Danger of mosquitoes

DANGER OF MOSQUITOES

Danger of mosquitoes is a manual about this insect and why it is so dangerous.

A story about mosquitoes and diseases.

What’s the most dangerous creature on earth? Without question the answer is: the mosquito. Mosquitoes and the diseases they spread have been responsible for killing more people than all the wars in history. Even today, 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.

The fact is that nearly half of the world’s population is at risk for malaria. Even residents of the United States are not immune. Malaria has occurred in the United States, and still does on rare occasions. Mosquitoes capable of carrying and transmitting malaria still inhabit most parts of this country. And an influx of malaria-infected persons has produced localized malaria transmission in some areas of the United States. Until 1960 malaria was a problem even in The Netherlands. The danger of mosquitoes is a worldwide problem.

Today, however, the threat of developing encephalitis from mosquitoes is far greater than the threat of malaria. Encephalitis, meningitis and other diseases can develop from the bites of mosquitoes infected with certain viruses. These include the viruses of West Nile, St. Louis encephalitis, LaCrosse (California) encephalitis, and Eastern equine and Western equine encephalitis.

THE MOSQUITO.

Mosquitoes belong to the group of insects known as diptera, or flies. In fact, mosquito means “little fly” in Spanish. Diptera means “two wings” – the characteristic that distinguishes flies from other types of insects. What distinguishes a mosquito from other types of flies are its proboscis (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. Males typically live only about a week. The immature stages of the mosquito are less familiar to us. 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, 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, such as leaves or sewage to serve as food for microorganisms that will be consumed by the developing mosquito larvae.

In less than a week, hatchling larvae can grow and develop into comma-shaped pupae. While larvae are commonly called “wigglers” because they wiggle violently when disturbed, mosquito pupae are known as “tumblers” because they tumble through the water when disturbed. While mosquito larvae and pupae breathe through siphon-like devices, the pupal stage does not feed. Usually within three days the pupa will transform into an adult mosquito.

There are some notable exceptions to the standard mosquito life cycle. The larvae of some mosquito species eat the larvae of other species, though the predatory larvae of some species will develop into blood-feeding adults.

Female mosquitoes can be particular about whose blood they consume, with each species having its own preferences. Most mosquitoes attack birds and mammals, though some feed on the blood of reptiles and amphibians. Only female mosquitoes bite, because a blood meal is usually required for egg laying. All male mosquitoes, and the females of a few species, do not bite. They feed on nectar and other plant juices instead of blood.

Various clues enable mosquitoes to zero in on people and other animals they seek to bite. They 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. A person sleeping in a mosquito-infested room may wake up with dozens of mosquito bites, while the person sleeping next to them has none. Similarly, people react differently to mosquito bites, some showing very little sign of being bitten, while others exhibit substantial redness, swelling and itching. This is an allergic reaction to the mosquito’s saliva, the severity of which varies among individuals.

Mosquitoes can fly long distances; some more than 20 miles from the water source that produced them. But they don’t fly fast, only about 4 miles an hour. And because they typically fly into the wind to help detect host odors, fewer mosquitoes are about on windy days.

As a mosquito flies closer to its target, it looks for the movement of dark objects. Once it finds you, it lands, inserts its proboscis and probes for blood vessels beneath the skin. When it finds one, it injects saliva into the wound. The saliva contains an anticoagulant that ensures a steady, smooth flow of blood. Unfortunately, the mosquito’s saliva also may contain pathogens such as malaria parasites or encephalitis virus. This is how mosquitoes transmit disease. Read more about the danger of mosquitoes.

CULEX MOSQUITOES.

The West Nile virus is transmitted predominantly by Culex mosquitoes. Culex are medium-sized mosquitoes that are brown with whitish markings on the abdomen. These include the house mosquitoes (C. pipiens and C. quinquefasciatus) that develop in urban areas, and the western encephalitis mosquito (C. tarsalis) more commonly found in rural areas. They typically bite at dusk and after dark. By day they rest in and around structures and vegetation.

Culex lay “rafts” of 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 of all sorts. They 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 MOSQUITOES.

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 areas where they live, 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.

ANAPHELES GAMBIAE.

Danger of mosquitoes. The Anopheles gambiae is the number one killer in Africa and has a prominent place in the chapter of “The danger of mosquitoes”. This mosquito is the most efficient vector of human malaria in the Afrotropical Region. Thus, it is commonly called the African malaria mosquito. The Anopheles gambiae complex of sibling species comprises eight reproductively isolated species that are almost indistinguishable morphologically: Anopheles amharicus, Anopheles arabiensis, Anopheles bwambae, Anopheles gambiae, Anopheles coluzzii, Anopheles melas, and Anopheles merus. Collectively they are sometimes called Anopheles gambiae sensu lato, meaning ‘in the wider sense.’
Members of the Anopheles gambiae complex are found throughout tropical Africa, south of the Sahara desert, with Anopheles arabiensis extending across southern Arabia. Anopheles gambiae is distributed throughout sub-Saharan Africa, including Madagascar.

Adults: Adult female Anopheles can be differentiated from other mosquito genera because the palps (appendages found near the mouth) are as long as their proboscis (feeding tube). Adult Anopheles also have a distinguishable resting position where their abdomen is raised into the air. Anopheles gambiae have a variable body color, but it typically ranges from light brown to grey with pale spots of yellow, white or cream scales, and dark areas on their wings. In comparison to other species, adults are considered small to medium-sized mosquitoes with average wing length varying from 2.8 to 4.4 mm.

Eggs: Eggs are between 0.47 and 0.48 mm long, convex below and concave above, and the surface is covered with a polygonal pattern. Similar to other Anopheles species, Anopheles gambiae lay their eggs singly and directly on the water, with each egg having floats on either side. Anopheles eggs are not drought resistant.

Larvae: All Anopheles larvae lack the respiratory siphons used as breathing tubes in most other mosquito genera, and therefore the larvae lie parallel to the water surface in order to breathe. Anopheles mosquitoes develop though four larval sizes or instars before pupating. Larvae are very small in the first instar and increase in size until reaching 5 to 6 mm by the completion of the fourth instar. They feed on organic matter and algae.

Pupae: Pupae of all mosquitoes are comma-shaped when viewed from the side. Unlike the pupae of many other insects, mosquito pupae are very mobile; they use the paddle at the end of their abdomen to quickly move through their aquatic habitat. The pupal stage does not feed.

Life Cycle and Biology.

As with all mosquitoes, Anopheles gambiae has four life stages: egg, larva, pupa, and adult. Both male and female adult mosquitoes feed on nectar from plants, but only the female bloodfeeds on vertebrates, where she obtains nutrients for her eggs. Although adults can survive for up to one month in captivity, they usually survive around one to two weeks in the wild.

Anopheles gambiae adults are active at night, with peak hours of activity from after midnight to 4:00 am, with activity continuing until just before dawn. The females prefer to bloodfeed indoors, but outdoor feeding also occurs. Anopheles gambiae are anthropophilic, meaning they prefer to bloodfeed on humans as opposed to other animals. This makes them very efficient vectors of the human malaria parasites and contributes to their status as one of the most important malaria vectors in the world.

Adult females lay their eggs on the surface of water in a variety of aquatic habitats, but prefer shallow sunlit pools of standing water. Larvae hatch from eggs and develop within the aquatic habitat. Studies have shown that Anopheles gambiae larvae can develop in permanent man-made structures such as concrete tanks and drainage canals and natural pools such as swamps, hoof prints and marshes.

As for all mosquitoes, the larvae of Anopheles gambiae pupate after the fourth instar once they acquire an appropriate amount of nourishment. The pupal stage is a non-feeding, mobile stage, during which the mosquito’s adult body is formed. Anopheles gambiae can develop from egg to adult in 10-11 days, but development is temperature dependent and can take as long as three weeks under colder conditions.

Seasonal abundance of Anopheles gambiae varies depending on location, but generally the population decreases during the dry season and peaks during the wet season. Populations begin to increase as the rainy season commences, peak in mid-season, and decline as water levels stabilize and aquatic predators establish themselves.

Medical Significance.

Danger of mosquitoes.In Africa in 2010, there were an estimated 174 million cases of malaria and 596,000 deaths (WHO 2012). Approximately 90% of all malaria deaths occur
in Africa (WHO 2012). Anopheles gambiae are considered to be one of the world’s most important human malaria vectors because of their susceptibility to the Plasmodium parasite, their preference for humans as a host and their indoor-feeding behavior. Due to their short development time and their preference for developmental habitats near human dwellings, Anopheles gambiae are considered effective vectors of human malaria, as well as lymphatic filariasis (elephantiasis).

Malaria.

It is important to note that the mosquitoes themselves do not cause the malaria disease. Malaria is caused by a protozoan parasite in the Plasmodium genus that the mosquitoes transmit through blood feeding. Different species of Plasmodium infect different vertebrate groups, including humans, other mammals, birds and reptiles. Not only is each Plasmodium specific for the vertebrate, many are transmitted to the non-human vertebrate by different mosquito species; for example, mosquitoes in the Culex genera are important vectors for transmission of Plasmodium among birds. Human malaria is caused by four human-specific species of Plasmodium: Plasmodium falciparum, Plasmodium malariae, Plasmodium ovale and Plasmodium vivax (WHO 2013), plus one zoonotic species, Plasmodium knowlesi, known from monkeys in southeastern Asia. The parasites are transmitted in the saliva of female Anopheles mosquitoes. When a female Anopheles mosquito takes a blood meal from an infected human, she ingests the Plasmodium parasite and, after a certain developmental time within the mosquito, she can transmit the parasite through subsequent blood feedings.
The human malaria disease is characterized by sudden fever and chills at regular intervals. Other symptoms can include fatigue, headache, and sweating after each fever.

Management.

Various management techniques are used to control mosquito populations and limit malaria transmission in an area. Effective and currently used management practices include education of the community about malaria and the role of mosquitoes in transmission, house and environment modifications to prevent mosquito entry and to reduce larval development site availability, and the use of bed nets, spatial repellents, and indoor residual spraying (IRS) of insecticides. Proposed management practices include the introduction of biological controls such as predators, sterile insect technique (SIT), and the release of genetically-modified mosquitoes.

Greatly assisted by multiple organizations such as The President’s Malaria Initiative and The Bill and Melinda Gates Foundation, the distribution of insecticide-treated bed nets in Africa has profoundly decreased the incidence of malaria. About 145 million treated bed nets were delivered to sub-Saharan Africa in 2010 alone.

Heavy reliance on insecticides in an area can lead to the development of insecticide resistant mosquito populations . Studies in eastern Uganda have found that Anopheles gambiae are extremely resistant to pyrethroid insecticides. Such high resistance levels could decrease the efficacy of pyrethroid treated bed nets. For this reason, studies monitoring resistance to insecticides in mosquito populations should be conducted regularly.
Modifications of the home and environment also can lead to dramatic changes in mosquito abundance. Screens placed over doors and windows can prevent mosquito entrance, reducing malaria cases in an area. Making small environmental changes to decrease the number of breeding sites also can lead to substantial decreases in mosquito populations. The periodic draining of rice fields and fish ponds was a very effective method used in Java, Indonesia.

Wizz Africa hopes that this chapter “The danger of mosquitoes” can contribute to stop the spread of malaria.