Mosquitoes and the diseases they spread
have been responsible for killing more people than all the wars in history.
Mosquitoes transmitting malaria kill 2 million to 3 million people and infect another 200 million or more every year.
Tens of millions more are killed and debilitated by a host of other mosquito-borne diseases, including filariasis, yellow fever, dengue and encephalitis.
Nearly half of the world’s population is at risk for malaria.
Malaria has occurred in the United States, and still does on rare occasions.
An influx of malaria-infected persons has produced localized malaria transmission in some areas of the United States.
The threat of developing encephalitis from mosquitoes is far greater than the threat of malaria in the United States.
Encephalitis, meningitis and other diseases can develop from the bites of mosquitoes infected with certain viruses: West Nile, St. Louis encephalitis, LaCrosse (California) encephalitis, and Eastern equine and Western equine encephalitis.
Mosquitoes belong to the group of insects known as diptera, or flies.
Two wing the characteristic that distinguishes flies from other types of insects.
The proboscis distinguishes a mosquito from other types of flies.
The proboscis, a long tubular mouthparts for sucking up fluids and the hair-like scales on its body.
The female mosquito’s life is often measured in weeks or months, while ales typically live only about a week.
Mosquitoes hatch from eggs laid in places that are or will be filled with water.
The eggs hatch into worm-like larvae that usually lie just beneath the water’s surface.
Larvae breathe through tubes on the tail end of their bodies, and feed on microscopic organisms, such as bacteria.
Thus most mosquito larvae require water containing organic material: leaves or sewage to serve as food for microorganisms that will be consumed by the developing mosquito larvae.
Larvae can grow and develop into comma-shaped pupae in less than 1 week.
While larvae wiggle violently when disturbed, while mosquito pupae tumble through the water when disturbed.
While mosquito larvae and pupae breathe through siphon-like devices.
Usually within three days the pupa will transform into an adult mosquito.
Most female mosquitoes attack birds and mammals, though some feed on the blood of reptiles and amphibians.
Only female mosquitoes bite.
A blood meal is usually required for egg laying.
All male mosquitoes, and the females of a few species, do not bite, and feed on nectar and other plant juices instead of blood.
Mosquitoes can detect carbon dioxide exhaled by their hosts many feet away.
Mosquitoes also sense body chemicals, such as the lactic acid in perspiration.
Some people are more attractive to mosquitoes than others, and people react differently to mosquito bites, some showing very little sign of being bitten, while others exhibit substantial redness, swelling and itching.
The reaction to a mosquito bite is an allergic reaction to the mosquito’s saliva.
Mosquitoes can fly long distances from their water source: some more than 20 miles.
Mosquitos fly only about 4 miles an hour.
Flies typically fly into the wind to help detect host odors.
There are fewer mosquitoes present on windy days.
When a mosquito finds its target, it lands, inserts its proboscis and probes for blood vessels beneath the skin, and injects saliva into the wound.
The saliva contains an anticoagulant that ensures a smooth flow of blood.
The mosquito’s saliva also may contain pathogens such as malaria parasites or encephalitis virus, transmitting disease.
The West Nile virus is transmitted predominantly by Culex mosquitoes.
Culex mosquito are medium-sized mosquitoes that are brown with whitish markings on the abdomen.
Culex mosquitos include the house mosquitoes (C. pipiens and C. quinquefasciatus).
Mosquitos that develop in urban areas, and the western encephalitis mosquito (C. tarsalis) more commonly found in rural areas, typically bite at dusk and after dark.
By day they rest in and around structures and vegetation.
Culex lay eggs on still water in a variety of natural and man-made containers, including tree holes, ditches, sewage and septic system water, catch basins, storm drains, non-chlorinated swimming and wading pools, decorative ponds, bird baths, flower pots, buckets, clogged gutters, abandoned tires, and water-retaining junk and debris.
Culex mosquito cannot develop in running water and water that is present less than a week.
Therefore, every effort should be made to prevent water from accumulating in containers or, at least, empty water out of them on a weekly basis.
Adult Culex mosquitoes do not fly far from where they develop as larvae. And unlike other mosquitoes that die with the coming of the first hard frost in autumn, the house mosquito can “over-winter” in protected places like sewers, crawlspaces and basements.
Aedes Mosquitos
The Aedes group of mosquitoes includes many nuisance mosquitoes, as well as species that transmit disease to humans. This is a diverse group that includes the inland floodwater mosquito (Aedes vexans), the Asian tiger mosquito (Aedes albopictus) and the tree hole mosquito (Ochlerotatus triseriatus*) – all of which prefer to feed on the blood of mammals. Floodwater mosquitoes lay their eggs on soil that becomes flooded, allowing the eggs to hatch and larvae to develop in temporary pools. Asian tiger and tree hole mosquitoes are container-breeding mosquitoes, laying their eggs in small, water-filled cavities, including tree holes, stumps, logs, and artificial containers, such as discarded tires.
Inland floodwater mosquitoes are brown with pale B-shaped marks on their abdomens. They can become particularly bothersome after areas, such as river backwaters and other low lying places, become flooded.
They are often the first mosquito noticed in spring, and later after heavy rainfall. Adults emerging together from flooded areas are often so numerous that natural controls, such as predators and parasites, are overwhelmed.
Unlike some other Aedes mosquitoes, inland floodwater mosquitoes may fly more than 10 miles from their larval development sites in search of blood meals. In Illinois, they may bite more people than any other species. They typically begin flying in late afternoon and are most active after dark, but will bite any time of day if disturbed while resting in shaded, heavily vegetated areas. Fortunately, in the United States they rarely, if ever, transmit disease, and typically die in autumn with the first hard frost.
Asian tiger mosquitoes are distinctive, black and white mosquitoes that bite by day (see picture on page 1). They were brought to this country in 1985, hidden in shipments of tires, and have since been found in many states including Illinois. The Asian tiger mosquito is capable of carrying LaCrosse encephalitis and West Nile viruses, though it is unclear whether the mosquito transmits these to humans. For more information, see the IDPH fact sheet on Asian tiger mosquitoes.
The primary vector (carrier) of LaCrosse encephalitis is the tree hole mosquito. It is a dark mosquito with silvery white spots on the sides of its thorax and abdomen. Like the Asian tiger mosquito, the tree hole mosquito bites by day and lays its eggs in small containers where water will pool, such as tree holes, discarded tires, cans, buckets and barrels. They often are found in and around wooded areas.
* Ochlerotatus triseriatus, the tree hole mosquito, was formerly known as Aedes triseriatus.
Mosquito-Borne Encephalitis Diseases
The cycles of mosquito-borne viral encephalitis and meningitis diseases are similar. Most involve various bird species that are said to be reservoirs. Once infected by a mosquito bite, the reservoir species are usually not seriously affected. They will, at least for a time, produce enough virus in their bodies to infect mosquitoes. In this manner, mosquitoes pick up the virus and may become vectors, or organisms that transmit the disease to other animals, such as birds, horses or humans. Horses and humans are generally thought of as “dead-end” hosts because they do not produce enough virus to infect mosquitoes. Thus, dead-end hosts are not involved in the spread of disease.
For any particular season, the number of human encephalitis cases is not easily predicted. The occurrence of some encephalitides seems to be cyclic. This may be due to variations in the condition of infected birds in a reservoir population. Birds may harbor enough virus to facilitate transfer to mosquitoes that bite them for only a few days after being infected. After this, they do not serve as a reservoir for the virus. When more and more birds in an area have passed the infective stage, fewer birds are around to pass the virus to mosquitoes. Thus, fewer mosquitoes will carry the virus, and fewer people will be infected.
The variation in numbers of infective vs. non-infective birds in a population may play a big part in the cyclic nature of some viral encephalitis diseases. Of course, many other factors are involved, such as the availability of food and other resources influencing the size of bird populations; the availability of sites for the development of mosquito larvae, which influences the size of mosquito populations; as well as weather, including rainfall and temperature.
West Nile Disease
In 2002, Illinois led the nation in West Nile disease cases with 884 and 67 deaths.
Like all encephalitis producing viruses, West Nile virus survives in birds and/or mammals, using them as reservoirs. Most birds and mammals survive infection, while the mosquitoes that bite them can ingest the virus and infect other animals they bite, including humans. The virus can affect some birds and mammals, such as crows, blue jays, squirrels, horses and humans, more seriously than others, producing severe illness and death. However, about 80 percent of humans develop no symptoms after being infected with the virus, developing at least a temporary immunity. Persons older than 50 years of age, and those with compromised immune systems, are much more likely to develop West Nile fever, a flu-like disease that may last for weeks, or life-threatening nervous system complications such as meningitis or encephalitis.
St. Louis Encephalitis
Similar to West Nile virus and also transmitted by Culex mosquitoes, the virus that causes St. Louis encephalitis is known for periodic outbreaks in the human population. In the United States, its occurrence has been limited compared to that of West Nile virus, and St. Louis encephalitis is usually confined to the southern portion of the United States, especially the Mississippi Valley. Compared to West Nile virus, St. Louis encephalitis has a lesser potential for producing epidemics. First noticed in St. Louis, Missouri, the largest epidemic occurred in the mid-70s when nearly 2,000 human cases were reported. Birds serve as a reservoir for the virus.
Eastern and Western Equine Encephalitides
Outbreaks of these related encephalitis viruses are rare. Several different mosquito species are suspected vectors of Eastern equine encephalitis, while Culex tarsalis, known as the western encephalitis mosquito, is the vector of Western equine encephalitis. Their uncommon occurrence is fortunate because, of all encephalitides, the equine encephalitides may have the highest potential for human mortality.
California Encephalitides
This group of encephalitis viruses is unique in several ways — it usually produces relatively mild illness in humans, and mammals, rather than birds, act as reservoirs for the virus.
The most common California encephalitis virus in the Midwest is LaCrosse. The LaCrosse virus is unique in that it primarily affects children, and because the virus can pass from a female mosquito to her offspring, mosquitoes can become infected without having to feed on an infected host.
Death from LaCrosse encephalitis is rare, but affected children may suffer from seizures and other nervous system complications that can persist for years. The disease is carried byOchlerotatus triseriatus, the tree hole mosquito. It bites humans after feeding on squirrels and chipmunks. LaCrosse encephalitis is associated with wooded areas inhabited by these rodents. Because the tree hole mosquito develops in natural cavities and in artificial containers, and because adults do not fly far from their larval development sites, there is increased potential for LaCrosse encephalitis where tires and other debris accumulate near wooded areas.
Preventing Mosquito Bites
One strategy to prevent mosquito bites is avoidance. But even if one were to remain indoors throughout the mosquito season, they might still encounter mosquitoes. Mosquitoes, such as the house mosquito, are adept at getting into structures to feed on the inhabitants, and also to use crawlspaces, basements and cellars as quiet spots in which to shelter themselves for the winter. It is important to keep structures in good repair, maintaining the integrity of window and door screens and weather stripping, and screening or sealing all gaps through which mosquitoes might enter, such as spaces around utility lines, vents, foundation cracks, and gaps around windows and doors.
Repellents are the first line of defense against mosquito bites. Many products provide somedegree of protection against mosquito bites. However, certain active ingredients provide better protection. For many years, DEET (N,N-diethyl-meta-toluamide) has been the standard by which products are measured. When applied according to label direction, products containing 20 percent to 30 percent DEET provide protection against mosquitoes that lasts several hours. Products containing much higher percentages of DEET are available, but generally do not provide significantly longer protection.
Recently, products containing another active ingredient, picaridin, have been shown to provide a similar degree of protection, and without the familiar odor and stickiness of DEET products. A third ingredient, lemon oil of eucalyptus, is a plant-derived compound that also is capable of providing protection, though not as long-lasting as that provided by products containing DEET or picaridin.
Whatever repellent you choose, be sure to read the label directions before applying to yourself or to children. Products containing lemon oil of eucalyptus should not be applied to younger children.