Mosquitoes, Zika Virus, and Transmission Ecology

Mosquitoes, Zika Virus, and Transmission Ecology

Updated April 18, 2016
Mosquitoes, Zika Virus, and Transmission Ecology
The Zika virus is transmitted primarily by mosquitoes, and
Controlling Mosquito Numbers
the Centers for Disease Control and Prevention (CDC) have
Control methods for the two species are varied and include
identified some cases of human-to-human transmittal. The
elimination of breeding sites and pesticide applications, as
two species of mosquitoes most commonly associated with
well as screening and well-sealed homes. Issues include
transmitting the virus are the yellow fever mosquito (Aedes
efficacy, cost, and human or environmental health.
aegypti, or YFM) and Asian tiger mosquito (Aedes
albopictus, or ATM). This report will focus on the ecology
Figure 1. Approximate U.S. Distribution of Zika
of the two mosquito species and briefly discuss methods
Virus Vectors, 2016
used or proposed either to control their populations or to
limit their ability to transmit viruses. Control methods could
require compliance with a variety of federal or state statutes
and regulations; such statutes generally have special
provisions regarding human health and safety. For a
discussion of human health issues, see CRS Report R44368,
Zika Virus: Basics About the Disease, and CRS Insight
IN10433, Zika Virus: Global Health Considerations.
Mosquito Ecology
Neither mosquito is native to the Western Hemisphere.
YFM is native to Africa, and ATM is from Southeast Asia.
Only females bite; blood proteins are then used in egg
development. After a blood meal from an infected person, a
virus may be transmitted to a person bitten later. Both
mosquitoes are vectors for other human viruses, including
dengue fever, chikungunya, and yellow fever. As a result of
their association with these major and established diseases,
considerable research on their ecology has been performed.
Both species are strongly associated with human habitat,
particularly YFM, which is a weaker flyer and rarely travels
more than a few hundred yards in its lifetime. However,
both are hitchhikers and may spread via cars, planes, and
boats.
Both species are already found in suitable habitat along the
U.S. Gulf Coast, and both occur sporadically in parts of
California, Arizona, and New Mexico. However, ATM has
adapted to cooler temperatures, allowing it to spread into
higher elevations and farther north. Its range currently
extends north into Pennsylvania and west to parts of Iowa
and Nebraska.

Source: Centers for Disease Control and Prevention, National
Both species breed in small containers of fresh water.
Center for Emerging and Zoonotic Infectious Diseases, “Surveillance
Breeding sites may include roadside rubbish, discarded
and Control of Aedes aegypti and Aedes albopictus in the United
tires, flower pots, gutters, potholes, and even bottle caps.
States,” at http://www.cdc.gov/chikungunya/resources/vector-
The larvae of both are aquatic, and at the larval stage their
control.html.
predators include other insects. When they emerge from the
larval stage, predators include dragonflies, birds, and bats.
Reducing the Sources: Breeding Sites
Both species are primarily diurnal (i.e., active in daytime).
Both avoid biting humans who have recently used various
Reduction of breeding sites as a mosquito-control method
was most famously tested in 1904, not long after the
registered repellants.
discovery of the link between mosquitoes and disease
transmission, with the building of the Panama Canal. It
continues to be used, especially in the South. The method is
still effective but requires broad public participation and
education for success. Public funding for cleanup of
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Mosquitoes, Zika Virus, and Transmission Ecology
roadside litter, destruction of old tires, and similar measures
In 2002, scientists at the British firm Oxitec genetically
may be required.
modified some YFM so that larvae die unless they are
exposed to the antibiotic tetracycline. When males are
Mosquitoes and Registered Pesticides
released into the wild, where tetracycline is generally
absent, males mate with wild females to produce larvae that
If habitat elimination or modification and other abatement
die before they reach adulthood. These genetically modified
approaches are not feasible or successful, requisite control
mosquitoes were field tested by the company in 2009-2011
strategies may require communities to rely on pesticides,
in Grand Cayman Islands, Malaysia, and Brazil. Oxitec
particularly those used at the aquatic, larval stage. The U.S.
reported that YFM populations were significantly reduced
Environmental Protection Agency (EPA) has registered a
within the test areas. Similar plans to field test these
number of pesticides under the Federal Insecticide,
engineered mosquitoes in the Florida Keys in 2012 were
Fungicide, and Rodenticide Act (FIFRA) specifically for
halted after environmental, regulatory, and ethical issues
the domestic control of mosquitoes. (Some additional
were raised. The U.S. Food and Drug Administration’s
unregistered pesticides manufactured in the United States
Center for Veterinary Medicine is currently reviewing the
may be exported and used abroad but not domestically.)
potential environmental and human health effects of a U.S
CDC guidelines recommend that decisions to use pesticides
field test of the mosquitoes under its Investigational New
to control mosquitoes should be based on surveillance data
Animal Drug protocol.
and the risk of human disease. Registered pesticides include
those to control mosquitoes at the larval stage (larvicides)
Mosquito Bacteria to Suppress Virus
in the breeding habitat before they can mature, ranging
from bacterial insecticides, growth inhibitors, and
Another approach focuses primarily on reducing
organophosphate insecticides, to mineral oils and
mosquitoes’ ability to transmit viruses. Researchers in
monomolecular films. There are also registered pesticides
Australia infected YFM with a bacterium called Wolbachia,
for controlling adult mosquitoes (adulticides), including
which naturally infects many other insect species, though
synthetic pyrethroid insecticides and organophosphate
not YFM. The bacterium appears to limit the multiplication
insecticides. A more detailed list and discussion of
of the dengue virus inside the mosquito, thus limiting the
registered pesticides are available from the National
chance of transmitting dengue to the next victim. The
Pesticide Information Center (http://npic.orst.edu/pest/
bacterium is too big to pass through a mosquito’s
mosquito/mosqcides.html) and from the EPA
mouthparts into a human, but infected females pass it via
(http://www.epa.gov/mosquitocontrol).
their eggs into their offspring.
Further, an exemption process exists for circumstances in
When Wolbachia-infected males mate with uninfected
which no efficacious pesticide is currently registered. Under
females, their offspring fail to hatch. But because infected
Section 18 of FIFRA (7 U.S.C. 136p; regulations in 40
females pass the bacterium on to their offspring, the
C.F.R., Part 166), state or federal agencies may request an
bacterium eventually spreads through the entire population,
Emergency Public Health exemption to use a non-registered
both in the laboratory and in certain field trials. Field trials
pesticide when they deem that a pest will cause a significant
in Australia appeared to be successful in ending chronic
risk to human health. Emergency Public Health exemptions
dengue fever outbreaks. Further trials are continuing in
must be approved by EPA and may be authorized for up to
Southeast Asia, and one trial began in Brazil in 2015.
one year. An Emergency Public Health exemption was
Because the dengue, chikungunya, and Zika viruses are
granted for the control for ticks carrying Lyme disease, for
similar (all RNA viruses), Wolbachia has been suggested as
example.
a potentially useful tool against Zika as well.
Preventing Transmission of the Virus
Conclusion
Alternatives to attempting to kill substantial numbers of
No single method of controlling YFM or ATM seems likely
mosquitoes directly include attempting to reduce the ability
to be effective in completely eliminating transmission of
of the mosquitoes to transmit viruses or to reduce the
Zika virus, nor in extirpating either mosquito species from
number of viable offspring. Neither method has been fully
the Western Hemisphere. Surveillance for the presence of
tested in the field with respect to Zika virus.
YFM and ATM and public cooperation in reducing
breeding sites are prerequisites for local policy choices.
Release of Genetically Modified Mosquitoes
Multiple tools are available or being tested; in conjunction
they may lessen or break transmission of the Zika virus
Genetic engineering has been used to create mosquitoes that
through mosquitoes.
either are not able to carry the disease-causing pathogens or
are unable to produce viable offspring. For example,
researchers are genetically engineering mosquitoes to make
M. Lynne Corn, Specialist in Natural Resources Policy
them more resistant to the Plasmodium parasite that causes
Tadlock Cowan, Analyst in Natural Resources and Rural
malaria. Other researchers are developing mosquitoes to
Development
express antimalarial peptides and enzymes that inhibit
Robert Esworthy, Specialist in Environmental Policy
parasite development. These varieties have not been field
IF10353
tested and hence are not approved.

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Mosquitoes, Zika Virus, and Transmission Ecology

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