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 infectious diseases.[1][2][3]
Length varies but is rarely greater than 16 mm (0.6 inch)[4], 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[5] 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 residences via deactivated air conditioning units.[6]
Feeding Habits
Both male and female mosquitoes are nectar feeders, but the females of many species are also capable of hematophagy (drinking blood ]). Females do not require blood for their own 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. Female mosquitoes hunt their blood host by detecting carbon dioxide (CO2) and 1-Octen-3-ol from a distance.
Mosquitoes of the genus Toxorhynchites never drink blood.[7] This genus includes the largest extant mosquitoes, the larvae of which prey on the larvae of other mosquitoes. These mosquito eaters have been used in the past as mosquito control agents, with varying success.[8]
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 affects vascular constriction, blood clotting, platelet aggregation, inflammation, immunity, and angiogenesis.[9] 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[10] and antimicrobial agents to control bacterial growth in the sugar meal.[11] The composition of mosquito saliva is relatively simple as it usually contains fewer than 20 dominant proteins.[12] 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.[12]
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.[9] 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.[13] 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.[14] Cellular proliferation in response to IL-2 is clearly reduced by prior treatment of cells with SGE.[14] 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.[15] 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.[15]
T cell populations are decidedly susceptible to the suppressive effect of mosquito saliva, showing enhanced mortality and decreased division rates.[16] 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.[17] 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.[18] A recent study suggests that mosquito saliva can also decrease expression of interferon−α/β during early mosquito-borne virus infection.[19] 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,[20] and recent research suggests that mosquito saliva exacerbates West Nile virus infection,[21] as well as other mosquito-transmitted viruses.[22]
Life Cycle of a Mosquito
Mosquitoes of different species lay their eggs in a variety of water sources that range from small containers to vast expanses of marshland. The larval stage is always aquatic and shuttles from the subsurface where it filter feeds on micro-organisms to the surface to obtain oxygen through a snorkel-like breathing apparatus. The pupal stage does not feed but unlike most Insect pupae is extremely active. The adult emerges from the pupal case using air pressure and assume a terrestrial existence.
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."[23]
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.[24] 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.[25] 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.[26][27] 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 2 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.[28] 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.
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 pesticides but specialized organisms that eat mosquitoes, or infect them with a disease that kills them.[citation needed] 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 products 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.[citation needed] 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.
