Mosquito Habitats — Mosquito World

Mosquito Habitats

Mosquitoes can live in almost any environment, with the exception of extreme cold weather. They favor forests, marshes, tall grasses and weeds, and ground that is wet at least part of the year. Because they must have water in order to thrive, their habitats break down into two basic types:

Permanent water mosquitoes tend to lay their eggs in clumps, called rafts, of 50 to 300 on the surface of standing water at the edges of lakes and ponds and among the vegetation in swamps and marshes. Some species prefer clean water, while Culex pipiens, the northern house mosquito, prefers stagnant or polluted water.

Culex and Anopheles mosquitoes are among the most common permanent water mosquitoes. These mosquitoes are most active when the average temperature is above 70 degrees. Their eggs must stay in water in order to survive and usually will hatch within a couple of days, releasing larvae to begin the development process.

Many permanent water mosquitoes can also breed in containers that collect and hold water, such as wading pools, buckets or toys left outside.

Floodwater mosquitoes lay their eggs in moist soil. The eggs, as many as one million per acre, will dry out as the ground does, then hatch when rains saturate the ground and water levels begin to rise. Floodwater habitats include:

  • Drainage ditches that fill during storms.
  • Woodland pools created by melting snow, or spring and early summer rains.
  • Floodplains along the banks of streams and rivers.
  • Irrigated pastures and fields.
  • Meadows and other soft ground where depressions form.

Common species include the Aedes vexans, also known as the inland floodwater mosquito. Mosquitoes that breed in floodwater habitats usually become a problem about seven to 10 days after a heavy rain, and subside in about a week or two.

Floodwater mosquitoes also breed in containers. Aedes albopictus, the Asian tiger mosquito, prefers the insides of old tires where dirty water collects, and Aedes triseriatus prefers treeholes that gather rainwater.

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Malaria mosquito — features of life and development, nutrition, habitat, enemies, danger to humans

Anopheles gambiae Giles is the most efficient vector of human malaria in the Afrotropical Region (CDC 2010). Thus, it is commonly called the African malaria mosquito. The Anopheles gambiae complex of sibling species (White 1974; Fanello et al. 2002; Coetzee et al. 2013) comprises eight reproductively isolated species that are almost indistinguishable morphologically: Anopheles amharicus Hunt et al. 2013, Anopheles arabiensis Patton 1905, Anopheles bwambae White 1985, Anopheles gambiae Giles 1902, Anopheles coluzzii Coetzee & Wikerson 2013, Anopheles melas Theobald 1903, and Anopheles merus Dönitz 1902. Collectively they are sometimes called Anopheles gambiae sensu lato, meaning ‘in the wider sense.’ None of these species occur in North America.

Figure 1. Female Anopheles gambiae Giles taking a blood meal. Photograph by Jim Gathany, CDC.

Synonymy (Back to Top)

Anopheles gambiae Giles, 1902 sensu stricto (White 1975; Mattingly 1977)

Anopheles gambiae species A (Davidson et al. 1967; White 1974)

Anopheles costalis Loew 1866 nomen dubium

Distribution (Back to Top)

Members of the Anopheles gambiae complex are found throughout tropical Africa (WHO 1989), south of the Sahara desert, with Anopheles arabiensis extending across southern Arabia. Anopheles gambiae s.s. is distributed throughout sub-Saharan Africa, including Madagascar (WHO 1989).

Figure 2. The approximate distribution of Anopheles gambiae s.s. Giles in tropical Africa. Diagram by Sabrina White, University of Florida, based on maps and data from Sinka et al. 2010.

Description (Back to Top)

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) (Foster and Walker 2009). Adult Anopheles also have a distinguishable resting position where their abdomen is raised into the air (Foster and Walker 2009). 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 (Gillies and de Meillon 1968). 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 (Gillies and de Meillon 1968).

Figure 3. Adult female African malaria mosquito, Anopheles gambiae Giles. Photograph by Lyle Buss, University of Florida.

Figure 4. Female Anopheles gambiae Giles taking a blood meal. Photograph by James Gathany, CDC.

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 (Gillies and de Meillon 1968). Similar to other Anopheles species, Anopheles gambiae lay their eggs singly and directly on the water, with each egg having floats on either side (Foster and Walker 2009). Anopheles eggs are not drought resistant (CDC 2010).

Figure 5. Unhatched (a) and hatched (b) eggs of Anopheles stephensi Liston, showing the characteristic singly laid Anopheles eggs with floats on both sides, and the split where the larva emerged. Photograph by Mark Benedict, CDC.

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 (Foster and Walker 2009). Anopheles mosquitoes develop though four larval sizes or instars before pupating (Foster and Walker 2009). 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 (Gillies and de Meillon 1968). They feed on organic matter and algae (Garros et al. 2008).

Figure 6. Larva of the African malaria mosquito, Anopheles gambiae Giles. Photograph by Ray Wilson, Bird and Wildlife Photography.

Pupae: Pupae of all mosquitoes are comma-shaped when viewed from the side (CDC 2010). 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 (Foster and Walker 2009). The pupal stage does not feed (Foster and Walker 2009).

Figure 7. Pupa of the African malaria mosquito, Anopheles gambiae Giles. Photograph by Ray Wilson.

Life Cycle and Biology (Back to Top)

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 (Foster and Walker 2009). Although adults can survive for up to one month in captivity, they usually survive around one to two weeks in the wild (CDC 2010).

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 (Gillies and de Meillon 1968). The females prefer to bloodfeed indoors, but outdoor feeding also occurs (Tuno et al. 2010). Anopheles gambiae are anthropophilic, meaning they prefer to bloodfeed on humans as opposed to other animals (White 1974). 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 (CDC 2010).

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Adult females lay their eggs on the surface of water in a variety of aquatic habitats, but prefer shallow sunlit pools of standing water (Gillies and de Meillon 1968). 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 (Mala et al. 2011) and natural pools such as swamps, hoof prints (Kweka et al. 2012), and marshes (Mala et al. 2011).

As for all mosquitoes, the larvae of Anopheles gambiae pupate after the fourth instar once they acquire an appropriate amount of nourishment (Foster and Walker 2009). 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 (Gillies and de Meillon 1968).

Seasonal abundance of Anopheles gambiae varies depending on location, but generally the population decreases during the dry season and peaks during the wet season (Gillies and de Meillon 1968, Yaro et al. 2012). Populations begin to increase as the rainy season commences, peak in mid-season, and decline as water levels stabilize and aquatic predators establish themselves (Gillies and de Meillon 1968).

Figure 8. An example of the sunlit pools that are ideal larval habitats for Anopheles gambiae Giles. Photograph by CDC.

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Malaria

Cause

Malaria is caused by the protozoan parasite Plasmodium. Human malaria is caused by four different species of Plasmodium: P. falciparum, P. malariae, P. ovale and P. vivax.

Humans occasionally become infected with Plasmodium species that normally infect animals, such as P. knowlesi. As yet, there are no reports of human-mosquitohuman transmission of such “zoonotic” forms of malaria.

Transmission

The malaria parasite is transmitted by female Anopheles mosquitoes, which bite mainly between dusk and dawn.

Nature of the disease

Malaria is an acute febrile illness with an incubation period of 7 days or longer. Thus, a febrile illness developing less than 1 week after the first possible exposure is not malaria.

The most severe form is caused by P. falciparum; variable clinical features include fever, chills, headache, muscular aching and weakness, vomiting, cough, diarrhoea and abdominal pain. Other symptoms related to organ failure may supervene, such as acute renal failure, pulmonary oedema, generalized convulsions, circulatory collapse, followed by coma and death.The initial symptoms, which may be mild, may not be easy to recognize as being due to malaria.

It is important that the possibility of falciparum malaria is considered in all cases of unexplained fever starting at any time between 7 days after the first possible exposure to malaria and 3 months (or, rarely, later) after the last possible exposure. Any individual who experiences a fever in this interval should immediately seek diagnosis and effective treatment, and inform medical personnel of the possible exposure to malaria infection. Falciparum malaria may be fatal if treatment is delayed beyond 24 h after the onset of clinical symptoms.

Young children, pregnant women, people who are immunosuppressed and elderly travellers are particularly at risk of severe disease. Malaria, particularly P. falciparum, in non-immune pregnant travellers increases the risk of maternal death, miscarriage, stillbirth and neonatal death.

The forms of human malaria caused by other Plasmodium species cause significant morbidity but are rarely life-threatening. Cases of severe P. vivax malaria have recently been reported among populations living in (sub)tropical countries or areas at risk. P. vivax and P. ovale can remain dormant in the liver. Relapses caused by these persistent liver forms (“hypnozoites”) may appear months, and rarely several years, after exposure. Relapses are not prevented by current chemoprophylactic regimens, with the exception of primaquine. Latent blood infection with P. malariae may be present for many years, but it is very rarely life-threatening.

In recent years, sporadic cases of travellers’ malaria due to P. knowlesi have been reported. Humans can be infected with this “monkey malaria” parasite while staying in rainforests and/or their fringe areas in south-east Asia, within the range of the natural monkey hosts and mosquito vector of this infection. These areas include parts of Cambodia, China, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Singapore, Thailand and Viet Nam. The parasite has a life-cycle of 24 h and can give rise to daily fever spikes occurring 9–12 days after infection. Symptoms may be atypical. Severe P. knowlesi malaria with organ failure may occur, and sporadic fatal outcomes have been described. P. knowlesi has no persistent liver forms and relapses do not occur. Travellers to forested areas of south-east Asia where human P. knowlesi infections have been reported should protect themselves against mosquito bites between dusk and dawn to prevent infection and take the usual chemoprophylaxis where indicated (see Country list).

Geographical distribution

The current distribution of malaria in the world is shown on the map in this chapter; affected countries and territories are listed both at the end of this chapter and in the Country list. The risk for travellers of contracting malaria is highly variable from country to country and even between areas in a country, and this must be considered in any discussion of appropriate preventive measures.

In many countries or area at risk, the main urban areas – but not necessarily the outskirts of towns – are free of malaria transmission. However, malaria can occur in the main urban areas of Africa and, to a lesser extent, India. There is usually less risk at altitudes above 1500 m, although in favourable climatic conditions the disease can occur at altitudes up to almost 3000 m. The risk of infection may also vary according to the season, being highest at the end of the rainy season or soon after.

There is no risk of malaria in many tourist destinations in south-east Asia, the Caribbean and Latin America.

Risk for travellers

During the transmission season in countries or areas at risk, all non-immune travellers exposed to mosquito bites, especially between dusk and dawn, are at risk of malaria. This includes previously semi-immune travellers who have lost or partially lost their immunity during stays of 6 months or more in countries or areas of no risk. Children who have migrated to countries and areas of no risk are particularly at risk when they travel to malarious areas to visit friends and relatives.

Most cases of falciparum malaria in travellers occur because of poor adherence to, or complete failure to use medicines, or use of inappropriate prophylactic malaria drug regimens, combined with failure to take adequate precautions against mosquito bites. Studies on travellers’ behaviour have shown that adherence to treatment can be improved if travellers are informed of the risk of infection and believe in the benefit of prevention strategies. Late-onset vivax and ovale malaria may occur despite effective prophylaxis, as they cannot be prevented with currently recommended prophylactic regimens which act only against blood-stage parasites.

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Malaria risk is not evenly distributed where the disease is prevalent. Travellers to countries where the degree of malaria transmission varies in different areas should seek advice on the risk in the particular zones that they will be visiting. If specific information is not available before travelling, it is recommended that precautions appropriate for the highest reported risk for the area or country should be taken; these precautions can be adjusted when more information becomes available on arrival. This applies particularly to individuals backpacking to remote places and visiting areas where diagnostic facilities and medical care are not readily available. Travellers staying overnight in rural areas may be at highest risk.

Precautions

Travellers and their advisers should note the four principles – the ABCD – of malaria protection:

  • Be Aware of the risk, the incubation period, the possibility of delayed onset, and the main symptoms.
  • Avoid being Bitten by mosquitoes, especially between dusk and dawn.
  • Take antimalarial drugs (Chemoprophylaxis) when appropriate, to prevent infection from developing into clinical disease.
  • Immediately seek Diagnosis and treatment if a fever develops 1 week or more after entering an area where there is a malaria risk and up to 3 months (or, rarely, later) after departure from a risk area.

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How Forest Loss Is Leading To a Rise in Human Disease

A growing body of scientific evidence shows that the felling of tropical forests creates optimal conditions for the spread of mosquito-borne scourges, including malaria and dengue. Primates and other animals are also spreading disease from cleared forests to people.

By Jim Robbins • February 23, 2016

In Borneo, an island shared by Indonesia and Malaysia, some of the world’s oldest tropical forests are being cut down and replaced with oil palm plantations at a breakneck pace. Wiping forests high in biodiversity off the land for monoculture plantations causes numerous environmental problems, from the destruction of wildlife habitat to the rapid release of stored carbon, which contributes to global warming.

But deforestation is having another worrisome effect: an increase in the spread of life-threatening diseases such as malaria and dengue fever. For a host of ecological reasons, the loss of forest can act as an incubator for insect-borne and other infectious diseases that afflict humans. The most recent example came to light this month in the Journal of Emerging Infectious Diseases, with researchers documenting a steep rise in human malaria cases in a region of Malaysian Borneo undergoing rapid deforestation.

This form of the disease was once found mainly in primates called macaques, and scientists from the London School of Tropical Medicine and Hygiene wondered why there was a sudden spike in human cases. Studying satellite maps of where forest was being cut down and where it was left standing, the researchers compared the patchwork to the locations of recent malaria outbreaks. They realized the primates were concentrating in the remaining fragments of forest habitat, possibly increasing disease transmission among their own populations. Then, as humans worked on the new palm plantations, near the recently created forest edges, mosquitoes that thrived in this new habitat carried the disease from macaques to people.

Such phenomena are not uncommon. “In years when there is a lot of land clearance you get a spike in leptospirosis [a potentially fatal bacterial disease] cases, and in malaria and dengue,” says Peter Daszak, the president of Ecohealth Alliance, which is part of a global effort to understand and ameliorate these dynamics. “Deforestation creates ideal habitat for some diseases.”

The Borneo malaria study is the latest piece of a growing body of scientific evidence showing how cutting down large swaths of forests is a major factor in a serious human health problem — the outbreak of some of the world’s most serious infectious diseases that emerge from wildlife and insects in forests. Some 60 percent of the diseases that affect people spend part of their life cycle in wild and domestic animals.

The research work is urgent — land development is rapidly taking place across regions with high biodiversity, and the greater the number of species, the greater the number of diseases, scientists say. They are deeply concerned that the next global pandemic could come out of the forest and spread quickly around the world, as was the case with SARS and Ebola, which both emerged from wild animals.

Mosquitoes are not the only carriers of pathogens from the wild to humans. Bats, primates, and even snails can carry disease, and transmission dynamics change for all of these species following forest clearing, often creating a much greater threat to people.

The risk of disease outbreaks can be greatly magnified after forests are cleared for agriculture and roads.

Throughout human history pathogens have emerged from forests. The Zika virus, for example, which is believed to be causing microencephaly, or smaller than normal heads, in newborns in Latin America, emerged from the Zika forest of Uganda in the 1940s. Dengue, Chikungunya, yellow fever, and some other mosquito-borne pathogens likely also came out of the forests of Africa.

Forests contain numerous pathogens that have been passed back and forth between mosquitoes and mammals for ages. Because they evolved together, these viruses often cause few or no symptoms in their hosts, providing “a protective effect from a homegrown infection,” says Richard Pollack of the T.H. Chan School Public Health at Harvard. But humans often have no such protection.

What research is demonstrating is that because of a complex chain of ecological changes, the risk of disease outbreaks, especially those carried by some mosquitoes, can be greatly magnified after forests are cleared for agriculture and roads.

A flood of sunlight pouring onto the once-shady forest floor, for example, increases water temperatures, which can aid mosquito breeding, explained Amy Vittor, an assistant professor of medicine at the University of Florida. She is an expert in the ecology of deforestation and malaria, which is where this dynamic is best understood.

Deforestation creates other conditions conducive to mosquito breeding. Leaves that once made streams and ponds high in tannins disappear, which lowers the acidity and makes the water more turbid, both of which favor the breeding of some species of mosquito over others. Flowing water is dammed up, deliberately and inadvertently, and pools. Because it is no longer taken up and transpired by trees, the water table rises closer to the forest floor, which can create more swampy areas.

As agriculture replaces forest, “re-growth of low lying vegetation provides a much more suitable environment” for the mosquitoes that carry the malaria parasite, Vittor says.

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A man sleeps inside a mosquito net in his home in West Papua, Indonesia. Ulet Ifansasti/Getty Images

The link between deforestation and increases in malaria has been known for some time, but research in the last two decades has filled in many of the details. Much of the work has been done in Peru, where in one region in the 1990s cases of malaria went from 600 per year to 120,000, just after a road was built into virgin forest and people began clearing land for farms.

The cascade of human-induced ecological changes dramatically reduces mosquito diversity. “The species that survive and become dominant, for reasons that are not well understood, almost always transmit malaria better than the species that had been most abundant in the intact forests,” write Eric Chivian and Aaron Bernstein, public health experts at Harvard Medical School, in their book How Our Health Depends on Biodiversity. “This has been observed essentially everywhere malaria occurs.”

Mosquitoes can adapt fairly quickly to environmental change. In response to a push to use bed nets to prevent nighttime bites in malaria-prone regions of the world, for example, researchers are seeing a change in the time of day mosquitoes bite — many now target their human quarry in the hours before bed.

A study by Vittor and others found that one malaria-carrying mosquito species, Anopheles darlingi, in a deforested area in Peru was radically different than its cousins in intact forests; the Anopheles darlingi in deforested areas bit 278 times more frequently than in an intact forest, according to a study published in the American Journal of Tropical Medicine and Hygiene in 2006.

“In the forest, we found almost no breeding whatsoever, and no biting by the adult mosquitoes,” Vittor said. That’s probably because the ecology of the deforested landscape — short vegetation and deep water — favored their breeding, and they need human blood to grow their eggs.

The types of mosquitoes that do well in this radically altered ecosystem are more “vector competent,” which means their systems are particularly good at manufacturing a lot of the pathogen that causes malaria. A study in Brazil, published in the Journal of Emerging Infectious Diseases in 2010, found that clearing four percent of the forest resulted in a nearly 50-percent increase in human malaria cases.

The ecology of the viruses in deforested areas is different. As forests are cut down, numerous new boundaries, or edges, are created between deforested areas and forest. A mosquito called Aedes africanus, a host of the yellow fever and Chikungaya viruses, often lives in this edge habitat and bites people working or living nearby. Other primates, which are also reservoirs for the pathogens, gather in the borders of these different ecosystems, providing an ongoing source of virus for the insects.

Insects are not the only way that deforestation can exacerbate infectious diseases. For some unknown reason, the species of snails that can better adapt to warm open areas that occur after a forest is cut down are better hosts for parasites called flatworms, some of which cause schistosomiasis, a disease which damages human organs.

Scientists are concerned that these outbreaks exacerbated by human alteration of landscapes could cause the next pandemic. The Roman Empire once stretched from Scotland to Africa and lasted for more than 400 years. No one knows exactly why the empire collapsed, but one contributing factor may have been malaria. A mass grave of babies from that era, excavated in the 1990s, found, through DNA analysis, that many of them had died from malaria, according to a study published in 2001 in the journal Ancient Biomolecules. Some researchers speculate that the malaria outbreak may have been exacerbated by deforestation in Rome’s surrounding Tiber River Valley to supply timber to the growing city.

One piece of the puzzle is to know what pathogens might come out of the forest in the future.

Once a disease has left a forested region, it can travel in human beings, crossing the world in a matter of hours by airplane before the person even shows symptoms. How well it does in its new homes depends on several factors. Once Zika traveled to Brazil from Africa, for example, it flourished because Aedes aegypti mosquitoes hang out around people and love to lay their eggs in small containers of water. Many people in Brazil’s large slums store water in buckets, and standing water also collects in tarps, old tires, and trash.

A key question about the Zika virus is whether it will enter the primate populations in South America, which means it might become a permanent resident and an ongoing source of infection. “Is it going to set up shop there?” asks Vittor. “We don’t know.”

Mosquitoes aren’t the only creatures that bring fever out of the forest. Angolan free-tailed bats were believed to harbor the Ebola virus that broke out and killed more than 11,000 people last year. And AIDS, which has killed more than 25 million people worldwide, came from people eating bush meat, likely chimpanzees.

A wild card in this disease scenario is the rapidly changing climate. If spring comes early, mosquitoes hatch earlier and summer populations are larger. In Southeast Asia, the spike in temperatures during El Niño weather cycles correlates with dengue fever outbreaks, because the warmer weather allows mosquitoes to breed faster and expand the population, which spreads the virus further, according to a study last year in the Proceedings of the National Academy of Sciences.

Part of the solution is to recognize and understand these connections and teach people that keeping nature intact has protective effects. And where people do cut down forests or build roads, numerous steps can be taken to lessen the chance of mosquito-borne disease outbreaks — education campaigns, more clinics, health training, and medical monitoring.

Another piece of the puzzle is to know what pathogens the world might be up against in the future as they come out of the forest. Ecohealth Alliance is cataloging wildlife-borne viruses in wild places where there is new encroachment into undisturbed nature and health care is poor or non-existent. The goal is to better understand how these viruses might spread and to potentially develop vaccines.

“If we could deal with the trade in wildlife and deforestation we wouldn’t need to stop an outbreak,” like Zika or Ebola, said Daszak, the organization’s president. “We would have already dealt with it.”

Jim Robbins is a veteran journalist based in Helena, Montana. He has written for the New York Times, Conde Nast Traveler, and numerous other publications. His latest book is the The Wonder of Birds: What they Tell Us about the World, Ourselves and a Better Future. More about Jim Robbins →

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