Control of disease-carrying mosquito vectors is the most effective method of preventing the transmission of some of the planet's most devastating diseases, including malaria and dengue.
The purpose of vector control is to reduce illness and death from diseases such as malaria, dengue, and West Nile virus (WNV), which are transmitted to and among humans by mosquito bites. Controlling the transmission of these and other vector-borne diseases requires controlling or eliminating their vectors. It has been estimated that mosquitoes, especially those that transmit the malaria parasite, have been responsible for half of the deaths in human history.
Mosquitoes also transmit many other life-threatening infections, including yellow fever, chikungunya, lymphatic filariasis, Rift Valley fever, and several encephalitis viruses. With the exception of yellow fever, there are no vaccines for preventing these diseases, and malaria parasites quickly develop resistance to most antimalarial drugs. There are no medications for effectively treating dengue or WNV, so mosquito control is the only option. Furthermore, the mosquitoes that carry these diseases are expanding their ranges.
Although various animals function as disease-carrying vectors, mosquitoes are the most important for human disease. Female mosquitoes of key species must consume vertebrate blood to provide nourishment for egg production. In the process, they can carry viral, bacterial, or parasitic disease-causing organisms between human and animal hosts.
Understanding the life cycles of disease agents and the habits of the mosquitoes that carry them is essential to vector control. Malaria is caused by five species of protozoan parasites of the genus Plasmodium that are transmitted from infected to uninfected people by Anopheles mosquitoes.
Dengue and yellow fever are viral infections carried by the Aedes aegypti mosquito. A. aegypti thrives in urban environments, breeding in manmade containers. Unlike other mosquitoes, it feeds in the daytime, in early morning and just before dusk. The female bites multiple people during each feeding period. Aedes albopictus or Asian tiger is a secondary dengue vector in Asia that, like A. aegypti, has spread to Europe and North America. Aedes mosquitoes are highly adaptive. They can survive temperatures below freezing by hibernating or sheltering in microhabitats, such as under houses where they are safe from aerial sprays. They can lie dormant in containers for months. The international trade in used tires, a prime breeding habitat, is a major factor in their expanding range. The mosquitoes also are resistant to many chemicals.
More than 130 bird species are infected with WNV. Mosquitoes that feed on the birds transmit the virus to humans. The birds and mosquitoes are attracted to the same pools of stagnant water that accumulate in droughts. Mosquitoes carrying WNV tend to feed at dusk and dawn.
A. aegypti probably arrived in the Western Hemisphere in water casks on slave ships from Africa, carrying yellow fever that caused a severe epidemic along the eastern seaboard in the eighteenth century. Although eradicated in North America in the mid-twentieth century, A. aegypti is back, this time carrying dengue.
One of the earliest large-scale vector-control campaigns occurred during the construction of the Panama Canal. In the French attempt to build the canal through mosquito-infested jungles, thousands of lives were lost to malaria and yellow fever. When the United States set out to construct the canal in the early twentieth century, thousands of workers and millions of dollars were invested in two years of vector control. In addition to identifying and isolating patients with malaria and yellow fever, swamps were drained, buildings were fumigated by burning sulfur or pyrethrum, and breeding areas were sprayed with oil and larvicides. Roads were paved and water systems replaced cisterns to eliminate stagnant water. Windows were covered with screens and beds with netting. The spread of mosquito-borne disease was nearly eliminated.
From the 1940s through the 1960s, massive vectorcontrol programs using the pesticide DDT (dichlorodiphenyltrichloroethane) almost completely eradicated mosquito vectors in the United States and many other countries. However DDT, which persists in the environment, caused immense damage to wildlife—decimating bird populations around the world by weakening their eggshells—and threatened human health. Furthermore, DDT-resistant mosquito strains soon emerged. Rachel Carson's 1962 book Silent Spring brought the devastating consequences of DDT to the world's notice. DDT was banned in the United States in 1972, a major factor in the recovery of the bald eagle and many other North American birds. However, DDT is still used in many developing countries where mosquito infestations pose an even greater threat.
Global mosquito control consumes tremendous financial resources, time, and labor, and can have significant environmental impacts. The choice of a control method depends on local circumstances—the magnitude of the disease burden, the ability to correctly apply interventions in a timely manner, and the likelihood that effective control can be sustained.
Insecticide-treated bed nets (ITNs) are one of the most effective means for controlling mosquitoes that bite people while they sleep. If used by an entire population, ITNs can reduce malaria transmission by 90%, malaria incidence by 50%, and child mortality by 18%. ITNs can reduce mortality from all causes by 20%. It has been estimated that for every 1,000 children protected with an ITN, five or six lives are saved each year. Pyrethroid-treated nets in malaria-endemic areas are a major component of the Roll Back Malaria initiative of the World Health Organization (WHO).
WHO recommends integrated vector management (IVM) that employs a range of methods based on scientific evidence and local circumstances. IVM requires collaboration between public and private sectors and healthcare workers, the involvement of local communities, good management practices, and rational use of insecticides. In addition to ITNs, IVM employs indoor residual spraying (IRS) to destroy malaria vectors in homes and sleeping areas, as well as larviciding and/or environmental management. Environmental management or modification prevents mosquitoes from accessing egg-laying habitats. It includes proper disposal of solid waste, removal of manmade habitats, draining areas where mosquitoes breed, filling in low-lying areas to prevent water from stagnating, and ongoing vector monitoring. These methods require more local technical expertise and are applicable to far fewer situations than ITNs and IRS.
Avoiding mosquito bites and mosquito-proofing homes, workplaces, and communities are effective methods of vector control.
In the early 2000s much research was devoted to vector control by biological and GM methods. An example of biologic control is infection of Anopheles mosquitoes with Wolbachia bacteria that can decrease malaria parasite transmission. The bacteria are passed on to mosquito offspring, causing a self-sustaining infection.
The Grand Challenges in Global Health (an initiative of the Bill & Melinda Gates Foundation and the Foundation for the National Institutes of Health in the United States, the Canadian Institutes of Health Research, and the Wellcome Trust in the United Kingdom) support the development of genetic tools for controlling dengue transmission. For example, a group at the University of California-Irvine has engineered male A. aegypti mosquitoes that carry a gene that destroys the flight muscles of female offspring.
Pyrethroid sprays and other insecticides kill desirable insects, such as butterflies, dragonflies, and ladybugs, and essential insect pollinators including bees, although public health officials argue that aerial spraying at night reduces harm to honeybees and other beneficial insects. Pyrethroid sprays can also kill fish, birds, bats, and geckos that eat mosquitoes. Some insecticides may be harmful to humans, especially when they are sprayed from the air. Health officials recommend that children, pets, and those with respiratory problems remain indoors until aerial sprays have dried. Some physicians tell their chemically sensitive patients to leave areas of aerial spraying. Furthermore, vector control methods never eliminate every mosquito, so personal and household protection will continue to be mainstays of control.
GM vector control is very controversial, particularly since some GM vectors have been released in the wild in secret. There are many concerns about the introduction of new GM species. Critics have also argued that eradicating a mosquito species could have unintended consequences on ecosystems and the food chain. Some critics worry that wiping out A. aegypti could cause a takeover by the invasive dengue-carrying Asian tiger, which is very hard to control with insecticides. However, Oxitec claims to have already engineered mutant Asian tigers.
Many mosquito vector populations are resistant, not only to DDT, but also to other commonly used insecticides, including pyrethroids, and insecticide-resistance is spreading. WHO reported resistance to ITN and IRS insecticides in 64 countries.
Some countries that had low transmission rates, such as Tunisia and the United Arab Emirates, have been able to eradicate malaria with vector control, strict monitoring, and aggressive malaria treatment. However, malaria is reemerging in most countries where the disease is endemic. In some parts of Africa, transmission rates are so high that even reducing mosquito bites by 99% would leave about ten infectious bites per person every year.
Vector control is one of the four basic elements of WHO's global malaria control strategy. WHO also develops new insecticides and application technologies. In the United States, the Centers for Disease Control and Prevention (CDC) monitors and surveys for malaria and other vector-borne diseases. However, vector control is primarily the responsibility of local officials. Insecticide spraying of breeding areas remains the most cost-effective method of mosquito control. Cities and counties also conduct mosquito surveillance, and local authorities may be alerted to standing water, such as storm sewers, ditches, and abandoned properties that are potential breeding grounds.
In the summer 2012, there were hundreds of WNV infections and multiple deaths in Texas and Louisiana. Dallas, Texas, declared a state of emergency and aerial sprayed for the first time in 45 years. The spraying was controversial, not only because of risks to beneficial insects, animals, and humans, but because it killed only adult mosquitoes, leaving the larvae. The online social-action group, Change.org, petitioned to halt the spraying.
In 2009, Key West, Florida, experienced its first major outbreak of dengue since 1934. Despite blanket spraying with pesticides, the outbreak lasted for 15 months, sickening 93 people. By 2012, the Florida Keys Mosquito Control Board was awaiting approval from the U.S. Food and Drug Administration (FDA) to release Oxitec's GM mosquitoes to combat dengue. As a major tourist destination, the area spends more than $1 million a year on truck and helicopter spraying. Still, approximately 20% of Key West homes have dengue-infected mosquitoes, reaching as high as 40% or more in the summer rainy season.
See also Dengue fever ; Malaria ; West Nile virus .
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Margaret Alic, PhD