Definitions

epidemic polyarthritis

Mosquito

[muh-skee-toh]

Mosquitoes are insects in the family Culicidae. They have a pair of scaled wings, a pair of halteres, a slender body, and long legs. The females of most mosquito species suck blood (hematophagy) from other animals, which has made them the most deadly disease vector known, killing millions of people over thousands of years and continuing to kill millions per year by the spread of diseases.

Length varies but is rarely greater than 16 mm (0.6 inch), and weight up to 2.5 mg (0.04 grain). A mosquito can fly for 1 to 4 hours continuously at up to 1–2 km/h travelling up to 10 km in a night. Most species are nocturnal or crepuscular (dawn or dusk) feeders. During the heat of the day most mosquitoes rest in a cool place and wait for the evenings. They may still bite if disturbed. Mosquitos are adept at infiltration and have been known to find their way into homes via deactivated air conditioning units.

Feeding Habits

Both male and female mosquitoes are nectar feeders, but the female of many species is also capable of hematophagy (drinking blood). Females do not require blood for survival, but they do need supplemental substances (like protein and iron) to develop eggs. Prior to and during blood feeding, they inject saliva into the bodies of their source(s) of blood. The Toxorhynchites genus of mosquito never drink blood. This genus includes the largest of the extant mosquitoes, the larvae of which are predatory on the larvae of other mosquitoes. These mosquito eaters have been used in the past as mosquito control agents with varying success.

Female mosquitoes hunt their blood host by detecting carbon dioxide (CO2) and 1-Octen-3-ol from a distance.

Mosquito saliva

In order for the mosquito to obtain a blood meal it must surmount the vertebrate physiological responses. The mosquito, as with all blood-feeding arthropods, has evolved mechanisms to effectively block the hemostasis system with their saliva which contains a complex mixture of secreted proteins. Mosquito saliva's effects are vascular constriction, blood clotting, platelet aggregation, inflammation, immunity, and angiogenesis. Universally, hematophagous arthropod saliva contains at least one anticlotting, one anti-platelet, and one vasodilatory substance. Mosquito saliva also contains enzymes that aid in sugar feeding and antimicrobial agents to control bacterial growth in the sugar meal. The composition of mosquito saliva is relatively simple as it usually contains fewer than 20 dominant proteins. Despite the great strides in knowledge of these molecules and their role in bloodfeeding achieved recently, scientists still cannot ascribe functions to more than half of the molecules found in arthropod saliva.

It is now well recognized that the feeding ticks, sandflies, and, more recently, mosquitoes have an ability to modulate the immune response of the animals (hosts) they feed on. The presence of this activity in vector saliva is a reflection of the inherent overlapping and interconnected nature of the host hemostatic and inflammatory/immunological responses and the intrinsic need to prevent these host defenses from disrupting successful feeding. The mechanism for mosquito saliva-induced alteration of the host immune response is unclear, but the data has become increasingly convincing that such an effect occurs. Early work described a factor in saliva that directly suppresses TNF-α release, but not antigen-induced histamine secretion, from activated mast cells. Experiments by Cross et al. (1994) demonstrated that the inclusion of Ae. aegypti mosquito saliva into naïve cultures led to a suppression of interleukin (IL)-2 and IFN-γ production, while the cytokines IL-4 and IL-5 are unaffected by mosquito saliva. Cellular proliferation in response to IL-2 is clearly reduced by prior treatment of cells with SGE. Correspondingly, activated splenocytes isolated from mice fed upon by either Ae. aegypti or Cx. pipiens mosquitoes produce markedly higher levels of IL-4 and IL-10 concurrent with suppressed IFN-γ production. Unexpectedly, this shift in cytokine expression is observed in splenocytes up to 10 days after mosquito exposure, suggesting that natural feeding of mosquitoes can have a profound, enduring, and systemic effect on the immune response.

T cell populations are decidedly susceptible to the suppressive effect of mosquito saliva, showing enhanced mortality and decreased division rates. Parallel work by Wasserman et al. (2004) demonstrated that T- and B-cell proliferation was inhibited in a dose dependent manner with concentrations as low as 1/7th of the saliva in a single mosquito. Depinay et al. (2005) observed a suppression of antibody-specific T cell responses mediated by mosquito saliva and dependent on mast cells and IL-10 expression. A recent study suggests that mosquito saliva can also decrease expression of interferon−α/β during early mosquito-borne virus infection. The contribution of type I interferons (IFN) in recovery from infection with viruses has been demonstrated in vivo by the therapeutic and prophylactic effects of administration of IFN-inducers or IFN, and recent research suggests that mosquito saliva exacerbates West Nile virus infection, as well as other mosquito-transmitted viruses.

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Mosquitoes and health

Mosquitoes are a vector agent that carries disease-causing viruses and parasites from person to person without catching the disease themselves. Female mosquitoes suck blood from people and other animals as part of their eating and breeding habits. When a mosquito bites, she also injects saliva and anti-coagulants into the blood which may also contain disease-causing viruses or other parasites. This cycle can be interrupted by killing the mosquitoes, isolating infected people from all mosquitoes while they are infectious or vaccinating the exposed population. All three techniques have been used, often in combination, to control mosquito transmitted diseases. Window screens, introduced in the 1880s, were called "the most humane contribution the 19th century made to the preservation of sanity and good temper.

Mosquitoes are estimated to transmit disease to more than 700 million people annually in Africa, South America, Central America, Mexico and much of Asia with millions of resulting deaths. In Europe, Russia, Greenland, Canada, the United States, Australia, New Zealand, Japan and other temperate and developed countries, mosquito bites are now mostly an irritating nuisance; but still cause some deaths each year. Historically, before mosquito transmitted diseases were brought under control, they caused tens of thousands of deaths in these countries and hundreds of thousands of infections. Mosquitoes were shown to be the method by which yellow fever and malaria were transmitted from person to person by Walter Reed, William C. Gorgas and associates in the U.S. Army Medical Corps first in Cuba and then around the Panama Canal in the early 1900s. Since then other diseases have been shown to be transmitted the same way.

The mosquito genus Anopheles carries the malaria parasite (see Plasmodium). Worldwide, malaria is a leading cause of premature mortality, particularly in children under the age of five, with around 5.3 million deaths annually, according to the Centers for Disease Control. Some species of mosquito can carry the filariasis worm, a parasite that causes a disfiguring condition (often referred to as elephantiasis) characterized by a great swelling of several parts of the body; worldwide, around 40 million people are living with a filariasis disability. The viral diseases yellow fever and dengue fever are transmitted mostly by Aedes aegypti mosquitoes. Other viral diseases like epidemic polyarthritis, Rift Valley fever, Ross River Fever, St. Louis encephalitis, West Nile virus (WNV), Japanese encephalitis, La Crosse encephalitis and several other encephalitis type diseases are carried by several different mosquitoes. Eastern equine encephalitis (EEE) and Western equine encephalitis (WEE) occurs in the United States where it causes disease in humans, horses, and some bird species. Because of the high mortality rate, EEE and WEE are regarded as two of the most serious mosquito-borne diseases in the United States. Symptoms range from mild flu-like illness to encephalitis, coma and death. Viruses carried by arthropods such as mosquitoes or ticks are known collectively as arboviruses. West Nile virus was accidentally introduced into the United States in 1999 and by 2003 had spread to almost every state with over 3,000 cases in 2006.

A mosquito's period of feeding is often undetected; the bite only becomes apparent because of the immune reaction it provokes. When a mosquito bites a human, she injects saliva and anti-coagulants. For any given individual, with the initial bite there is no reaction but with subsequent bites the body's immune system develops antibodies and a bite becomes inflamed and itchy within 24 hours. This is the usual reaction in young children. With more bites, the sensitivity of the human immune system increases, and an itchy red hive appears in minutes where the immune response has broken capillary blood vessels and fluid has collected under the skin. This type of reaction is common in older children and adults. Some adults can become desensitized to mosquitoes and have little or no reaction to their bites, while others can become hyper-sensitive with bites causing blistering, bruising, and large inflammatory reactions, a response known as Skeeter Syndrome.

Mosquito control

There are many methods used for mosquito control. Some target the larval stage, while others are used to kill or repel adults. Much of modern mosquito control is no longer dependent on dangerous pesticides but specialized organisms that eat mosquitoes, or infect them with a disease that kills them. Such methods can even be used in Conservation Areas, like the "Forsyth refuge", where some major mosquito control is performed and monitored using "killifish" and juvenile eels. The success is documented with most advanced underwater microscopes like the ecoSCOPE. However, outbreaks of human mosquito-borne diseases may still result in fogging with chemicals that are less toxic than those used in the past.

A few varieties of the natural soil bacteria Bacillus thuringiensis, especially Bt israelensis (Bti), are used to interfere in the digestion systems of larvae. It can be dispersed by hand or dropped by helicopter in large areas. Unfortunately Bti is no longer effective after the larvae turn into pupae, because they stop eating. At this point larviciding oils, such as Golden Bear, can be used which increase the water tension until the pupae and larvae cannot break the surface to obtain air and therefore drown. A chemical commonly used in the United States is methoprene, considered slightly toxic to larger animals, which mimics and interferes with natural growth hormones in mosquito larvae, preventing development. Methoprene is frequently distributed in time-release briquette form in breeding areas. Malathion has also been sprayed in metropolitan areas like New York City to decrease the mosquito population and prevent the spread of West Nile Virus.

Dragonflies, also known as mosquito hawks, are excellent control agents. Dragonfly naiads consume mosquito larvae in the breeding waters, and adult dragonflies eat adult mosquitoes, particularly the day flying Asian tiger mosquitoes. Fogging for adult mosquitoes can backfire and increase long term populations if it removes dragonflies and other natural controls. Lizards are also useful predators which eat mosquitoes indoors.

Mosquito repellents generally contain one of the following active ingredients: DEET, Catnip oil extract, nepetalactone, citronella, or eucalyptus oil extract. Often the best "repellent" is a fan or gentle breeze as mosquitoes do not like moving air.

The most effective solutions for malaria control efforts in the third world are: mosquito nets (Transwiki:Klamboe), insecticide-laced mosquito nets, and DDT. Plain mosquito nets are cheap, they are completely effective in protecting humans within the net, they do not adversely affect the health of natural predators such as dragonflies, and do not require sophisticated public health capacity on the part of the government. The role of DDT in combating mosquitoes has been the subject of considerable controversy. While some argue that DDT deeply damages biodiversity, others argue that DDT is the most effective weapon in combating mosquitoes and hence malaria. While some of this disagreement is based on differences in the extent to which disease control is valued as opposed to the value of biodiversity, there is also genuine disagreement amongst experts about the costs and benefits of using DDT. Moreover, DDT-resistant mosquitoes have started to increase in numbers, especially in tropics due to mutations, reducing the effectiveness of this chemical.

Other popular methods of household mosquito control are the use of small electrical mats plugged into a socket, mosquito repellent vapour and mosquito coil all having a form of allethrin—a chemical commonly used to combat mosquitos and other pests in general. Mosquito repellent candles containing Citronella oil is another method to keep mosquitoes at bay. Some more lesser known methods use the cultivation of plants like wormwood or sagewort, lemon balm, lemon grass, lemon thyme and the mosquito plant (Pelargonium) which act against mosquitoes. However scientists have added that these plants are effective only when the leaves are crushed and used.

A newer approach to killing mosquitoes in a non-toxic way is to use a device that burns propane, thus generating carbon dioxide, warmth, and water vapor. These three elements, often coupled with a chemical attractant heated in this process, draws the mosquitoes toward the propane flame, where they are then sucked into a net or holder where they collect. Given that the typical mosquito's lifespan ranges from days to weeks, simply trapping them long enough allows them to die without any further effort. One such product is marketed by the Coleman Company.

One of the unconventional methods of mosquito control is the use of electromagnetic, ultrasound devices designed to confuse mosquitoes. It has found use in a variety of devices including a downloadable software which is claimed to thwart the insects using PC speakers. However the effectiveness is questionable since computer speakers aren't designed for high frequency sounds (15-20 kHz range).

Bats are an extremely effective form of natural mosquito control. One brown bat can consume 1200 mosquitoes in an hour. It's easy to build a bat box to encourage bats to live nearby. One averaged size bat box can provide a home for dozens of bats. Care must be taken, however, to ensure no humans are bitten by the bats, as bats are the number-one vector for rabies, an always-fatal disease in the absence of immediate vaccination. The purple martin swallow (Progne subis) is not, however, a prodigious consumer of mosquitoes as is so often claimed by companies that manufacture martin housing.

One of the main, non-chemical ways to prevent mosquito bites is the mosquito net. Mosquito netting if properly used and maintained (no holes), provides the maximum possible personal protection against biting insects. In many areas of the world, mosquitoes are not only a nuisance, but also pose a serious health threat. Sleeping under a bednet is highly recommended by the World Health Organization (WHO) and the U.S. Center for Disease Control (CDC) if staying in these areas.

One of the most popular chemical skin treatments is N,N-diethyl-meta-toluamide, commonly known as DEET. It has been used widely since its invention by the U.S. Department of Agriculture in 1945. DEET products have been widely used for many years but these products have occasionally been associated with some minor to moderate adverse reactions. DEET concentrations in repellents range from 5% up to 100%.

Other less commonly used mosquito repellents include: catnip oil extract, nepetalactone (no known credible tests), citronella 10% solution (84% effective for about 1 hour), or eucalyptus oil extract. A soybean oil-based product worked for about 1.5 hours and a lemon eucalyptus-based solution worked for about 3 hours.

Picaridin, first used in Europe in 2001, has been reported to be effective by Consumer Reports (7% solution) and the Australian Army (20% solution). Consumer Report retests in 2006 show that a 7% solution of picaridin now has a protection time of about 0 minutes and a 15% solution was only good for about one hour. So far DEET is the champion effective repellent against mosquitoes, especially when worn in conjunction with light coloured clothing, long sleeved pants and shirts and a hat.

Mosquitoes use carbon dioxide (CO2) and 1-octen-3-ol from human and animal breath and sweat as odor cues and DEET inhibits the detection of the latter in insects. Other potent body and breath odor cues include lactic acid, dimethyldisulfide,and acetone.

Other commercial products offered for household mosquito "control" include small electrical mats, mosquito repellent vapor, DEET-impregnated wrist bands, and mosquito coils containing a form of the chemical allethrin. Mosquito-repellent candles containing citronella oil are sold widely in the U.S. All of these have been used with mixed reports of success and failure. Some claim that plants like wormwood or sagewort, lemon balm, lemon grass, lemon thyme and the mosquito plant (Pelargonium) will act against mosquitoes. However, scientists have determined that these plants are “effective” for a limited time only when the leaves are crushed and applied directly to the skin.

There are several, widespread, unproven theories about mosquito control such as the assertion that Vitamin B, in particular B1 Thiamine, garlic, ultrasonic devices, incense, can be used to repel or control mosquitoes. Moreover, some manufacturers of "mosquito repelling" ultrasonic devices have been found to be fraudulent, and their devices were deemed "useless" in tests by the UK Consumer magazine Which? Application of other organic materials directly to the skin -- aloe vera, lime juice, and peppermint oil, for example -- is touted as somewhat effective, though the results have not been scientifically evaluated. One explanation is that such materials, with their strong odors, help mask the natural skin and sweat odors that initially attract mosquitos.

Bug zappers kill a wide range of flying insects including many beneficial insects that eat mosquitoes as well as some mosquitoes. Bug zappers have not been proven effective at controlling overall mosquito population.

Some newer mosquito traps or known mosquito attractants emit a plume of carbon dioxide together with other mosquito attractants such as sugary scents, lactic acid, octenol, warmth, water vapor and sounds. By mimicking a mammal’s scent and outputs, female mosquitoes are drawn toward the trap, where they are typically sucked into a net or holder by an electric fan where they are collected. According to the American Mosquito Control Association, "these devices will, indeed, trap and kill measurable numbers of mosquitoes," but their effectiveness in any particular case will depend on a number of factors such as the size and species of the mosquito population and the type and location of the breeding habitat. They are useful in specimen collection studies to determine the types of mosquitoes prevalent in an area but are typically far too inefficient to be useful in reducing mosquito populations.

Natural predators

The dragonfly eats mosquitoes at all stages of development and is quite effective in controlling populations. Although bats and Purple Martins can be prodigious consumers of insects, many of which are pests, less than 1% of their diet typically consists of mosquitoes. Neither bats nor Purple Martins are known to control or even significantly reduce mosquito populations. In the shape of a larva, the toxorhynchites (mosquito hawks) are known as natural predators of the culicidae species. Each larva can eat an average of 10 to 20 mosquitoes larvae per day. During its entire development, a toxorhynchites can consume an equivalent of 5,000 larvae of the first stage (L1) or 300 larvae stage 4 (L4) (Steffan & Evenhuis, 1981; Focks, 1982). However, it can consume all types of prey, of organic debris (Steffan & Evenhuis, 1981) or even exhibit cannibalistic behavior.

Treatment of mosquito bites

Visible, irritating bites are due to an immune response from the binding of IgG and IgE antibodies to antigens in the mosquito's saliva. Some of the sensitizing antigens are common to all mosquito species, whereas others are specific to certain species. There are both immediate hypersensitivity reactions (Types I & III) and delayed hypersensitivity reactions (Type IV) to mosquito bites (see Clements, 2000). There are several commercially available anti-itch medications. These are usually orally or topically applied antihistamines and, for more severe cases, corticosteroids such as hydrocortisone and triamcinolone. Many home remedies and recipes exist, most of which are effective against itching, including calamine lotion, baking soda, salt, rubbing alcohol, vinegar, and toothpaste. Also rubbing nail polish (preferably clear) on the bite will stop it from itching most of the time. Ammonia has been clinically demonstrated to be an effective treatment.

Scratching, cooling, and heat are effective but bring relief only during the application, although scratching a mosquito bite usually serves to irritate and inflame the area further and increase the risk of infection and scarring.

Applying streaming hot water to bitten areas can immediately eliminate itching if carried out for several minutes. However, this method is only a home remedy and not proven to work on every bite.

Cultural views

According to the “Mosquitoes” chapter in Kwaidan: Stories and Studies of Strange Things, by Lafcadio Hearn (1850–1904), mosquitoes are seen as reincarnations of the dead, condemned by the errors of their former lives to the condition of Jiki-ketsu-gaki, or "blood-drinking pretas".

The Babylonian Talmud (Gittin 56b) asserts that the Roman Emperor Titus was punished by God for having destroyed the Temple in Jerusalem by having a mosquito fly into Titus' nose, picking at his brain, ceaselessly buzzing, driving him crazy and eventually causing his death. No such account appears in any Roman source. Titus died prematurely from unclear causes after only two years in power.

Systematics

* Anopheles
* Cellia
* Bironella
* Chagasia

* Toxorhynchites

Identification

  • Brunhes, J.; Rhaim, A.; Geoffroy, B. Angel G. Hervy P. Les Moustiques de l'Afrique mediterranéenne French/English. Interactive identification guide to mosquitoes of North Africa, with database of information on morphology, ecology, epidemiology, and control. Mac/PC Numerous illustrations. IRD/IPT [12640] 2000 CD-ROM. ISBN 2-7099-1446-8 Mosquito species can also be identified through their DNA, however this is relatively expensive so it is not commonly performed. See the Use of DNA in forensic entomology.

See also

References

  • Clements, A.N. 2000. The Biology of Mosquitoes. Volume 1: Development, Nutrition and Reproduction. CABI Publishing, Oxon. ISBN 0-85199-374-5
  • Davidson, E. (ed.) 1981. Pathogenesis of Invertebrate Micorobial Diseases. Allanheld, Osmun & Co. Publishers, Inc., Totowa, New Jersey, USA. 562 pages.
  • Jahn, G. C., Hall, D.W., and Zam, S. G. 1986. A comparison of the life cycles of two Amblyospora (Microspora: Amblyosporidae) in the mosquitoes Culex salinarius and Culex tarsalis Coquillett. J. Florida Anti-Mosquito Assoc. 57, 24–27.
  • Kale, H.W., II. 1968. The relationship of purple martins to mosquito control. The Auk 85: 654-661.

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