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Thou Shalt Not Suffer a Mosquito to Live.
Nasty little blighters really, mosquitoes. To most people in civilised areas, mosquitoes are merely considered pests. They buzz in our ears when we are trying to sleep. They suck our blood, leaving an irritating itch. They drive us to distraction by flying around inside our cars as we're trying to concentrate on driving. However, something you may not know is they have the deadliest bite known to man. A shark might bite you in half, lions can tear you limb from limb and a snake can inject you with a toxin that will leave you a quivering mass in minutes. Nevertheless, they just don't have the numbers. More people die every year from the bite of the lowly mosquito than the bites of all other beasts put together.
What Exactly Is A Mosquito?
In Spanish and Portuguese, the name 'mosquito' means 'little fly', and was first used in North America around 1583. The English refer to them as 'gnats', the Germans call them 'stechmucken' and the French refer to them as 'les moucherons'. The mosquito is a member of the order Diptera, or 'true flies', since they have two wings, and the family Culicidae. However, mosquitoes are different from most other flies as their wings have scales. These scales reduce friction over their surfaces, increasing the efficiency of the mosquito's relatively undersized wings. There are over 2,700 species of mosquito. The most common mosquitoes belong to the genera Aedes, Anopheles and Culex, all of which are disease carriers. Only female mosquitoes are biters. Both male and female mosquitoes feed primarily on plant nectar, but the female needs a blood meal to provide protein for her eggs. Mosquitoes, like other insects, have triple-sectioned bodies and compound eyes. Their legs extend from the middle section, called the 'thorax'. A mosquito's headgear consists of a pair of antennae (which act as feelers, used in sensing direction) and a tube-like funnel called a 'proboscis1', with which it wreaks wanton havoc upon mankind and other similarly unfortunate prey. Unlike many other flying insects, mosquitoes have a pair of vibrating knobs at the back of their wings instead of a second pair of wings. These knobs - called 'halters' - help mosquitoes to balance when they fly. One can differentiate the males from the females by the male's fluffy antennae.
The mosquito goes through four stages before she becomes an adult and starts sucking our blood. The first stage is as an egg. Culex and Anopheles mosquitoes lay their eggs in 'rafts' of up to 200 eggs which float on the surface of stagnant or still waters. Aedes lay their eggs in damp soil where they will await flood waters. All species of mosquito eggs hatch in water. Most will hatch within 48 hours; some species can remain dormant through the winter freezes to hatch with the spring thaws.
The larvae, often called 'wigglers', hatch from the eggs. Most species will hang upside down from the water's surface with a breathing tube, or siphon, extending from their abdomens. Anopheles mosquito larvae do not have these siphons; instead, they lie parallel to the surface, breathing through holes in their abdomens. Some other species attach themselves to aquatic plants to obtain their oxygen. During this stage the larvae feed by filtering the water for organic materials. The larva will shed its skin, or moult, four times, growing to a length of 0.5 to 2cm before becoming a pupa.
During their pupal stage, mosquitoes do not feed, but are quite active. They usually float at the surface, breathing through two 'trumpets' and will react to light and movement by diving and swimming away. The pupae, or 'tumblers' as they are called, will remain in this stage for one to four days, depending on water temperature and species.
After the pupa has fully developed, it will use air pressure to burst open its pupal case. The adult will emerge on the surface of the water and scurry away to find a safe place to let its exoskeleton and wings dry and harden. Then, it will fly away to find a mate as quickly as it can before it feeds. While the males typically live for only a few days after mating, the females can live for several days or even a couple of weeks, repeating the mating, feeding and egg-laying cycle several times.
If there is water nearby, there are probably mosquitoes. They are found at altitudes from sea level to 3,600m (10,800ft). From Tierra Del Fuego to Anchorage, Alaska, from the tropical rainforests to the Siberian2 tundra, mosquitoes are buzzing and biting. While most mosquitoes never fly further than a mile from where they were hatched, some, like the Florida salt marsh mosquito, can migrate 75 to 100 miles. Currently, mosquitoes from Papua New Guinea ride the winds across the Torres Strait Islands into the Cape York Peninsula of Australia. While Australia already has plenty of mosquitoes of its own, these are bringing Japanese Encephalitis with them.
Many species are specifically adapted to their environments. There are species acclimated to nearly every type of environment on earth, except the driest deserts and the Antarctic. A mosquito used to prowling the temperate forests would not survive long in more arid or colder regions. However, of course, she will have cousins who love it there. Mosquitoes are usually most active around dusk and dawn. This is when their appetites are at their most voracious. This is partly due to the mosquitoes' prey being active at this time, but also because many species tend to dry out a bit in the middle of the day and become somewhat lethargic.
A Mosquito's Bite
Before a mosquito can bite you, it needs to find you. The mosquito comes well-equipped for this task. A mosquito's first clue to your whereabouts is your scent. On the mosquito's antennae are chemical receptors that sense primarily for carbon dioxide expelled from the breath of its prey. It specifically senses for high concentrations of CO2 to differentiate prey from the background levels of CO2 in the environment. Mosquitoes are also attracted by certain alcohols such as octenol, which is created by fermenting vegetable matter in the bodies of herbivores such as cattle stocks, and by lactic acid which is created in the muscles. Mosquitoes are also attracted by other chemicals in our skin, which is why some people are bitten more often than others. People who do not sweat as profusely as others are not bitten as often. A mosquito can detect its prey by scent from as far as 36m (100ft) away. While a mosquito's vision is not very good, they can see contrast3 and movement from about ten metres (32 feet) away. As the mosquito gets closer it then uses infrared sensors to detect body heat4.
Now that the mosquito has found her prey, she uses her highly-adapted mouth to obtain the blood meal she'll need to produce eggs. The mosquito will insert her proboscis through the skin to extract blood. She doesn't always get it on the first try and may make repeated stabs, which increases the irritation of the skin. Once she does strike blood, she secretes saliva into the skin. The saliva contains anticoagulants to keep the blood from clotting. The saliva may also carry any of the several diseases mosquitoes transmit. A typical meal will run to 5 microlitres of blood. Along with the irritation of the skin caused by the stabbing of the proboscis, an immune reaction to the saliva occurs and is the cause of most of the pain associated with a mosquito bite. The red swollen patch at the site of the bite is referred to as a 'wheal'. The body's immune system reacts to the saliva by sending histamines and white blood cells to the site. Until the saliva is broken down, the itching will continue. Washing the wheal with soapy water and applying anti-itch ointments, such as Calamine, can alleviate some of this.
The Diseases They Spread
If mosquitoes were merely content to feed on our blood, they would be, at most, a nuisance to mankind. However, nature also has the sense of humour to make mosquitoes vehicles for the transmission of a number of diseases that range from severely inconveniencing to deadly.
Named after a Spanish version of the Swahili phrase ki denga popo, or 'sudden possession of a person by a spirit', the dengue virus is responsible for the most important arthropod-borne viral diseases in the world, which causes between 50 and 100 million cases worldwide every year. This is an appallingly high rate of infection if you consider the fact that there are approximately 2.5 billion people living in dengue-epidemic areas5. The virus is spread by two species of mosquitoes - Aedes aegypti and Aedes albopictus - and causes two forms of disease: dengue fever and dengue haemorrhagic fever.
Classic dengue fever (DF) is, happily, usually a self-limiting infection with an incubation period of three to 15 days, and is characterised by the sudden onset of fevers, chills, muscle aches6, headaches and malaise. There is usually an interim period between the onset of the symptoms and the second phase, during which time the patient may begin to feel better. Younger children who are infected do not usually display symptoms of infection or, if they do, do not experience the muscle and joint pain associated with this disease.
Dengue haemorrhagic fever (DHF), however, is another kettle of fish. It is the most serious manifestation of dengue infection, and its victims are usually those under 15 years of age. These victims will generally suffer high fever that may hit 40-41°C, violent muscle contractions, hypovolemia (a blood disorder characterised by a decrease in the volume of circulating blood), thrombocytopoenia (a decrease in the number of platelets in the blood), haemorrhage and shock. There are four grades of DHF:
- Grade I, in which the victim suffers the symptoms of normal DF along with thrombocytophoenia and haemoconcentration due to capillary leakage.
- Grade II, where the patient may begin to ooze blood.
- Grade III, where the victim suffers from circulatory collapse due to the haemorrhage.
- Grade IV, in which pulse and blood pressure may be so low as to be undetectable.
While people in the Western world may take comfort in the knowledge that they are at very low risk of being attacked by A aegypti since it cannot tolerate cold winters, perhaps it would be pertinent to also point out that it was the mosquito A albopictus7 that invaded the United States in the 1980s, when they sneaked into the country from Asia in water collecting in used car and truck tyres shipped to the US for recapping.
Rift Valley Fever
Rift Valley Fever (RVF) is yet another deadly disease caused by an arthropod-borne virus called Phlebovirus, which is spread by a variety of mosquitoes and other insects. The virus was first identified after an epidemic amongst sheep in the Rift Valley, Kenya, in 1930, and has been the cause of several outbreaks in sub-Saharan and North Africa ever since. However, it was not until after the Aswan High Dam8 was constructed in Egypt in 1971 that this disease emerged as a human epidemic; it has caused periodic epidemics in that region ever since. In September, 2000, it finally escaped the borders of Africa and spread to Saudi Arabia and Yemen. Some see this as a sign that the disease may eventually spread to Asia and Europe.
RVF is primarily an animal disease. Many different types of animals are susceptible to the disease, especially domestic animals and livestock. Over 90% of lambs infected with RVF succumb to the disease, and the abortion rate among infected ewes is almost 100%. The virus can also easily cross over to humans via mosquitoes or through contact with contaminated body fluids. Within six days, human patients will display flu-like symptoms, and may develop neck stiffness, photophobia (an aversion to light) and vomiting. This can cause the disease to be mistaken for meningitis in the early stages. The disease is, fortunately, usually self-limiting, and can be treated with the drug ribavirin; however, a small proportion of individuals who come down with this disease may develop a more severe disease with symptoms such as eye disease, meningoencephalitis (inflammation of the brain and surrounding tissue) or haemorrhagic fever. The fatality rate for those who develop haemorrhagic fever is almost 50%.
The success of this nasty little virus depends greatly upon the part played by its vectors. For example, the infamous Aedes mosquito will acquire the virus during a blood meal and harbour the virus in her ovum. Then the virus is passed on to her offspring, which will play host to the virus until they pass the pathogen on to a human or animal host. This provides the virus with a survival mechanism, as the eggs of these mosquitoes are hardy and may survive for several years in inhospitable conditions, before the rainy season makes conditions suitable for them to hatch.
There is a reason why dog-lovers bring their pets for yearly vaccinations: canine heartworm. This nasty disease is caused by a roundworm called Dirofilaria immitis and can be life-threatening at its worst. Like the malaria and Rift Valley Fever pathogens, this virus requires both a mammal9 host and mosquito host to complete its life cycle.
An infection begins when a mosquito carrying the worm bites a dog or similarly susceptible host, depositing the worms in the skin. These worms make their way into the bloodstream where they mature into adults, and then travel to the right side of the heart through a vein. Then they multiply, and multiply, and multiply even further, infesting the chamber of the right side of the heart and the lung arteries, sometimes even lodging themselves in the veins of the liver and veins entering the heart. The disease progresses from faint coughing to extreme fatigue, difficulty in breathing, weight loss and even bloody cough. Unless treatment is started in time, congestive heart failure will follow, and the animal will eventually die.
Encephalitis refers to acute inflammation of the brain. Although some encephalitis may be caused by chemical poisoning, the majority of encephalitis cases are caused by viruses and many are spread by - yep, you've guessed it - mosquitoes.
There are two classes of viruses that cause mosquito-borne encephalitis - togaviridae and flaviviridae. Alpha viruses from the family togaviridae are responsible for equine encephalitis, which is named by geographic region: Western equine encephalitis (WEE - western US and Canada), Eastern equine encephalitis (EEE - eastern US) and Venezuelan equine encephalitis (VEE - South and Central America, and southern US). The flaviviridae viruses also cause encephalitis, with names based on geographic location, the most famous ones being Japanese B encephalitis and St Louis encephalitis. These are usually spread by Culex and Culiseta mosquitoes. Since 1999, West Nile Virus (similar to St. Louis encephalitis) has been spreading throughout almost all of the contiguous US, infecting humans, birds, horses and other mammals. Most people infected with WNV exhibit flu-like symptoms. Extreme cases can lead to West Nile encephalitis, meningitis or meningoencephalitis.
Encephalitis progresses very quickly from general symptoms such as headache, fever and quickened pulse to confusion, stupor and coma. Death may occur within four hours if the person is not treated in time, especially if the case is misdiagnosed when concurrent with an influenza epidemic. Even when treated, the disease may cause after-effects such as retardation, epilepsy, hydrocephalus (enlargement of the head caused by the accumulation of fluid) and deafness, depending on at what stage therapy is begun.
The infamous Aedes aegypti is famous for transmitting another dreadful virus infection - Yellow Fever, which occurs predominantly in the southern US, Latin America and Africa. Characterised by hepatitis with jaundice, fever, headache, muscle pain, nausea and vomiting (which may be black in colour due to mucosal bleeding), this disease struck down workers at the Panama Canal at the turn of the last century. It is an excellent example of what happens when there is no adequate, effective mosquito control at places where urbanisation of the tropics10, and therefore an increase in air travel, occur. The mortality rate for this disease is between five and ten percent, and death usually occurs between four and eight days after the onset of symptoms. Happily, most people spontaneously recover from this disease and gain lifelong immunity.
Not so Hideyo Noguchi, the man who argued vehemently that Yellow Fever was caused by a spirochaete11 bacterium, not a virus. He went down to Accra to scrutinise Yellow Fever blood samples for evidence of his bacteria, and was later struck down by the Yellow Fever virus. His last words were, apparently, 'I don't understand.'
If there is anything worse than a deadly disease caused by a virus and spread by a mosquito, it is a disease with gruesome manifestations caused by a parasite and spread by a mosquito.
Elephantiasis (also known as filariasis) is an example of Mother Nature's gruesome sense of humour. This disease is spread by the Culex mosquito, which thrives in places with filthy, stagnant water. The culprit for this disease, however, is an abominable worm called a filaria. There are two significant species: Wuchereria bancrofti, which causes filariasis in the Pacific Islands and Africa, and Brugia malayi, which is endemic in Southeast Asia. These parasites mature in the lymphatic vessels and lymph nodes of the genitals and lower extremities, then mate and produce equally nasty offspring that enter nearby blood vessels. If you're lucky enough to get a small infection, you may only wind up with enlarged lymph nodes and headaches. If you're unlucky enough to be infected multiple times, you get dead parasites that clog up your lymph nodes and form fibrous tissue, (caused by an allergic reaction to the filaria, be it dead or alive) around their corpses, plugging up your lymphatic systems so that your legs and genitals become grotesquely swollen from the accumulation of tissue fluids and covered with thick scaly skin, thus making it look as though you have elephant body parts. One man afflicted with this disease was unable to move unless he placed his testicles, which had swollen to the size of watermelons, in a wheelbarrow and carted them around before him.
Although this disease is curable by a drug called diethylcarbamazine, the lymphatic damage usually requires surgical correction. People who have little or no access to medical services, however, are often disfigured for life, and are subsequently ostracised. Also, although the disease itself is not life-threatening, the intense and unrelenting itch caused by the disease can be so severe that there are those who choose to commit suicide rather than cope with it.
This is probably the most well-known disease of the tropics. Almost everybody who was posted to a tropical country back in the British colonial times caught it, or at the very least knew somebody who had. It was probably the most fashionable as well - everybody and anybody who was writing a book about the humid paradise that was Asia was expected to devote at least a few chapters to this malady12.
Despite all the careless patter about mosquito bites and quinine shots, however, malaria is far from a disease that should be taken lightheartedly. An endemic disease throughout the world, malaria is estimated to cause between one and three million deaths a year, although the exact number is unknown, owing to the fact that most fatalities occur in third world countries. 90% of the deaths caused by malaria occur in sub-Saharan Africa, the majority of which are young children. History has documented the wave of malaria in 79 AD that swept across Europe, devastating Greece and Italy and severely crippling the Roman Empire. When Columbus crossed the ocean to America, he and his crew unwittingly brought the deadly disease with them. The disease quickly became endemic all the way from Chile to Montreal. This is clearly a disease to be reckoned with.
Malaria is caused by a type of sporozoan13 called Plasmodium, of which there are four species: Plasmodium falciparum, P malariae, P ovale and P vivax. Of these four, P falciparum is the most deadly, causing over 95% of all malarial deaths, although all four have similar life cycles. Spread by the Anopheles mosquito (which predominantly attacks by night), these parasites enter the human bloodstream and make their way to host liver cells where they enlarge and divide many times to form more parasites. Eventually these infected cells burst, releasing armies of the parasite into the bloodstream, where they then proceed to infect red blood cells and continue proliferating. These cells will also burst, releasing more parasites into the bloodstream where they will continue with the senseless slaughter of red blood cells until they are stopped or until the host is dead.
It should be explained at this point that the Anopheles mosquito is not simply a vehicle of transmission for malaria, but it also plays a major role in the life cycle of the malaria parasite. When she preys upon an infected host, the Anopheles mosquito brings the parasite into herself. Inside this mosquito, the sporozoan mates and produces offspring to complete its life cycle. It is then the mosquito that provides a place for these parasites to mature into infective parasites (sporozoites) that will subsequently be transferred to many hosts.
The first signs of malaria are a low-grade fever, headache and anorexia - these quickly progress to chills, fever, flushed skin and pain in the back and limbs as the parasites burst the victim's red blood cells. Sweating is common as the fever subsides. If left to progress, the disease may become severe enough to cause anaemia and, where there is no access to medication, even coma and death.
Happily, malaria is no longer a serious problem in the United States, following the introduction of DDT, and the disease is curable by medications such as choloroquine, mefloquine and doxycycline. These drugs have replaced quinine, which was found to be toxic and responsible for causing deafness. However, DDT-resistant strains are now on the rise, as are drug-resistant ones, and we would do well not to dismiss the potential threat of malaria in the future!
Even if mosquitoes weren't the horrible knaves of infectious diseases that they are, they would still be perfectly capable of causing us bodily harm, if only indirectly. To put it shortly: a mosquito bites a human. The human frantically scratches the bite area in an attempt to relieve the itch. The skin around the bite area breaks, thus conferring a breach in the body's first line of defence and allowing pathogenic microbes to enter the body. The bite area becomes septic and infected. When was the last time you gave that mosquito bite a satisfying scratch?
One Last Note on Disease
To those of you who are afraid for your lives: please note that AIDS cannot be transmitted by mosquitoes. This is because any organism that is transmitted by insects must be able to survive inside its insect host to begin with. Fortunately, the HIV virus is digested by mosquitoes (who consider the virus to be food), which means that its life cycle ends right there. Anyway, mosquitoes do not ingest enough HIV particles to transmit AIDS by contamination. These mosquitoes may be nasty little blighters, but they're certainly not living hypodermic needles.
There's nothing more dangerous than a wounded mosquito.
- Monty Python
The best time to kill mosquitoes is when they're young. The best way to do this is to remove their breeding habitats. Old tyres, trash bins, tin cans, etc. should be dumped or destroyed. Tyres are especially bad since they retain water very well. If possible, cut or drill holes into the sidewalls to allow drainage. Tyres transported for recycling are believed to be what the Asian Tiger Mosquito used to enter the US in Houston, Texas. The Asian Tiger Mosquito, so called because of its distinctive black and white stripes, also used this method to get to Jacksonville, Florida, and has also been found in Canada. Also clear debris from gutters, change the water in wading pools and bird baths often, fill or drain holes in trees and stumps and arrange tarps and covers so that they drain. If there are ditches in your area that do not drain, they should be reported to mosquito control authorities or public health authorities. Do not fill or drain these areas on your own since they may fall under wetland protection regulations. If you have decorative ponds or water features, the introduction of top-feeding predacious minnows or other similar fish native to your area can be effective.
The use of larvacides can be quite effective but also runs the risk of causing substantial environmental damage. For instance, the application of petroleum oils leaves a film on the surface of water through which the mosquito larva cannot extend their siphons, thus causing them to suffocate. However, this film can also kill other insects and fish the same way. Some municipalities fog sewer and drainage systems, which shouldn't contain much aquatic life, with petroleum exhaust to limit mosquito larvae.
The use of insecticides should be left to mosquito control departments. These organisations use several integrated control measures such as source reduction, chemical and biological control methods, and monitoring in concert to control mosquito larvae and adults with minimal impact on the environment. Home-use insecticide sprays and foggers kill insects indiscriminately and are therefore not recommended.
Bug zappers have been popular for the last few decades. These devices use ultraviolet light to attract insects through an electrically-charged grid. As the insects pass through the grid they are given a strong electric shock which explodes the insect. While this seems like a fair treatment for mosquitoes, these devices do little to control them. Bug zappers destroy mostly beneficial insects and rarely kill significant numbers of mosquitoes and biting flies. Also, if they are located near picnic areas, the explosion of so many insects could lead to contamination of foods prepared near them.
New devices on the market use CO2, octenol and other chemicals to target female mosquitoes. While these mosquito traps do kill significant numbers of mosquitoes, they may attract much greater numbers into an area than they can trap. Some studies show that people using these devices near their homes may be bitten more often that those who do not. These devices are usually expensive both in initial cost and in power cost as most of these either require an electrical connection or burn propane to create CO2 and to power fans. People interested in their use should research each device thoroughly before investing in one. Much of the independent research on these devices is currently inconclusive.
The erection of bat14 and martin houses should be encouraged for their own reasons. Unfortunately, while bats and purple martins do eat mosquitoes, they don't consume enough to provide significant mosquito control. Bats and martins are indiscriminate insect-eaters and will consume beneficial insects as well as mosquitoes and biting flies. As fliers, bats and martins have to economise their energy and will prefer to eat a few larger insects rather than many small mosquitoes.
Repellents, when used properly, are highly effective in protecting us from mosquitoes. The most common repellent in use is N,N-diethyl-meta-toluamide (DEET). DEET has been in use for over 45 years and has proven itself to be more effective than any other insect repellent. However there are some things to be aware of when using DEET:
- You should always follow the label instructions when applying, especially when applying it to children.
- It should not be over-applied. A light application is just as effective as a thick application and minimises any adverse effects.
- It should never be applied to the eyes, nose, and mouth or to cuts, abrasions or irritated skin. Do not apply DEET or other repellents to children's hands as they may rub their eyes or touch their mouths.
- Use the lowest concentration of DEET that will be effective for your exposure to mosquitoes. When mosquito exposure will be brief and light, 10% DEET is adequate. When spending extended time in heavily-infested regions, a concentration of 20% to 30% may be required.
- Do not use DEET-based repellents with sunscreen or sunblock as the effectiveness of both are reduced significantly when used together.
Permethrin is a repellent and insecticide for use on clothing and mosquito netting. Permethrin should not be applied to the skin as it is ineffective in this usage and is a neurotoxin. After drying on clothing, permethrin bonds to the fibres of the clothing and remains an effective insecticide and repellent for extended periods of time. Permethrin-treated clothing will retain its properties through several washings and when worn with DEET applied to the skin can very effectively protect the user from mosquito bites. As always, carefully follow label instructions before using any repellent or insecticide.
Citronella oil is touted as a safe, alternative mosquito repellent. When applied to the skin, citronella, and other plant oil-based repellents, do not provide nearly as much protection as DEET. Citronella candles and torches do repel mosquitoes, but only directly in the smoke plumes they provide. Any part of the body not in the smoke plume will not be protected. Planting citronella plants around a house does not create enough oil to act as an effective repellent.
In April, 2005 the US Centers for Disease Control began suggesting two alternatives to DEET based repellents, picaridin and oil of lemon eucalyptus. Both have been shown to provide protection similar to that provided by low concentration DEET formulas; both also avoid some of the negative aspects of DEET based formulas; and neither of them have the bad odour many associate with DEET repellents, nor do they have an unpleasant skin-feel. Many users do not like DEET repellents because it can harm plastics found in clothes and in artificial fingernails. Also many users may be more apt to use repellents based on oil of lemon eucalyptus as it is a natural substance. With these alternatives, the US CDC are hoping that more people will use repellents to control the spread of diseases, particularly West Nile Virus.
Considering the diseases that it spreads and the resultant number of deaths, the economic costs both in livestock and public funds spent on its control and the mere irritation and pain its bite inflicts, it's little wonder that the mosquito is one of the most loathed creatures on planet Earth. When you consider the fact that a mosquito hovering near you could directly produce thousands of offspring, you should show little hesitation as you reach for the fly swatter or rolled-up newspaper. Unless you have the reflexes of Pat Morita, from The Karate Kid,do not use chopsticks as the mosquito will likely escape. If you don't have anything else, swat it with your hand; you can wash it off afterwards. Whatever you do, don't let it get away. That mosquito must die!
- The American Mosquito Control Association.
- The United States Department of Health and Human Services Centres for Disease Control and Prevention
- The How Stuff Works Website
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- Trattler. 1997. Clinical microbiology made ridiculously simple. McGraw-Hill International Editions
- Madigan, T, JM Martinko and J Parker. 1997. Brock Biology of Microorganisms. Prentice Hall, USA
- Shimeld, LA and AT Rodgers. 1999. Essentials of Diagnostic Microbiology. Delmar Publishers, USA
- Zimmerman, BE and DJ Zimmerman.1996. Killer germs: Microbes and diseases that threaten humanity. Contemporary Books, Inc, Illinois
- Dog Owners' Guide: Heartworm Disease
- WHO Information Fact Sheet: Rift Valley Fever