The main mosquito that transmits Zika virus—and also Dengue, Chikungunya and Yellow Fever—is Aedes aegypti, a particularly wily foe.
It prefers urban areas and bites mainly people, making it very efficient at spreading disease. It bites in the day, so bed nets, a common way to protect people against the night-biting malaria mosquitoes, have little effect. It breeds in small containers of water, such as flower pots, cans, and tires that collect rainwater.
“I’ve seen Aedes aegypti merrily breeding in discarded soda caps,” said Joseph M. Conlon, technical adviser to the American Mosquito Control Association.
The New York Times, January 30, 2016
Understanding the mosquito lifecycle is the first step in designing effective prevention measures. Aedes aegypti and other mosquitoes have a complex life-cycle with dramatic changes in shape, function, and habitat. This makes it difficult for infection prevention specialists to determine all the places they reproduce.
Female mosquitoes lay their eggs on the inner, wet walls of containers with water. Eggs can withstand desiccation (extreme dryness) for several months, which means that even if all larvae, pupae, and adults are eliminated, repopulation will occur as soon as the eggs are flooded with water.
Larvae hatch (see diagram below, number 1) when water inundates the eggs as a result of rain or the addition of water by people. In the following days, the larvae (number 2) will feed on microorganisms and particulate organic matter, shedding their skins three times before metamorphosis is triggered, changing the larva into a pupa (number 3) (CDC, 2012).
Pupae do not feed; they just change in form until the body of the adult, flying mosquito is formed. Then the newly formed adult emerges from the water after breaking the pupal skin (number 4). The entire life cycle lasts 8 to 10 days at room temperature, depending on the level of feeding. Thus, there is an aquatic phase (larvae, pupae) and a terrestrial phase (eggs, adults) in the Aedes aegypti life-cycle (CDC, 2012).
Source: CDC, 2012
Public health campaigns aimed at reducing the spread of Zika virus focus on ridding communities of standing water; chemically, physically, or biologically destroying breeding mosquitoes; and encouraging the use of repellants and personal protective clothing.
Rain-filled cavities in trees, bamboo internodes, leaf-axils of plants. Source: CDC, 2012.
Aedes aegypti mosquitoes lay their eggs in areas where water collects or in water storage containers that are inadequately covered. An important step in control operations is identifying the types and abundance of containers producing mosquitoes. Different containers require specific control measures that depend on the type of container and how it is used. Disposing of unused containers, placing useful containers under a roof or protected with tight covers, and frequently changing the water of animal drinking pans and flower pots greatly reduces potential breeding sites. There are five general types of containers that serve as breeding grounds for mosquitoes:
Large discarded containers (tires, damaged appliances) and small discarded containers (paint cans). Source: CDC, 2012.
Ornamental or recreational containers (plant pots and dishes, plastic pools, rooting plants in water/ aquatic plants). Source: CDC, 2012.
Physical, biologic, and chemical control methods include:
Protective clothing and other personal protective measures against mosquitoes will reduce the risk of acquiring the diseases they transmit. Mosquito-bite prevention includes using air conditioning or window and door screens when indoors, wearing long sleeves and pants, using permethrin-treated clothing and gear, and using insect repellents. When used according to the product label, U.S. Environmental Protection Agency-registered insect repellents are safe for pregnant women. To protect yourself:
A man with Zika virus can pass it to his female or male sex partners via semen or blood. The virus can stay in semen longer than in blood, although it is not known for exactly how long. To prevent the spread of Zika during sex, use condoms correctly from start to finish during each sexual encounter; this includes vaginal, anal, and oral sex. Abstaining from sex is the only way to be sure that the Zika virus is not sexually transmitted (CDC, 2016a).
If your partner is pregnant, either use condoms correctly every time you have sex, or do not have sex during the pregnancy. Even if travelers returning to the United States from an area where Zika is present do not feel sick, they should take steps to prevent mosquito bites for 3 weeks so they do not spread Zika to mosquitoes that could spread the virus to other people (CDC, 2016a).
An eradication method already in use in Brazil to combat Dengue fever employs the release of mosquitoes infected with a bacterium called Wolbachia,* which reduces the ability of the Aedes mosquito to infect a host. In September 2014 the Brazilian government approved the release of Wolbachia mosquitoes in Rio de Janeiro to fight the spread of Dengue fever. This program will likely be expanded to combat the spread of the Zika virus.
*Wolbachia are bacteria that live within insect cells and are passed from one generation to the next through the insect’s eggs. Wolbachia is present in up to 60% of the different species of insects around us including some mosquitoes that bite people. Wolbachia is not found in Aedes aegypti—the primary mosquito species involved in the transmission of Dengue and Zika (Monash University, n.d.).
Attempts to eradicate or at least slow the spread of Zika also include the release of genetically engineered male mosquitoes that pass a lethal gene to their offspring. This is currently being done in small batches in Brazil.
The only way to prevent congenital Zika virus infection is to prevent maternal infection, either by avoiding areas where Zika virus transmission is ongoing or strictly following steps to avoid mosquito bites (Staples et al., 2016).
Men who reside in, or have traveled to, an area of active Zika virus transmission and who have a pregnant partner should abstain from sexual activity or consistently and correctly use condoms during sex (vaginal intercourse, anal intercourse, or fellatio) for the duration of the pregnancy. Pregnant women should discuss their male partner’s potential exposures to mosquitoes and history of Zika-like illness (Oster et al., 2016).
Several South and Central American countries have taken the unprecedented step of advising women to avoid becoming pregnant until the risks associated with Zika virus infection are more thoroughly understood.