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Zika Virus and Vectors

Zika disease in humans is caused by infection with the Zika virus which is transmitted most often by the bite of an infected mosquito.

See also the EDEN Zika in Humans page.


Vector  Mosquitoes 

The mosquito species most likely to be involved in transmission in the United States and its territories are two “container mosquitoes", Aedes aegypti (the yellow fever mosquito) and Aedes albopictus (the Asian tiger mosquito). The two species can be distinguished by white markings on the scutum (thorax). A. aegypti has white markings in the shape of a lyre; A. albopictus has a prominent white stripe running the length of the scutum.

An estimated distribution of the occurrence of these two mosquitoes in the United States was provided by the US Centers for Disease Control (CDC). See Vector Distribution map below.

Container mosquitoes are also responsible for the transmission of chikungunya and dengue viruses to humans and dog heartworm to dogs and cats. Another mosquito, Aedes triseriatus, occurs in the same type of habitat and is responsible for transmitting La Crosse virus to humans. It is important for public health that everyone remain vigilant about removing items that might be attractive for the container mosquitoes.

Pictures of Aedes aegypti and Aedes albopictus mosquitoes
Photo credits: Aedes aegypti. James Newman, University of Florida, FMEL. Aedes albopictus. Sean McCann, University of Florida. Used with permission.


Cycling between mosquitoes and humans 

The virus cycles between mosquitoes and humans, and non-human reservoirs are currently unknown.

At this time, most of the local mosquito populations in the continental United States are not known to be transmitting the virus. Many states are collecting and testing mosquitoes for presence of the Zika virus. While there have been no positive mosquitoes detected yet, there is evidence that local transmission of the virus has occurred in south Florida, in Miami-Dade and Broward Counties. Four cases of Zika were reported from people with no travel history. A majority of the infections reported in the U.S. to date have been in people who have travelled to areas where the local mosquitoes are transmitting the virus, and then returned home after being infected.

Graphic showing cycle of human to mosquito
Zika virus circulates between humans and mosquitoes. Credit: CDC (Public domain)


"Container Mosquitoes" 

Container mosquitoes, such as Aedes aegypti and Aedes albopictus deposit their eggs on the sides of water-holding containers, just above the water line. Individual eggs are about the size of a grain of pepper and can be difficult to see. The eggs undergo a drying out period, and can remain dormant until the container becomes flooded. Eggs will hatch into larvae when the water level in the container rises and comes into contact with the eggs. Immature mosquitoes will live in water until they emerge as adults.  Mosquito larvae (Photo) feed on organic materials including leaves, algae, and other nutrients that occur in the container. The larvae molt, or shed their skin, four times, increasing in size during each period between molts. The fourth time they molt, they undergo transformation to a pupa (Photo), which is very active in the water, but does not feed. Immature mosquitoes are also known as “wigglers” and “tumblers” because of the swimming movements they make in the water. After several days in the pupal stage, the adult mosquito emerges and begins a terrestrial life.

Photo credits: Mosquito larvae, Roxanne Connelly, University of Florida, FMEL. Mosquito pupa,  Michelle Cutwa, University of FL, FMEL. Used here with permission of University of Florida.

Photo of mosquito egg deposits in container
Mosquito eggs can be seen above the water line in this container. Credit: Roxanne Connelly, University of Florida, FMEL. Used here with permission.


Adult Mosquito Behavior 

Male and female adult mosquitoes feed on sugar sources (such as flower nectar) for energy. The females also feed on blood to provide nutrients to their developing eggs. Adult container mosquitoes don’t fly much over 500 feet from their aquatic habitat. Anyone being bitten while on their own property should check for water-holding containers as the mosquitoes are likely being generated very close to home. Generally, the female mosquito lays her eggs about 3 days after taking a blood meal, at which time, she seeks more blood to nourish the next batch of eggs. Each egg batch could include as many as 150 eggs. She can continue the blood-feeding and egg laying for as long as she survives. The females live about a month. Container mosquitoes are known to distribute their eggs amongst several containers. Often times containers will have eggs from more than one female.

Goes to photo of natural mosquito-breeding containers
Examples of “natural” containers include tree holes, bromeliad plants, and bamboo. Roxanne Connelly, University of Florida, FMEL. Used with permission of UF. Click for larger image.


More on mosquito containers 

Containers are classified as almost anything that can hold water. Examples of common container types where these mosquitoes have been detected include tires, potted plant saucers, tree holes, bottles, cups, pet dishes, flower vases, bird baths, tarps, cisterns, trash cans, buckets, corrugated extension spouts, and clogged or bent roof gutters. Container types can be further defined as “natural” or “human made”. Natural containers include tree holes and bromeliad plants that collect water in their tanks. Human-made containers can include anything that is not naturally occurring that might be found in the yard or community. Some examples of container mosquito habitat are shown here.
Goes to photo of man-made mosquito-breeding containers
Some examples of containers where Aedes albopictus and Aedes aegypti can be found. Credit: Creative Commons. Click for larger image.


Vector Distribution 

An estimated distribution of the occurrence of these two mosquitoes in the United States was provided by the US Centers for Disease Control (CDC). For US residents who live in the states where container mosquitoes are found, it is wise to begin mosquito source reduction efforts locally and within the community now even though local transmission of Zika virus has not been reported.

 

Distribution of Aedes aegypti and Aedes albopictus in the U.S.
Click for full sized image. This is a public domain image from the CDC website, acquired April 2016.


Vector Source Reduction 

Container mosquitoes are responsible for transmission of many pathogens that cause disease, and source reduction should be a regular task around the home. Source reduction means removing the source of the mosquitoes, which in this case means water-holding containers. For items that can be discarded, conduct yard and neighborhood cleanup routines weekly. Make sure that any items that can be removed (bottles, cans, any trash that holds water) are appropriately discarded.

For small water-holding items that need to remain in place (bird baths, plant saucers, buckets, watering cans), clean them out weekly from April – November. These can be cleaned by (1) dumping the water and replacing it with clean water and (2) scrubbing the inside wall of the container to remove any mosquito eggs that may be attached.

For larger bodies of water such as cisterns, water collecting drums, rain barrels, and planters that do not have drainage holes, insect screening as a cover will prevent mosquitoes from depositing eggs inside the container, or a larvicide with active ingredients Bacillus thuriengensis israelensis (Bti) or methoprene can be used. The larvicide products come in the form of granules, briquettes, and donuts and will last a few weeks in the water holding receptacle. Make sure to follow label recommendations for application rates and sites. Products with these active ingredients can be purchased at local hardware and do-it-yourself stores. For abandoned pools and ornamental ponds, mosquito fish can be added which will consume the immature stages of the mosquitoes. 

 

FMEL-Florida Medical Entomology Laboratory

 

Note: The Use Permission for the information and images made available on this page extend to all EDEN member Extension programs.




Last Updated:7/29/2016 10:33 PM
 

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