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Mosquito, Not Just A Nuisance In The Summer


This article discusses the insect mosquito and the diseases transmitted by them, specifically, West Nile Virus, Encephalitis and Malaria. Prevention of mosquito-borne diseases and their diagnoses are presented so the public may take proper precaution to avoid being infected or to get early treatment if were infected by the mosquito-borne disease.

Mosquito Information      

Mosquitoes are insects belonging to the order Dipteran, the True Flies. They have two wings, but unlike other flies, their wings have scales. The mouthparts of female mosquitoes form a long piercing-sucking proboscis which can pierce skin and suck blood. Males differ from females by having feathery antennae and mouthparts not suitable for piercing skin. While warm blooded animals, birds and human are prey to female mosquitoes, nectar is the principal food source for the male mosquitoes.

There are over 2500 different species of mosquitoes throughout the world, of which over 150 species occur in the United States and 73 species occur in Florida. Each mosquito has a Latin scientific name, such as Culex tarsalis. Culex is the "generic" name of a group of closely related mosquitoes and tarsalis is the "species" name. According to Tom Floore, The Spanish called the mosquitoes, "musketas," and the native Hispanic Americans called them "zancudos." "Mosquito" is a Spanish or Portuguese word meaning "little fly" while "zancudos," a Spanish word, means "long-legged." The use of the word "mosquito" is apparently of North American origin and dates back to about 1583. In Europe, mosquitoes were called "gnats" by the English, "Les moucherons" or "Les cousins" by French writers, and the Germans used the name "Stechmucken" or "Schnacke." In Scandanavian countries mosquitoes were called by a variety of names including "myg" and "myyga" while the Greeks called them "konopus." The Chinese call them "Wen-Ze".

Where Does Mosquito Come From?

The mosquito goes through four separate and distinct stages of its life cycle: Egg, Larva, Pupa, and Adult. Each of these stages can be easily recognized by its special appearance. The following diagram taken from Tom Floore's article shows the four stages which take place on non-turbulent water surfaces. Hence, any water surfaces such as in the small pond, marsh or swampy area and even water buckets are the breeding ground of mosquitoes. 

Facts About Mosquito?

Some facts about mosquitoes are collected below: 

1. Only female mosquitoes bite; using proteins from a blood meal to produce eggs. Most species can produce several batches of eggs
during their lifetime.

2. The mosquito's visual picture, produced by various parts of its body, is an infrared view produced by its prey's body temperature. So Warmer bodies attract more mosquitoes. 

3. The average life span of the female mosquito is 3 to 100 days; the male's is 10 to 20 days. 

4. Depending on species, female mosquitoes may lay 100 to 300 eggs at a
time and may average 1,000 to 3,000 during their lifespan. 

5. Worldwide, mosquito-borne diseases kill more people than any other single factor. In the United States, mosquitoes vector (spread) several types of encephalitis, dog heartworm, and malaria. World wide one million people die from malaria.

6. The larval and pupal stages can be found in a variety of aquatic habitats including: discarded containers, tires, temporary woodland pools, tree and
crab holes, salt marshes, and irrigation ditches.

7..Most mosquitoes remain within 1 mile of their breeding site. A few species may range up to 20 miles or more. 

8. Depending on temperature, mosquitoes can develop from egg to adult in as little as 4-7 days. So don't keep water standing around more than 3 days.

9. Presently, the cues used by mosquitoes to find their hosts are poorly understood. Carbon dioxide (CO2), heat, octenol and light have been
shown to be attractants. Other compounds tested such as lactic acid also have proved to attract certain species of biting insects. Bar-be-cue and grill in the woods are certainly attractive to mosquitoes.  .

10. Mosquitoes find new hosts by sight (they observe movement); by detecting infra-red radiation emitted by warm bodies; and by chemical signals (mosquitoes are attracted to carbon dioxide and lactic acid, among other chemicals). Mosquito can 'smell' you, or a cow or another host  20 to 35 meters away.

11. Letting a mosquito drink its fill may be smart, because if you brush it away too soon, it might come back for more. It is also possible to cause disease transmission (blood seeping from mosquito to human body) when the mosquito is slapped to death.

West Nile Virus (WNV) and Disease

West Nile virus (WNV) is a mosquito-borne virus and is closely related to St. Louis encephalitis (SLE) virus. SLE virus is most prevalent in the southeastern and mid-western United States. However, WNV is typically found in Africa, Europe, and Asia (primarily in countries bordering the Mediterranean Basin). Infection with this virus does not always result in a clinical disease. Studies have shown that normally only a small percentage of humans infected with the virus will show symptoms of disease. The general symptoms of West Nile fever, resulting from infection with WNV, range from fever, rash, and headache to meningitis, encephalitis, coma, and death. 

West Nile virus was first isolated from a febrile adult woman in the West Nile District of Uganda in 1937. The ecology was characterized in Egypt in the 1950s. The virus became recognized as a cause of severe human meningoencephalitis (inflammation of the spinal cord and brain) in elderly patients during an outbreak in Israel in 1957. Equine disease was first noted in Egypt and France in the early 1960s.  Recent outbreaks of WNV encephalitis in humans have occurred in Algeria in 1994, Romania in 1996-1997, the Czech Republic in 1997, the Democratic Republic of the Congo in 1998, Russia in 1999, and the United States in 1999. Epizootics of disease in horses occurred in Morocco in 1996, Italy in 1998,
and the USA in 1999, including Maryland, Connecticut, New Jersey, and New York City. Equine disease was clustered on Long Island, east of New York City. The appearance of WNV in North America in 1999, with encephalitis reported in humans and horses, may be an important milestone in the evolving history of this virus. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. After an incubation period of 10 days to 2 weeks, infected mosquitoes can then transmit West Nile virus to humans and animals while biting to take blood. The virus is located in the mosquito's salivary glands. During blood feeding, the virus may be injected into the animal or human, where it may multiply, possibly causing illness.  

In August and September, 1999, West Nile (WN) virus was recognized in the Western Hemisphere for the first time when it caused an epidemic of encephalitis and aseptic meningitis in the New York City metropolitan area. Intensive hospital-based surveillance identified 62 cases including 7 deaths in the region, with 9 cases residing within the outbreak's epicenter, an approximately 9 km2 area of the borough of Queens. To better estimate the public health impact of the epidemic, a household-based seroprevalence survey was conducted in this area. Last fall, the NY Department of Health (DOH), in collaboration with Center for Disease Control (CDC), conducted a door-to-door survey to determine how many people had been infected with West Nile virus in northern Queens. In the survey, which was anonymous, information was collected about recent illness and different types of exposure to mosquitoes, and a blood specimen was collected to look for evidence of a West Nile virus immune response. City blocks were selected at random from a 9 km2 area that encompassed parts of the neighborhoods of Auburndale, Linden Hill, Murray Hill, and Whitestone (mostly within Community Board 7), where the highest rates of West Nile encephalitis had  been found. The survey was intended to help answer the following public health question: "In the epicenter of the epidemic, what percentage of the population actually became infected with West Nile virus?" 

West Nile virus can cause a spectrum of illness which ranges from no symptoms to mild illness to fatal encephalitis. Previous studies of this infection have shown that people with severe disease represent the "tip of the iceberg" and that many more people will have had mild or asymptomatic illness. Therefore, to determine how many people were actually infected, more needs to be done than simply counting the severe cases. Blood tests can detect the presence of an immune response against West Nile virus, indicating the person had been infected; the tests can also determine whether the infection was recent. CDC reported to New York City DOH that 19 anonymous blood samples tested positive for evidence of previous infection with West Nile virus. Based on these results, it is estimated that approximately 2.6% (or between 1.2% and 4.1%) of the population of age 5 or greater in the surveyed area in northern Queens (population 46,220) had been infected with West Nile virus (or between 533 and 1903 individuals). Because the sampled area had the highest rate of West Nile encephalitis cases, researchers believe that infection rates elsewhere in the city were significantly lower. These findings, while not unexpected, demonstrate the potential impact of mosquito-borne diseases on a community, and the value of vigilance in monitoring and preventing the proliferation of urban mosquitoes. As described in the New York City West Nile plan, the New York City DOH is correctly emphasizing prevention -- by eliminating areas of standing water where mosquitoes can breed; using larvicides to kill mosquito larvae ("wrigglers", which live in standing water) before they can transform into adult mosquitoes; surveillance to rapidly detect infection in humans, animals, and mosquitoes; and education and outreach to the public. 

Serum samples were tested for two types of antibodies to West Nile virus: immunoglobulin M (IgM) and immunoglobulin G (IgG). The two tests help determine whether an individual was recently infected with West Nile virus (IgM antibodies) and whether an individual was ever infected with West Nile virus (IgG antibodies). Surveyed individuals whose blood tested positive for antibodies to West Nile virus (indicating a previous infection) were more likely to report recent illness characterized by fever, headache, muscle aches, joint pains, and fatigue, than persons who tested negative (30% vs. 11%).  The infection itself (and any associated illness) is short-lived. A person with antibodies to West Nile virus is likely to have life-long immunity to another infection with this virus. 

West Nile Encephalitis

"Encephalitis" means an inflammation of the brain and can be caused by viruses and bacteria, including viruses transmitted by mosquitoes. West Nile encephalitis is an infection of the brain caused by West Nile virus, a flavivirus commonly found in Africa, West Asia, and the Middle East. It is closely related to St. Louis encephalitis virus found in the United States. By the bite of a mosquito (primarily one of the Culex species) that is infected with West Nile virus, human can get West Nile Encephalitis. West Nile encephalitis is NOT transmitted from person-to-person. For example, one cannot get West Nile virus from touching or kissing a person who has the disease, or from a health care worker who has treated someone with the disease.

There is no evidence that a person can get the virus from handling live or dead infected birds. However, it would be smart to avoid bare-handed contact when handling any dead animals, including dead birds. Infected mosquitoes are the primary source for West Nile virus and caused the recent outbreak in the New York City metropolitan area. Ticks infected with West Nile virus have been found in Asia and Africa. Their role in the transmission and maintenance of the virus is uncertain. However, there is no information to suggest that ticks played any role in the New York area outbreak. 

The incubation period in humans (i.e., time from infection to onset of disease symptoms) for West Nile encephalitis is usually 3 to 15 days. Most infections of  West Nile encephalitis are mild and symptoms include fever, headache, and body aches, often with skin rash and swollen lymph glands. More severe infection may be marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis and, rarely, death. There is no specific therapy for treating West Nile encephalitis. In more severe cases, intensive supportive therapy is indicated, i.e., hospitalization, intravenous (IV) fluids, airway management, respiratory support (ventilator) if needed, prevention of secondary infections (pneumonia, urinary tract, etc.), and good nursing care. Case-fatality rates range from 3% to 15% and are highest in the elderly. (West Nile virus multiples in the person's blood system and crosses the blood-brain barrier to reach the brain. The virus interferes with normal central nervous system functioning and causes inflammation of brain tissue)  Unfortunately, there is no vaccine against West Nile encephalitis. Seek medical advice IMMEDIATELY if two of the following symptoms appear: 

Sustained fever above 103 ºF 
Severe headaches 
Muscle aches and pain, disorientation, neurological ticks and
tremors, or blurred vision 
Nausea and vomiting  

Dogs and cats can get WNV infection as humans do, however, in the New York area epidemic there was only a large die-off of American crows. A total of 18 native bird species have demonstrated morbidity or mortality. Also, domestic geese were reported as dying from West Nile virus infection in Israel in late 1999. West Nile virus was isolated from a dead cat in the New York area epidemic. A serosurvey of dogs and cats in the epidemic area showed a low infection rate. West Nile virus has been identified in the tissue of several horses that died on Long Island, NY, in October 1999, and specific West Nile antibody has been observed in others. Investigations by the US Department of Agriculture (USDA) and CDC indicate that West Nile virus was responsible for some of the horse deaths.

Prevention of Outbreak

Active sampling for West Nile virus (i.e., surveillance) in mosquito and bird populations will greatly enhance state and local government's early detection systems. When the first virus activity is detected in a community, prior to the occurrence of human disease, rapid mosquito control measures, such as targeted application of adulticides and larvacides, should be implemented. 

Arboviral encephalitis (blood feeding arthropods, mosquitoes, sand flies, ceratopogonids "no-see-ums", and ticks) can be prevented in two major ways: personal protective measures to reduce contact with mosquitoes and public health measures to reduce the population of infected mosquitoes in the environment. Every individual can reduce the risk of becoming infected with West Nile Virus (getting bitten by mosquitoes) by 

1. Stay indoors at dawn, dusk, and in the early evening when mosquitoes are most active. 

2. Wear long-sleeved shirts and long pants whenever you are outdoors.

3. Avoid shaded areas where mosquitoes may be resting  

4. Apply insect repellent sparingly to exposed skin. An effective repellent will contain 35% DEET (N,N-diethyl-meta-toluamide). DEET in high concentrations (greater than 35%) provides no additional protection. A recent study in the New England Medical Journal about mosquito repellent by Mark S. Fradin and John F. Day is worth noting: (The abstract of the study is quoted below) 

"Background The worldwide threat of arthropod-transmitted diseases, with their associated morbidity and mortality, underscores the need for effective insect repellents. Multiple chemical, botanical, and "alternative" repellent products are marketed to consumers. We sought to determine which products available in the United States provide reliable and prolonged complete protection from mosquito bites.

Methods We conducted studies involving 15 volunteers to test the relative efficacy of seven botanical insect repellents; four products containing N,N-diethyl-m-toluamide, now called N,N-diethyl-3-methylbenzamide (DEET); a repellent containing IR3535 (ethyl butylacetylaminopropionate); three repellent-impregnated wristbands; and a moisturizer that is commonly claimed to have repellent effects. These products were tested in a controlled laboratory environment in which the species of the mosquitoes, their age, their degree of hunger, the humidity, the temperature, and the light–dark cycle were all kept constant.

Results DEET-based products provided complete protection for the longest duration. Higher concentrations of DEET provided longer-lasting protection. A formulation containing 23.8 percent DEET had a mean complete-protection time of 301.5 minutes. A soybean-oil–based repellent protected against mosquito bites for an average of 94.6 minutes. The IR3535-based repellent protected for an average of 22.9 minutes. All other botanical repellents we tested provided protection for a mean duration of less than 20 minutes. Repellent-impregnated wristbands offered no protection.

Conclusions Currently available non-DEET repellents do not provide protection for durations similar to those of DEET-based repellents and cannot be relied on to provide prolonged protection in environments where mosquito-borne diseases are a substantial threat."

4. Repellents may irritate the eyes and mouth, so avoid applying repellent to the hands of children. Spray clothing with repellents containing permethrin or DEET, as mosquitoes may bite through thin clothing. Whenever you use an insecticide or insect repellent, be sure to read and follow the manufacturer's DIRECTIONS FOR USE, as printed on the product.  Note: Vitamin B and "ultrasonic" devices are NOT effective in preventing mosquito bites. 

Public health measures include elimination of larval habitats or spraying of insecticides to kill juvenile (larvae) and adult mosquitoes. The combination of mosquito control methods selected for use in a control program depends on the type of mosquitoes to be controlled and the habitat structure. In emergency situations, wide area aerial spraying is used to quickly reduce the number of adult mosquitoes. In many states aerial spraying may be available as a means to control nuisance mosquitoes. Finally, mosquito populations can be reduced on your property and in your neighborhood by eliminating standing water that collects in unused birdbaths, boats, buckets, tires, unused pools, roof gutters, and other containers. If you find dead crows or birds at home please contact your local health department who will send a representative to pick up the bird or request you dispose of the bird in an appropriate manner. While it is not likely that a person can get WNV from handling live or dead birds, people should always avoid barehanded contact with dead animals. Hands should also be washed with soapy water after handling dead animals even if gloves or plastic bags are used.

If you own a horse stable, the following are suggestions based upon knowledgeable persons' opinions. As always, consult your veterinarian in all matters regarding your horses health but you can do the following to reduce the risk of getting your horses infected by WNV:

1) Keep horses stabled from dusk to dawn. Do not turn out if mosquitoes are present or likely. If appropriate, consider the use of a sheet and/or fly bonnets on the horse during turn-out.
2) Use insect repellants on your horses and yourself.  
3) Consider a time released aerosol dispenser of insecticide in your horses stall.
4) Consider area spraying of any breeding grounds of mosquitoes.
5) Eliminate any standing water which could be mosquito breeding areas. Car tires, bird baths, gutters and clogged drains are often a source of mosquito breeding activity. Ensure screens on windows and doors fit tightly.
6) Avoid riding in or near wetlands which contain high numbers of insects.
7) Report any dead birds and any unusual animal deaths to health officials. Handle carefully! Call the Department of Animal Health or your veterinarian for further instructions.
8) Recognize signs of the equine virus: lethargy, hind end weakness, stumbling, 4 legged in-coordination, head tilt, facial twitching, head pressing convulsions, circling, hyper-excitability, partial paralysis and coma. Fever does not necessarily seem to be a common sign of the disease. Call your vet immediately if your horse evidences any of the above signs.
9) Co-operate with USDA and state officials suggestions and precautions as well as any possible testing. Fears involving government actions such as collecting or destroying positive horses are without basis.
10) Since the virus can and has affected humans, exposure to insects should be minimized and insect repellants should be used in any area where mosquitoes may be prevalent.
11) Since it appears the virus may survive the winter weather, implement the above procedures in the Spring/Summer of next year when the weather becomes conducive to mosquito activity.
12) Consider and evaluate carefully the use in stables of newer type "bug Zappers" developed in conjunction with the ARS USDA.
13) Exercise effective rodent control measures.
14) Pay attention to research on West Nile Virus especially development of a rapid diagnostic test and vaccine development.
15) Ensure swimming pools are not a breeding grounds for mosquitoes.
16) Consult with your local/state mosquito commission for additional suggestions!
17) Consider the use of larvae eating fish in ponds.
18) Avoid bats & discourage nesting as they can carry the virus. Consider utilizing professional assistance to remove and control nesting of bats.

Mosquitoes Can Not Transmit AIDS

Often people show misconception that mosquitoes can transmit AIDS. No, They can't. The following facts are helpful for understanding this issue. (reference: Rutgers Cooperative Extension Fact Sheet #FS736 by Wayne J. Crans, Associate Research Professor in Entomology)

Media releases concerning the possibility of mosquitoes transmitting AIDS (Acquired Immune Deficiency Syndrome) were common when the disease was first recognized, and the subject is still addressed by tabloids that seek captivating headlines to increase their circulation. The topic was initiated by reports from a small community in southern Florida where preliminary evidence suggested that mosquitoes may have been responsible for the higher on average incidence of AIDS in the local population. The media was quick to publicize claims that mosquitoes were involved in AIDS transmission despite findings of scientific surveys of the National Centers for Disease Control (CDC) that clearly demonstrated that mosquito transmission of AIDS in that community appeared highly unlikely. Nevertheless, media releases perpetuated the concept that mosquitoes transmitted AIDS, and many people still feel that mosquitoes may be responsible for transmission of this infection from one individual to another.

There are three theoretical mechanisms which would allow blood-sucking insects such as mosquitoes to transmit HIV.

1. In the first mechanism, a mosquito would initiate the cycle by feeding on an HIV positive carrier and ingest virus particles with the blood meal. For the virus to be passed on, it would have to survive inside the mosquito, preferably increase in numbers, and then migrate to the mosquito's salivary glands. The infected mosquito would then seek its second blood meal from an uninfected host and transfer the HIV from its salivary glands during the course of the bite. This is the mechanism used by most mosquito-borne parasites, including malaria, yellow fever, dengue, and the encephalitis viruses.

2. In the second mechanism, a mosquito would initiate the cycle by beginning to feed on an HIV carrier and be interrupted after it had successfully drawn blood. Instead of resuming the partial blood meal on its original host, the mosquito would select an AIDS-free person to complete the meal. As it penetrated the skin of the new host, the mosquito would transfer virus particles that were adhering to the mouthparts from the previous meal. This mechanism is not common in mosquito-borne infections, but equine infectious anemia is transmitted to horses by biting flies in this manner.

3. The third theoretical mechanism also involves a mosquito that is interrupted while feeding on an HIV carrier and resumes the partial blood meal on a different individual. In this scenario, however, the AIDS-free host squashes the mosquito as it attempts to feed and smears HIV contaminated blood into the wound. In theory, any of the mosquito-borne viruses could be transmitted in this manner providing the host circulated sufficient virus particles to initiate re-infection by contamination. 

Each of these mechanisms has been investigated with a variety of blood sucking insects and the results clearly show that mosquitoes cannot transmit AIDS. News reports on the findings, however, have been confusing, and media interpretation of the results has not been clear. The average person is still not convinced that mosquitoes are not involved in the transmission of a disease that appears in the blood, is passed from person to person and can be contracted by persons that share hypodermic needles. Here are just some of the reasons why the studies showed that mosquitoes cannot transmit AIDS:

Mosquitoes Digest the Virus that Causes AIDS

When a mosquito transmits a disease agent from one person to another, the infectious agent must remain alive inside the mosquito until transfer is completed. If the mosquito digests the parasite, the transmission cycle is terminated and the parasite cannot be passed on to the next host. Successful mosquito-borne parasites have a number of interesting ways to avoid being treated as food. Some are refractory to the digestive enzymes inside the mosquito's stomach; most bore their way out of the stomach as quickly as possible to avoid the powerful digestive enzymes that would quickly eliminate their existence. Malaria parasites survive inside mosquitoes for 9-12 days and actually go through a series of necessary life stages during that period. Encephalitis virus particles survive for 10-25 days inside a mosquito and replicate enormously during the incubation period. Studies with HIV clearly show that the virus responsible for the AIDS infection is regarded as food to the mosquito and is digested along with the blood meal. As a result, mosquitoes that ingest HIV-infected blood digest that blood within 1-2 days and completely destroy any virus particles that could potentially produce a new infection. Since the virus does not survive to reproduce and invade the salivary glands, the mechanism that most mosquito-borne parasites use to get from one host to the next is not possible with HIV.

Mosquitoes Do Not Ingest Enough HIV Particles to Transmit AIDS by Contamination

Insect-borne disease agents that have the ability to be transferred from one individual to the next via contaminated mouthparts must circulate at very high levels in the bloodstream of their host. Transfer by mouthpart contamination requires sufficient infectious particles to initiate a new infection. The exact number of infectious particles varies from one disease to the next. HIV circulates at very low levels in the blood--well below the levels of any of the known mosquito-borne diseases. Infected individuals rarely circulate more that 10 units of HIV, and 70 to 80% of HIV-infected persons have undetectable levels of virus particles in their blood. Calculations with mosquitoes and HIV show that a mosquito that is interrupted while feeding on an HIV carrier circulating 1000 units of HIV has a 1:10 million probability of injecting a single unit of HIV to an AIDS-free recipient. In laymen's terms, an AIDS-free individual would have to be bitten by 10 million mosquitoes that had begun feeding on an AIDS carrier to receive a single unit of HIV from contaminated mosquito mouthparts. Using the same calculations, crushing a fully engorged mosquito containing AIDS positive blood would still not begin to approach the levels needed to initiate infection. In short, mechanical transmission of AIDS by HIV-contaminated mosquitoes appears to be well beyond the limits of probability. Therefore, none of the theoretical mechanisms cited earlier appear to be possible for mosquito transmission of HIV. 

Malaria - A Serious Mosquito-Borne Disease

Malaria parasites have been with us since the dawn of time. They probably originated in Africa (along with mankind) and fossils of mosquitoes up to 30 million years old show that the vector for malaria was present well before the earliest history. The Plasmodium parasites are highly specific, with man as the only vertebrate host and Anopheles mosquitoes as the vectors. This specificity of the parasites also points towards a long and adaptive relationship with our species. 

At present, at least 300,000,000 people are affected by malaria globally, and there are between 1,000,000 and 1,500,000 malaria deaths per year . Malaria is generally endemic in the tropics, with extensions into the subtropics. Malaria in travelers arriving by air is now an important cause of death in non-malarious areas, and this is not helped by the common ignorance or indifference of travelers to prophylaxis. Distribution varies greatly from country to country, and within the counties themselves, as the flight range of the vector from a suitable habitat is fortunately limited to a maximum of 2 miles, not taking account of prevailing wind etc.

What Is Malaria?

Malaria is a serious, sometimes fatal, disease caused by a parasite. There are four kinds of malaria that can infect humans: Plasmodium falciparum
(plaz-MO-dee-um fal-SIP-a-rum), P. vivax (VI-vacks), P. ovale (o-VOL-ley), and P. malariae (ma-LER-ee-aa).

Malaria occurs in over 100 countries and territories. More than 40% of the people in the world are at risk. Large areas of Central and South
America, Hispaniola (Haiti and the Dominican Republic), Africa, the Indian subcontinent, Southeast Asia, the Middle East, and Oceania are
considered malaria-risk areas (an area of the world that has malaria).

The World Health Organization estimates that yearly 300-500 million cases of malaria occur and more than 1 million people die of malaria.
About 1,200 cases of malaria are diagnosed in the United States each year. Most cases in the United States are in immigrants and travelers
returning from malaria-risk areas, mostly from sub-Saharan Africa and the Indian subcontinent.

Humans get malaria from the bite of a malaria-infected mosquito. When a mosquito bites an infected person, it ingests microscopic malaria parasites
found in the person’s blood. The malaria parasite must grow in the mosquito for a week or more before infection can be passed to another
person. If, after a week, the mosquito then bites another person, the parasites go from the mosquito’s mouth into the person’s blood. The
parasites then travel to the person’s liver, enter the liver’s cells, grow and multiply. During this time when the parasites are in the liver, the person
has not yet felt sick. The parasites leave the liver and enter red blood cells; this may take as little as 8 days or as many as several months. Once
inside the red blood cells, the parasites grow and multiply. The red blood cells burst, freeing the parasites to attack other red blood cells. Toxins
from the parasite are also released into the blood, making the person feel sick. If a mosquito bites this person while the parasites are in his or her
blood, it will ingest the tiny parasites. After a week or more, the mosquito can infect another person.

Each year in the United States, a few cases of malaria result from blood transfusions, are passed from mother to fetus during pregnancy, or are
transmitted by locally infected mosquitoes.

What are the signs and symptoms of malaria? Symptoms of malaria include fever and flu-like illness, including shaking chills, headache, muscle aches, and tiredness. Nausea, vomiting, and diarrhea may also occur. Malaria may cause anemia and jaundice (yellow coloring of the skin and eyes) because of the loss of red blood cells. Infection with one type of malaria, P. falciparum, if not promptly treated, may cause kidney failure, seizures, mental confusion, coma, and death. For most people, symptoms begin 10 days to 4 weeks after infection, although a person may feel ill as early as 8 days or up to 1 year later. Two kinds of malaria, P. vivax and P. ovale, can relapse; some parasites can rest in the liver for several months up to 4 years after a person is bitten by an infected mosquito . When these parasites come out of hibernation and begin invading red blood cells, the person will become sick.

Malaria can be diagnosed by looking for the parasites in a drop of blood. Blood will be put onto a microscope slide and stained so that the
parasites will be visible under a microscope. Any traveler who becomes ill with a fever or flu-like illness while traveling and up to 1 year after returning home should immediately seek professional medical care. You should tell your health care provider that you have been traveling in a malaria-risk area.

Malaria can be cured with prescription drugs. The type of drugs and length of treatment depend on which kind of malaria is diagnosed, where
the patient was infected, the age of the patient, and how severely ill the patient was at start of treatment.

How can malaria and other travel-related illnesses be prevented?

Visit your health care provider 4-6 weeks before foreign travel for any necessary vaccinations and a prescription for an anti-malarial
drug. Take your anti-malarial drug exactly on schedule without missing doses. Prevent mosquito and other insect bites. Use DEET insect repellent
on exposed skin and flying insect spray in the room where you sleep. Wear long pants and long-sleeved shirts, especially from dusk to
dawn. This is the time when mosquitoes that spread malaria bite. Sleep under a mosquito bednet that has been dipped in permethrin
insecticide if you are not living in screened or air-conditioned housing. 


Mosquitoes are not just a summer nuisance appearing at picnics, ballparks and camping grounds. They are fairly mobile and some can live thorugh winter. They can carry viruses and parasites and transmit to birds, animals and human. They multiply in large quantity and reproduce several batches during their life time. They can produce epidemic disease such as the West Nile Virus (WNV) and  Encephalitis and Malaria. It is best to control their population by eliminating their breeding grounds, any form of standing water. People should take precaution to prevent (especially children and elderly) being bitten by mosquitoes. Use effective insect repellent when staying outdoors during summer. Just remember: squash a mosquito to a bloody death may give you a bit of pleasure of vengeance but it could be too late for preventing to be infected..     



Written by Ifay Chang, Ph.D. on July 7th, 2002

Information about the Author

Dr. Chang is the co-founder of Medical World Search which offers an intelligent medical search engine, called MWSearch. MWSearch is an independent search service without affiliation with any healthcare organization or drug companies. Medical World Search ( ) has been offered for public use since 1996.

In early 90's, while working as a research scientist at IBM T. J. Watson Resaerch Center, Dr. Chang led a group of researchers developing an advanced clinic information system with the purpose of supporting effiecient and reliable healthcare practice. The system has been adopted by Kaiser Permanente and other healthcare organizations.

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