Updated August 22, 2016
Genetically Engineered Mosquitoes: A Vector Control
Technology for Reducing Zika Virus Transmission
Background
yellow fever, dengue, and chikungunya. Aedes aegypti
In February 2016, the World Health Organization (WHO)
mosquitoes are non-native to the United States. A model
declared Zika virus a “public health emergency of
created by Toronto researchers found that approximately
international concern.” WHO defines such a public health
63% of the U.S. population lives in areas where Zika virus
emergency (1) to constitute a public health risk to other
might spread during seasonally warm months if mosquitoes
states through the international spread of disease, and (2) to
in the United States were to become vectors of Zika virus.
potentially require a coordinated international response.
As much as 7% of Americans live in areas where the cold
This definition implies a situation that is serious, unusual,
might not kill off the mosquito in the winter, leaving them
or unexpected; carries implications for public health beyond
vulnerable year round. (See CRS InFocus 10353,
the affected state’s national border; and may require
Mosquitoes, Zika Virus, and Transmission Ecology.)
immediate international action. (See CRS Insight IN10433,
Zika Virus: Global Health Considerations.)
No vaccine exists for Zika, and scientists have estimated
that it could take two years or more to develop such a
While only about one out of five persons infected with Zika
remedy. Mosquito control and bite prevention are the first
virus exhibit even the common symptoms of mild fever,
lines of defense. Controlling Aedes aegypti by conventional
rash, and joint pain, the U.S. Centers for Disease Control
methods such as truck and aerial spraying is only
and Prevention (CDC) have confirmed that Zika-infected
moderately effective in reducing mosquito populations––
pregnant women are at risk for delivering babies with
approximately 30%-50%––in part owing to the resistance
microcephaly, a birth defect of the cerebral cortex where a
the mosquitoes have developed to the more commonly used
baby’s head is smaller than expected when compared to
insecticides and to the limited area in which Aedes aegypti
babies of the same sex and age. While research is limited,
mosquitoes circulate (100-200 yards from where the larvae
pregnant women are considered at risk for delivering babies
emerge). Aedes aegypti mosquitoes also tend to favor house
with microcephaly no matter the stage of their pregnancy
interiors where spraying/fogging is not practical. Strategic
when they become infected with Zika virus. (See CRS
placement of several low-cost autocidal gravid ovitraps
Report R44368, Zika Virus: Basics About the Disease.)
(which mimic breeding sites) in house interiors can reduce
the Aedes aegypti population by about 50%.
Zika virus has now triggered outbreaks in 33 countries and
territories, although confirmed cases linking Zika virus to
Further contributing to the urgency of the pandemic, the El
babies with birth defects have thus far been seen in only
Niño weather phenomenon in 2015-2016 brought warmer
Brazil and French Polynesia. Several countries have also
temperatures and moisture to the regions most affected by
reported a spike in cases of Guillain-Barré syndrome, a
Zika virus, and with that weather pattern, a potential
neurological syndrome, also believed to be an effect of the
increase in the population of Aedes aegypti mosquitoes.
virus in some victims.
OX513A Genetically Engineered Mosquitoes
A Mosquito-Borne Virus
In this environment, the creation of a genetically engineered
Zika virus (so named for the Zika forest in Uganda, where it
(GE) Aedes aegypti mosquito by the British firm Oxitec in
was first identified in monkeys in 1947) is a mosquito-
2002, known as OX513A, is generating significant interest
borne flavivirus that has rapidly infected human populations
among public health officials. Developed originally to
in Latin America and the Caribbean, including outbreaks in
suppress the incidence of dengue fever, OX513A is now
the U.S. territories of Puerto Rico, the U.S. Virgin Islands,
regarded as a promising technology to reduce the incidence
and American Samoa. As of April 2016, over 400 cases in
of Zika virus transmission by reducing the population of
the United States have been confirmed, each acquired
mosquitoes. Oxitec is owned by Maryland-based Intrexon
through either travel to areas where the mosquito vectors
Corporation.
for Zika virus circulate or sexual contact with people who
had traveled to such areas. No confirmed cases of local
Oxitec’s OX513A are mosquitoes that have been
transmission have been confirmed in the contiguous United
genetically engineered with a dominant transgene that
States as yet.
produces a lethal protein that ties up the transcriptional
machinery in the cells. The gene is passed on to the
The first outbreak of Zika virus outside Africa, Asia, and
mosquito’s offspring so that they die before reaching
the Pacific Islands occurred in Brazil in May 2015. The
adulthood. Each OX513A mosquito is also engineered with
virus is spread predominantly by the female Aedes aegypti
a fluorescent marker that permits effective monitoring of
mosquito (and to a less effective extent by Aedes
larvae to assess the effectiveness of control. The fluorescent
albopictus), an aggressive day-biter that is also a vector for
marker is visible using a specialist microscope in all
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Genetically Engineered Mosquitoes: A Vector Control Technology for Reducing Zika Virus Transmission
OX5213A offspring. The OX513A male mosquitoes, which
0.01% females accidently released. This could lead to a
do not bit or spread the virus are reared in laboratories and
small but temporary increase in the number of biting
then released to mate with wild Aedes aegypti female
mosquitoes. A possible solution currently being explored by
mosquitoes.
Oxitec and the University of California-Irvine is a genetic
modification to make females unable to fly. If successful,
Since only the females bite, releasing millions of OX513A
this approach would make these 0.01% released females
males to mate with wild females, who would then produce
unable to mate or bite.
larvae that die, could reduce the population of Aedes
aegypti mosquitoes and, thereby, reduce the risk of Zika
U.S. Environmental Assessment of GE Mosquitoes
virus transmission to humans. This approach targets only
Oxitec applied for a permit to field test the OX513A
the Aedes aegypti mosquitoes that can spread disease,
mosquitoes in the Florida Keys in 2011. On April 3, 2012,
because the OX513A males produce offspring only with
the Key West City Commission passed a resolution
their own species.
objecting to the release of the OX513A mosquitoes. The
U.S. Food and Drug Administration Center for Veterinary
OX513A mosquitoes can be bred for generations and
Medicine (FDA-CVM) led an examination of OX513A
multiplied. Adult males with Oxitec’s lethal transgene
under its Investigational New Animal Drug regulatory
survive in the environment for only about a week. The
process. FDA’s review team was comprised of experts from
OX513A mosquitoes also have the advantage of repressing
the CVM, the CDC, and the U.S. Environmental Protection
populations of Aedes aegypti mosquitoes that carry
Agency. On March 11, 2016, FDA published its
insecticide resistance genes. According to peer-reviewed
Preliminary Finding of No Significant Impact (FONSI) for
studies, of the more than 150 million OX513A mosquitoes
proposed field testing the OX513A mosquito in the Florida
released to date in field trials, no effects on other species
Keys. After a 60-day comment period, FDA published its
have been observed, no evolution of resistance to the lethal
final EA and associated FONSI on August 5, 2016, which
transgene has been seen, and there has been no mating with
allows Oxitec to begin field trials.
non-target mosquitoes detected.
This review team examined Oxitec’s and independent
According to peer-reviewed studies, releases of OX513A
collaborators published evidence from their Brazil and
males in the Cayman Islands in 2010 led to 90%
Cayman Islands field trials and other data on safety studies.
suppression of the wild Aedes aegypti population. Isolated
FDA found that the probability that the release of OX513A
field demonstrations in Brazil have also achieved similarly
male mosquitoes would result in toxic or allergenic effects
successful results after six to nine months. In 2011, Oxitec
in humans or other animals is negligible. “Almost all of the
conducted a sustained series of OX513A field releases in
OX513A mosquitoes released for the investigational field
Itaberaba, a suburb of Juazeiro in the semi-arid northeast
trial will be male, and male mosquitoes do not bite humans
region of Brazil. Normal mosquito control continued during
or other animals. They are therefore not expected to have
the field study as public health agents continued to destroy
any direct impacts on human or animal health.” FDA also
breeding sites and treat homes with larvicide. According to
found that the “probability that the release or rearing of
peer-reviewed studies, the Aedes aegypti population was
OX513A mosquitoes would have adverse impacts on the
reduced by over 90% in a year based on data from multiple
ecosystem is largely negligible” and that the “probability of
locations.
OX513A mosquitoes and their progeny persisting and
Brazil’s National Biosafety Commission approved country-
establishing at the proposed trial site or spreading beyond
wide use of OX513A in 2014, making Brazil the first
its boundaries is extremely unlikely.”
country to approve the commercial use of the OX513A
With the FONSI, Oxitec plans to begin field testing in Key
mosquitoes. A year later, the OX513A mosquitoes were
Haven, Florida, in collaboration with the Florida Keys
released in the Brazilian city of Piracicaba and, in January
Mosquito Control District. However, the Florida Keys
2016, announced plans to scale up the program and expand
Environmental Coalition and others have petitioned the
their OX513A production capacity. Panama also field tested
Florida Commissioner of Agriculture and Consumer
the OX513A mosquitoes in 2014.
Services to halt any field testing of the OX513A
In April 2016, Brazil’s National Health Surveillance
mosquitoes in the state. As of August 2016, the Florida
Agency announced that it would grant Oxitec a temporary
Keys Mosquito Control Board has not approved the trial
registration to deploy the genetically engineered mosquitoes
release, instead putting it on the November ballot as a non-
throughout the country. The agency is now developing new
binding referendum. Oxitec has asked FDA to consider
rules to provide Brazil with a regulatory framework to
releasing the mosquitoes on an emergency basis elsewhere.
address the OX513A mosquitoes, as well as other
Congress has oversight of FDA regulations and
genetically engineered insects that may be developed in the
appropriations. Annual appropriations for the CDC are also
future. WHO has issued a positive recommendation in
under congressional authority. If the United States supports
support of the OX513A mosquitoes. In addition, the Pan-
stepped-up international efforts to reduce the incidence of
American Health Organization has also announced that it
Zika transmission, appropriations to the United States
will provide technical support for countries that wish to
Agency for International Development could play an
implement the OX513A mosquitoes.
important role.
Some researchers have raised questions about the OX513A
mosquitoes’ fitness for breeding, and whether the males
Tadlock Cowan, Analyst in Natural Resources and Rural
could evolve resistance to the lethal gene. Males are
Development
mechanically sorted in the laboratory, resulting in less than
IF10401
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Genetically Engineered Mosquitoes: A Vector Control Technology for Reducing Zika Virus Transmission


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https://crsreports.congress.gov | IF10401 · VERSION 5 · UPDATED

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