Mosquito-borne Diseases
Mosquito-borne Diseases
Disease History, Characteristics, and Transmission
Introduction
Mosquitoes have harmed more humans than any other group of insects. The scientific names conveyed upon mosquitoes reflect the torment that they can cause: Psorophora horrida, Culex perfidiosus, Mansonia perturbans, Aedes vexans, and Aedes tormentor. However, the annoyance caused by mosquitoes pales in comparison to the widespread suffering and millions of deaths that these insects cause. These flies from the order Diptera transmit some of the most devastating diseases.
There are over 2,500 different species of mosquitoes throughout the world. The vast majority of mosquitoes are harmless to humans, feeding only on nectar or other plant juices. Only females of some species consume blood. These mosquitoes transmit such diseases as malaria, yellow fever, dengue, filariasis and encephalitis (St. Louis encephalitis [SLE], Western Equine encephalitis [WEE], LaCrosse encephalitis [LAC], Japanese encephalitis [JE], Eastern Equine encephalitis [EEE] and West Nile virus [WNV]) to humans and to animals.
Disease History, Characteristics, and Transmission
A vector-borne disease results from an infection transmitted to humans and other animals by blood-feeding anthropods, such as mosquitoes. Female mosquitoes take a blood meal by bending back a troughlike protective scabbard to permit other mouthparts to penetrate the skin. The piercing “needle” is a long tube composed of six long, separate, stilettolike stylets wet with saliva that adhere to each other. The tube is traversed by two channels, a wide one through which the blood is sucked into the digestive system and a narrow one through which saliva containing an anticoagulant can be injected into the vertebrate host. Pathogens are transmitted from the mosquito to the host at this point.
IN CONTEXT: SCIENTIFIC, POLITICAL, AND ETHICAL ISSUES
Developed during the 1940s, the pesticide dichloro-diphenyltrichloroethane (DDT) was used to fight malaria and other insectborne diseases—and was considered by many to be a so-called miracle pesticide. During three decades of use, approximately 675,000 tons of DDT were applied in the United States.
Although DDT can greatly reduce the burden of mosquitoborne disease in the 1960s, DDT use remains controversial. DDT is an environmentally persistent chlorinated hydrocarbon that accumulates in the food chain and has significant environmental consequences that offset its benefits in the control of disease. The pesticide was banned in many developed nations, including the United States since 1972, because of its potential to damage ecosystems and wildlife. It is, however, still used for disease control in some countries. Proponents of DDT use assert that its environmental impact has been overstated, and that prohibitions on DDT use and manufacturing may have contributed to worldwide deaths from mosquito-borne diseases over the past several decades. It is again recommended for limited use in targeted areas because of its effectiveness in removing this severe public health hazard. Others assert that safer insecticides than DDT should be used in all locations.
American biologist and author Rachel Louise Carson (1907–1964), was a seminal figure in the environmental movement during the 1950s and early 1960s. Carson's book Silent Spring was an indictment of overzealous pesticide use and its effects on the environment, was published in 1962 and quickly became a controversial and enduring contribution to the environmental literature. Carson argued against indiscriminate pesticide use without consideration of its ecological consequences. Largely as a result of Silent Spring, DDT was banned by the United States in 1972 and is currently illegal in many other countries.
In 1878, English parasitologist Patrick Manson discovered the link between bloodsucking insects and disease. European and American infectious disease experts subsequently focused on the most common mosquitoborne diseases in these regions: malaria and yellow. The rapid worldwide movement of goods and people has also helped mosquitoes to cross the globe. By the end of the twentieth century, scientists battled mosquito-borne diseases that were worldwide plagues.
West Nile fever, caused by a mosquito-borne virus related to yellow fever, made its first appearance in the Western Hemisphere in New York City in 1999. First isolated and identified in Uganda in 1937, West Nile came to the United States via an infected mosquito, a sick person, or an infected bird. A human source is improbable because human blood generally contains too little virus to contaminate a mosquito. It may have come to New York in mosquitoes that stowed away on a airplane. However, the most likely scenario is that it arrived in illegally imported birds that had been not been quarantined before entering the country. It has subsequently spread by migrating birds, and is now present in all 48 contiguous states in the U.S.
Scope and Distribution
Malaria occurs in tropical areas of Central and South America, Africa, Asia, and the East Indies. Until the mid-twentieth century, it was far more widespread. Malaria was established in virtually all subtropical and tropical areas as well as some temperate areas. Malaria receded because breeding sites for mosquitoes were drained for agricultural and industrial purposes. Meanwhile, people moved into in better housing that was less open to mosquitoes. While the disease has been all but obliterated from most of the developed world, it continues to kill elsewhere. More than one million people, mostly in Africa, die annually from malaria with a child succumbing every 30 seconds.
Several different species of the Aedes and Haemogogus (South America only) mosquitoes transmit the yellow fever virus. While control programs successfully eradicated mosquito habitats in the past, particularly in South America, these programs have lapsed. As a result, mosquito populations have jumped and there is an accompanying rise in the risk of yellow fever epidemics. There are 200,000 estimated cases of yellow fever with 30,000 per year. However, the World Health Organization (WHO) suspects that yellow fever may be underreported.
Dengue is also spread by the Aedes mosquito. Dengue haemorrhagic fever (DHF) is a potentially lethal complication. WHO estimated in 2007 that there were 50 million cases of dengue annually, but the disease is rapidly spreading worldwide. DHF, mostly found in Asia, is a leading cause of hospitalization among children with over 500,000 requiring such care annually.
Treatment and Prevention
The best method of preventing mosquito-borne diseases is to kill mosquitoes. To eradicate mosquitoes, public health experts advise emptying containers of standing water that attract egg-laying females. Some governments kill mosquitoes through insecticide spraying programs or swamp-draining efforts. Exposure to mosquito-borne diseases can be minimized by limiting outdoor movement. Screens are effective at keeping mosquitoes from entering homes.
Infection with the parasite that causes malaria is treated with chloroquine, unless the parasite is resistant to this medication, in which case quinine sulfate and antibiotic combinations are used. As many other mosquito-borne diseases are viral in nature, treatment is mostly supportive or in some cases, involves antiviral medications.
Vaccines are available to protect against Japanese encephalitis and yellow fever. However, vaccine development for dengue and DHF is difficult because any of four different viruses may cause disease. Protection against only one or two dengue viruses could actually increase the risk of more serious illness. Other vaccines are in development. As of 2005, at least one potential vaccine for malaria was ready for clinical trial in humans.
WORDS TO KNOW
BEDNETS: A type of netting that provides protection from diseases caused by insects such as flies and mosquitoes. It is often used while sleeping to prevent insects from biting while still allowing air to flow through its mesh structure.
GENETIC ENGINEERING: Genetic engineering is the altering of the genetic material of living cells in order to make them capable of producing new substances or performing new functions. When the genetic material within the living cells (i.e., genes) is working properly, the human body can develop and function smoothly. However, should a single gene—even a tiny segment of a gene go awry—the effect can be dramatic: deformities, disease, and even death.
INSECTICIDE: A chemical substance used to kill insects.
MOSQUITO COILS: Mosquito coils are spirals of inflammable paste that, when burned, steadily release insect repellent into the air. They often used in Asia, where many coils release octachlorodipropyl ether, which can cause lung cancer.
MOSQUITO NETTING: Fine meshes or nets hung around occupied spaces, especially beds, to keep out disease-carrying mosquitoes are called mosquito netting. Mosquito netting is a cost-effective way of preventing malaria.
PESTICIDE: Substances used to reduce the abundance of pests, any living thing that causes injury or disease to crops.
Impacts and Issues
In some regions, mosquitoes have shown the ability to become resistant to pesticides. Anopheles mosquitoes in some areas are no longer killed by applications of DDT. Pesticide can also be prohibitively expensive. In 2007, Uganda announced that it could not spray DDT to fight malaria because it had failed to raise the 400 million United States dollars necessary to purchase the pesticide. An estimated 320 Ugandans die of malaria daily.
Several international organizations and charities have been instrumental in the fight against mosquitoborne diseases. The Bill and Melinda Gates Foundation supports mosquito-borne disease research and prevention efforts worldwide, including the development of effective and affordable drugs, improvement of existing preventative measures, and vaccine development. RAPIDS (Reaching HIV-Affected People with Integrated Development and Support), a consortium of several organizations, targets its efforts against mosquito-borne diseases in HIV/AIDS affected communities in Zambia. RAPIDS distributes protective netting and provides inhome follow-up care, ensuring that mosquito netting is properly used.
Promotion of specific sanitation measures is underway in areas where mosquito-borne diseases pose public health threats. Biological control methods, such as wasps that kill mosquitoes, are also being investigated. Researchers are also developing and investigating the use of genetically engineered mosquitoes to fight malaria. The modified mosquitoes are resistant to malaria, and breed at a faster rate than unmodified, non-resistant mosquitoes. Researchers hope that such mosquitoes can be introduced into malaria-prone regions and overtake wild disease-carrying mosquito populations. However, little is known about the impact genetically engineered mosquitoes could have on the transmission or development of other diseases.
Outdoor time-released insecticide misting systems are increasing in popularity, particularly in the United States, as means of controlling mosquitoes. These systems utilize various synergized formulations of natural pyrethrins or synthetic pyrethroids that are dispensed into the environment at intervals determined by the user. Some systems also utilize minimum risk pesticides to control or repel mosquitoes. The American Mosquito Control Association (AMCA) opposes this method of dispensing pesticides as inconsistent with the Integrated Mosquito Management practices approved by the Environmental Protection Agency as part of the Pesticide Environmental Stewardship Program. The AMCA specifically fears unnecessary insecticide use, indiscriminate killing of beneficial insects, pesticide exposure to humans, and promotion of insecticide resistance.
IN CONTEXT: LIVING WITH DISEASE
It was once argued that malaria could be eradicated. The draining of marshlands and the use of the pesticide DDT dramatically reduced the six million cases a year that the U.S. experienced in the first decades of the twentieth century. By 1960, the World Health Organization (WHO) had established antimalarial policies in 100 nations and was confident that the disease could be eradicated.
A number of sociopolitical factors, however, combined to slow the advance of medicine. People became complacent about malaria and public health programs were allowed to falter and lapse. Without outside aid, poor nations did not have the money for malarial control methods. Additionally, countries torn by war focused resources on fighting, not on medical care. Meanwhile, malarial microbes evolved in response to drugs, while the ready availability of air travel brought new strains into areas that lacked immunity to them. Global warming is expected to bring malaria back to northern Europe, and it never completely left southern Europe or the United States. WHO now forecasts a 16 percent growth rate in the disease per year.
The regular use of insecticide-impregnated curtains and bednets can reduce the rate of such diseases, particularly among children. The success of using bednets for sustained mosquito control is dependent upon regular treatment of the nets with pyrethroid insecticide once or twice a year. Dip-it-yourself kits have been distributed for this purpose in some countries, and researchers are developing better, longer-lasting insecticide-impregnated fabrics for netting, drapery, and clothing.
Travelers visiting areas known for major outbreaks of mosquito-borne diseases are advised to use mosquito repellent insecticide. The use of mosquito coils, and protective clothing and bedding is also often recommended, along with available vaccinations.
IN CONTEXT: PERSONAL RESPONSIBILITY AND PROTECTION
In October 2006 the Division of Global Migration and Quarantine at Centers for Disease Control and Prevention (CDC) issued an updated list of measures to prevent bites from mosquitoes, ticks, fleas and other insects and arthropods. The preventative measures were designed to “reduce the possibility of being bitten by insects or arthropods that can transmit diseases (vector-borne), such as malaria, dengue, and tickborne encephalitis (TBE).” CDV recommendation include:
- Use an insect repellent on exposed skin to repel mosquitoes, ticks, fleas and other arthropods. EPA-registered repellents include products containing DEET (N,N-diethylmetatoluamide) and picaridin (KBR 3023). DEET concentrations of 30% to 50% are effective for several hours. Picaridin, available at 7% and 15 % concentrations, needs more frequent application. DEET formulations as high as 50% are recommended for both adults and children over 2 months of age.
- Protect infants less than 2 months of age by using a carrier draped with mosquito netting with an elastic edge for a tight fit.
- When using sunscreen, apply sunscreen first and then repellent. Repellent should be washed off at the end of the day before going to bed.
- Wear long-sleeved shirts which should be tucked in, long pants, and hats to cover exposed skin. When you visit areas with ticks and fleas, wear boots, not sandals, and tuck pants into socks.
- Inspect your body and clothing for ticks during outdoor activity and at the end of the day. Wear light-colored or white clothing so ticks can be more easily seen. Removing ticks right away can prevent some infections.
- Apply permethrin-containing (e.g., Permanone) or other insect repellents to clothing, shoes, tents, mosquito nets, and other gear for greater protection. Permethrin is not labeled for use directly on skin. Most repellent is generally removed from clothing and gear by a single washing, but permethrin-treated clothing is effective for up to 5 washings.
- Be aware that mosquitoes that transmit malaria are most active during twilight periods (dawn and in the evening).
- Stay in air-conditioned or well-screened housing, and/or sleep under an insecticide treated bed net. Bed nets should be tucked under mattresses and can be sprayed with a repellent if not already treated with an insecticide. Daytime biters include mosquitoes that transmit dengue and chikungunya viruses and sand flies that transmit leishmaniasis.
SOURCE: Centers for Disease Control and Prevention, National Center for Infectious Diseases, Division of Global Migration and Quarantine.
BIBLIOGRAPHY
Speilman, Andrew, and Michael D'Antonio. Mosquito: A Natural History of Our Most Persistent and Deadly Foe. New York: Hyperion, 2001.
Web Sites
Centers for Disease Control Division of Vector-Borne Infectious Diseases. “Division of Vector-Borne Infectious Diseases.” February 23, 2007 <http://www.cdc.gov/ncidod/dvbid/> (accessed April 25, 2007).
Caryn E. Neumann