************************************************
ProMED-mail makes every effort to verify the reports that
are posted, but the accuracy and completeness of the
information, and of any statements or opinions based
thereon, are not guaranteed. The reader assumes all risks in
using information posted or archived by ProMED-mail. ISID
and its associated service providers shall not be held
responsible for errors or omissions or held liable for any
damages incurred as a result of use or reliance upon posted
or archived material.
************************************************************
Donate to ProMED-mail. Details available at:
<http://www.isid.org/donate/>
************************************************************
Visit ProMED-mail's web site at <http://www.promedmail.org>.
Send all items for posting to: promed@promedmail.org (NOT to
an individual moderator). If you do not give your full name
name and affiliation, it may not be posted. You may unsub-
scribe at <http://ww4.isid.org/promedmail/subscribe.php>.
For assistance from a human being, send mail to:
<postmaster@promedmail.org>.
############################################################
############################################################

List-Unsubscribe: http://ww4.isid.org/promedmail/subscribe.php

0 comments:

Post a Comment