Mosquito life cycle:

The mosquito goes through four separate and distinct stages of its life cycle and they are as follows: Egg, Larva, pupa, and adult. Each of these stages can be easily recognized by their special appearance. There are four common groups of mosquitoes living in the Bay Area. They are Aedes, Anopheles, Culex, and Culiseta.

Egg: Eggs are laid one at a time and they float on the surface of the water. In the case of Culex and Culiseta species, the eggs are stuck together in rafts of a hundred or more eggs. Anopheles and Aedes species do not make egg rafts but lay their eggs separately. Culex, Culiseta, and Anopheles lay their eggs on water while Aedes lay their eggs on damp soil that will be flooded by water. Most eggs hatch into larvae within 48 hours.

Larva: The larva (larvae - plural) lives in the water and come to the surface to breathe. They shed their skin four times growing larger after each molting. Most larvae have siphon tubes for breathing and hang from the water surface. Anopheles larvae do not have a siphon and they lay parallel to the water surface.

The larva feed on micro-organisms and organic matter in the water. On the fourth molt the larva changes into a pupa.

Pupa: The pupal stage is a resting, non-feeding stage. This is the time the mosquito turns into an adult. It takes about two days before the adult is fully developed. When development is complete, the pupal skin splits and the mosquito emerges as an adult.

Adult: The newly emerged adult rests on the surface of the water for a short time to allow itself to dry and all its parts to harden. Also, the wings have to spread out and dry properly before it can fly. The egg, larvae and pupae stages depend on temperature and species characteristics as to how long it takes for development. For instance, Culex tarsalis might go through its life cycle in 14 days at 70 F and take only 10 days at 80 F. Also, some species have naturally adapted to go through their entire life cycle in as little as four days or as long as one month.²

Figure-1: Picture of Female mosquito Anopheles

Figure-2: General life-cycle of Mosquito

INTRODUCTION:

There seem to be more mosquito borne diseases and epidemic fears sweeping the nation than there ever were before. This is understandable - especially when there is a possibility that the small bug sucking on your arm could be transferring one of these diseases:

· Malaria

· Yellow Fever

· Dengue Fever

· West Nile Virus

· Arboviral Encephilitides

That is not to say that every bite will be lethal. Most mosquito bites are nothing but annoying. However, in a time when there are so many diseases, it is best to protect yourself the best you can to ensure that you don't become the next victim of a mosquito borne illness. This article discusses the four main mosquito-borne diseases that can be transmitted to humans, and reviews each diseases symptom. That is the malaria, yellow fever, dengue fever, west Nile virus and somewhat Encephalitis. Mosquitoes can cause more than itchy, annoying bumps. These flying insects have the potential to infect humans with several different potentially dangerous and deadly diseases. In the United States, mosquito-related deaths are rare, but they can and do occur. Those who are planning to travel abroad, should be aware of the types of mosquito transmitted diseases, preventative measures one can take, as well as the symptoms and urgency of treatment should infection occur. Here we will discuss about the four main mosquito-borne diseases.³

Malaria:

Malaria is quite rare in the United States, but affects 300 to 500 million people worldwide, and can have dangerous and even deadly consequences. This parasitic disease can be transmitted from infected mosquitoes to humans.

There are three stages of malaria infection.

The cold stage can cause symptoms such as fever, shaking and chills.

The hot stage can cause symptoms such as nausea and vomiting, high fever, dizziness, delirium, headache and pain.

The sweating stage can cause symptoms such as splenomegaly, decreased body temperature, hepatomegaly, sweating, fever and chills, fatigue, shortness of breath, anemia, pale skin and extreme exhaustion. Those who suspect that they have malaria need to get medical attention immediately so that they can begin treatment and have the best chance of avoiding serious health complications and death.⁴

LIFE CYCLE OF MALARIA:

Ÿ Female Anopheles mosquito carrying malaria-causing parasites feeds in a human and injects the parasites in the form of sporozoites into the bloodstream. The sporozoites travel to the liver and invade liver cells.

Ÿ Over 5-16 days, the sporozoites grow, divide, and produce tens of thousands of haploid forms, called merozoites, per liver cell. Some malaria parasite species remain dormant for extended periods in the liver, causing relapses weeks or months later.

Ÿ The merozoites exit the liver cells and re-enter the bloodstream, beginning a cycle of invasion of red blood cells, asexual replication, and release of newly formed merozoites from the red blood cells repeatedly over 1-3 days. This multiplication can result in thousands of parasite-infected cells in the host bloodstream, leading to illness and complications of malaria that can last for months if not treated.

Ÿ Some of the merozoites -infected blood cells leave the cycle of asexual multiplication. Instead of replicating, the merozoites in these cells develop into sexual forms of the parasite, called male and female gametocytes that circulate in the bloodstream.

Figure-3: Life cycle of malaria parasite

Ÿ When a mosquito bites an infected human, it ingests the gametocytes. In the mosquito gut, the infected human blood cells burst, releasing the gametocytes, which develop further into mature sex cells called gametes. Male and female gametes fuse to form diploid zygotes, which develop into actively moving ookinetes that burrow into the mosquito midgut wall and form oocysts.

Ÿ Growth and division of each oocyst produces thousands of active haploid forms called sporozoites. After 8-15 days, the oocyst bursts, releasing sporozoites into the body cavity of the mosquito, from which they travel to and invade the mosquito salivary glands. The cycle of human infection re-starts when the mosquito takes a blood meal, injecting the sporozoites from its salivary glands into the human bloodstream.

The mosquitoes that transmit malaria are found in some areas, not any of the major cities. These mosquitoes generally bite around sunset and sunrise. Malaria transmitting mosquitoes are usually found near the Cambodian border (especially near the seaside) and the Burma border.⁵

Symptoms of malaria: sudden high fever and chills, muscle aches and headaches. They usually show up after an incubation time of 7 to 30 days. To be properly diagnosed with malaria, you must undergo a blood smear examination, which all major hospitals and many clinics can administer. There are drug-resistant strains of malaria, and it's important to tell the doctor where you've been because the particular drug resistance varies by region.⁶

Yellow Fever:

Aedes aegypti are pests. Bites cause minor localized itching and irritation to the skin, and can make an outdoor adventure very unpleasant. Most bites are not medically significant, but can be annoying. While many mosquitoes bite at night, dawn or dusk, A. aegypti readily bite during the day and indoors as well as outdoors. As the name suggests, the yellow fever mosquito is the primary vector of the yellow fever virus.⁷

Yellow fever, viral diseases, is another mosquito-borne illness:

The early symptoms of this disease typically last two to five days and can include headache, nausea and vomiting, muscle pain, high fever, kidney inflammation, furry tongue, irritability, slowed pulse, decreased urine, bloodshot eyes, constipation, facial flushing and proteinurea.

Those with yellow fever will often experience a remission lasting a few hours or a few days. Once the remission is over the yellow phase begins and lasts for about three to nine days.

The symptoms associated with the yellow phase can include most of the early symptoms plus; jaundice, convulsions, bleeding, confusion, hemorrhage, renal damage, bruising, weakness, acute fever, slowed heartbeat, diarrhea, bloody vomit or black vomit, hematuria, liver inflammation, chest and abdominal pain and yellow skin. Coma and death can also occur. Those who suspect that they have this disease need to seek medical attention immediately to begin treatment and to try and avoid serious medical complications, coma and death.⁸

Transmission of the yellow fever virus in the animals:

· Sylvatic (jungle) cycle: In tropical rainforests, yellow fever virus is endemic among lower primates. Infected monkeys pass the virus to mosquitoes that feed on them. Persons who subsequently enter the forest (often workers, eg, loggers, and travelers) are infected with this form of disease. In Africa, the principal vector of the jungle cycle is Aedes africanus; in South America, Haemogogus janthinomys is the primary vector for jungle transmission. Nonhuman primates remain the preferred host in this setting.

Figure-4: Transmission of yellow fever virus

· Intermediate (savannah) cycle: In moist and semihumid areas of Africa, semi-domestic mosquitoes (that breed in the wild and around households) will feed primarily on monkeys but also on humans when the opportunity arises. This cycle likely reflects the evolution of yellow fever into an epidemic human disease. It is the most common cycle present in Africa and frequently leads to small-scale outbreaks within villages. It can potentially develop into large-scale epidemics if an infected individual carries the disease into an urban region. This cycle has not been identified in South America.

· Urban cycle: Aedes aegypti is responsible for the transmission of urban yellow fever in both Africa and South America. This mosquito has the ability to transmit the virus from person to person and infect large populations of unvaccinated individuals. Urban outbreaks are rare in South America, yet they are still occasionally reported in densely populated regions in Africa.⁹

Dengue Fever:

The disease "dengue fever" is carried by a particular species of mosquito, the Aedes aegypti mosquito, which is active all day. It is on the rise to becoming one of the top public health problems in the tropics.

The mosquito which carries this virus is most common in urban areas around human dwellings, and is most active during the daytime. The Aedes mosquito breeds in clear water.

Dengue fever is an acute viral disease and an infectious disease that can be transmitted from mosquitoes to humans. Epidemics of dengue are growing, and it is currently estimated that 50 to 100 million people develop this disease each year throughout more than 100 countries.¹⁰

Symptoms usually start to appear between 5 and 7 days after being bitten. The symptoms include severe headaches, high fever which may fluctuate, bone aches (hence the slang name), joint and muscle pains, nausea and vomiting, and a rash of small red spots a few days after the onset of fever. Dengue fever is a very serious illness abd require immediately for treatment. It can become fatal if the symptoms are allowed to progress over time to DHF (Dengue Hemorrhagic Fever), which is characterized by heavy bleeding, though DHF is thought to affect mainly longterm residents who have picked up more than one strain of the virus. Aspirin should be avoided, because aspirin reduces blood clotting and thus makes hemorrage more likely. There is no vaccine against dengue fever, and no specific treatment. You'll have to follow the doctor's orders. Dengue fever is often mistaken for malaria, the flu (influenza) or something else. Malaria can be excluded by a blood test, and the indication of dengue can be inferred by a blood test. Recovery will take time, with tiredness sometimes lasting several weeks. Research revealed that the Aedes aegypti mosquito that carries dengue does not travel far from its breeding place, unlike other species of mosquito, which explains why dengue outbreaks tend to occur in localized areas of a few hundred meters radius. It is believed that dengue spread by infected human migrant workers more than migrant mosquitoes. Therefore, when an outbreak can be determined in a particular location, the authorities may come and spray. However, given that urban dwellers tend to travel around the city and symptoms start around 5 to 7 days later, it's not always clear where the infection occured.¹¹

There are several clinical forms of dengue fever:

- “Asymptomatic” dengue fever: the patience does not feel ill.

- “Classical” dengue : high fever with debilitating fatigue ; intense muscle aches and pain, occasionally unbearable headaches, the appearance of itchy red patches on the body. The illness lasts from 4 to 8 days but convalescence may take longer.

- Dengue “haemorrhagic” fever: the same as above but with persistent fever and external and internal haemorrhagic symptoms (bleeding).

- Dengue “shock” syndrome: the same as above but with the onset of a state of shock.

The latter two forms, more frequent in children, can be fatal if left untreated. Any of the four serotypes can cause any of the various clinical forms. A diagnosis of dengue fever can only be given with certainty after lab tests (risk of confusion with influenza, leptospirosis, malaria, etc.) that either directly look for the virus or proceed by detecting antibodies to it. While it is possible to alleviate the symptoms of dengue, there is no known remedy to fight the virus itself. In addition, no safe and effective vaccine is available currently. The only way to control this disease is to reduce the vector mosquito population as much as possible. Such control is all the more necessary since more and more regions of the world are being affected by dengue fever and the haemorrhagic form seems to be appearing with increasing frequency.¹²

Transmission of Dengue fever:

Mosquitoes usually pick up the Dengue virus while feeding on the blood of an infected person, the virus incubates for a period of up to 10 days and an infected mosquito is capable, during probing and blood feeding, of transmitting the virus for the rest of its life. It is also suspected that infected female mosquitoes may transmit the virus to their offspring by transovarial (via the eggs) transmission, but how relevant this in sustaining transmission of the virus to humans remains unclear.¹³

So while infected mosquitoes are the culprits in spreading the disease it is infected humans who are the main carriers and multipliers of the virus, as they provide a source for uninfected mosquitoes. The virus circulates in the blood of infected humans for two to seven days and this is when symptoms appear and the Aedes mosquitoes may acquire the virus when they feed on an individual during this period - some research has shown that monkeys in some parts of the world also play a similar role in the transmission of the dengue virus.

1. When drinking the blood of a viraemic human (virus present in the peripheral blood), the female mosquito takes in a certain number of viral particles.

2. The viruses move up with the ingested blood into the oesophagus.

3. They end up in the stomach where they resist the mosquito’s digestive juices.

4. Given the high degree of adaptation between the virus and its vector, the viral particles are able to break through the stomach wall barrier and after a latency phase, they multiply with great intensity inside the mosquito’s cells.

5. While invading all parts of the mosquito’s body, the virus enters the salivary glands where it concentrates.

6. The next time the female A.aegypti bites a healthy person to take a blood meal; it injects saliva that acts as an anaesthetic, a lubricant for the mobile mouth parts, an anticoagulant and an aid to digestion due to the enzymes it contains.

7.While doing this, it also injects a certain amount of virus which, if the person is not immune, will bring on the disease.x

Figure-5: Transmission of dengue virus

(Phase 1) and Phase 6) are separated by a period of about 10 to 14 days called the extrinsic incubation period. The length of time does, however, depend on the temperature. After that, the female remains infectious for the rest of its life. This diagram applies to almost all the arboviruses transmitted by mosquitoes. In some cases, vertical tranmission may occur, which means that an infected female can transmit the virus to its eggs and therefore to its offspring.¹⁴

West Nile Virus:

The term West Nile mosquito or mosquito West Nile Virus does not mean that one must travel to the other side of the world to come into contact with this insect. Year by year, it seems like more and more West Nile mosquito bite cases are popping up all over the United States. Now viewed as a seasonal epidemic in North America, summer and fall seem to be the times when it shines. West Nile virus is the most common mosquito-borne illness in the United States. The Center for Disease Control (CDC) tracks the activity and spread of this illness and has worked to raise awareness of simple protective measures one can take, such as the elimination of common breeding grounds for mosquitos and the application of mosquito repellent. First making an appearance in North America in 1999, coming with reports of infected humans and horses, it claimed 18 lives in its first year. Reports show that there were 3,598 humans infected with the mosquito West Nile Virus by the year 2007. While not everyone who is infected by a diseased mosquito's bite will meet death, at least 200 of those people are expected to develop some type of serious illness. Everyone should protect themselves from the possibility of the West Nile Virus. A constant awareness of one's surroundings should assist in that protection. This is even truer for children and for those over 50. The older or younger one is, the more susceptible they are to this growing deadly disease. The symptoms of West Nile virus can include fever, seizures, neck pain, lethargy, headache, coma, body aches, malaise, rash, fatigue, muscle aches, stiff neck, swollen lymph nodes, diarrhea, weakness, photophobia, nausea and vomiting, altered mental state and maculopapular rash. This is a very serious disease that needs to be treated by a doctor immediately for the best outcome.¹⁵

Transmission of West Nile virus:

The first step in the transmission cycle of West Nile virus (WNV) happens when a mosquito bites an infected bird or animal and gets the virus while feeding on the animal's blood. The infected mosquito can then transmit the virus to another bird or animal when it feeds again.

Crows are highly susceptible to lethal infection, as are robins, blue jays, and other birds. Scientists have identified more than 138 bird species that can be infected and more than 43 mosquito species that can transmit WNV. Although the virus usually cycles between mosquitoes and birds, infected female mosquitoes also can transmit WNV through their bites to humans and other "incidental hosts," such as horses. With so many susceptible hosts to amplify the virus and so many types of mosquitoes to transmit it, WNV has spread rapidly across the United States. Most cases of human disease occur in elderly people and in people with impaired immune systems. WNV also can be transmitted through blood transfusions and organ transplants from WNV-infected donors. Health experts also believe it is possible for WNV to be transmitted from a mother to her unborn child and through breast milk.¹⁶

Figure-6: Transmission of Wet Nile virus

Encephalitis:

Encephalitis in various forms such as St. Louis, Western Equine, La Crosse, Eastern Equine, and West Nile, which was recently discovered in the Northeast, is endemic to the United States and increasing in incidence. Although extremely rare, Eastern Equine Encephalitis has a 30% - 60% mortality rate once contracted. Severe damage to the central nervous system occurs in those that survive the illness. Eastern Equine Encephalitis (EEE) is maintained in nature through a cycle between the Culiseta melanura mosquito and birds that live in freshwater swamps. Although Culiseta melanura do not bite humans, some mosquitoes will "cross bite"; i.e., bite an infected bird and then bite a human or animal (horse, emu, and other exotic birds), thereby spreading the disease. These mosquitoes are also known as "bridge vectors". A vector is a species that transmits a disease from one host to another. These bridge vectors may take a meal from a bird and later take another meal from a mammal. Symptoms usually occur within two to ten days after being bitten by an infected mosquito. These symptoms include high fever, stiff neck, headache, confusion, and lethargy. Encephalitis, swelling of the brain, is the most dangerous symptom. Rhode Island has confirmed five cases of EEE with two deaths in the last thirteen years. The last death was reported in 1993.

Encephalitis is an acute inflammation of the brain. Encephalitis with meningitis is known as meningoencephalitis. Symptoms include headache, fever, confusion, drowsiness, and fatigue. More advanced and serious symptoms include seizures or convulsions, tremors, hallucinations, and memory problems.¹⁷

Viral

Viral encephalitis can be due either to the direct effects of an acute infection, or as one of the sequelae of a latent infection. A common cause of encephalitis in humans is herpes simplex virus type I (HSE) which may cause inflammation of the brain. This can result in death. Others include infection by Flaviviruses such as St. Louis encephalitis or West Nile virus, or Togaviruses such as Eastern equine encephalitis (EEE), Western equine encephalitis (WEE) and Venezuelen equine encephalitis (VEE).

Bacterial and other

It can be caused by a bacterial infection, such as bacterial meningitis, spreading directly to the brain (primary encephalitis), or may be a complication of a current infectious disease syphilis (secondary encephalitis). Certain parasitic or protozoal infestations, such as toxoplasmosis, malaria, or primary amoebic meningoencephalitis, can also cause encephalitis in people with compromised immune systems. Lyme disease and/or Bartonella henselae may also cause encephalitis. Another cause is granulomatous amoebic encephalitis.¹⁸

Diagnosis:

Adult patients with encephalitis present with acute onset of fever, headache, confusion, and sometimes seizures. Younger children or infants may present irritability, poor appetite and fever. Neurological examinations usually reveal a drowsy or confused patient. Stiff neck, due to the irritation of the meninges covering the brain, indicates that the patient has either meningitis or meningoncephalitis. Examination of the cerebrospinal fluid obtained by a lumbar puncture procedure usually reveals increased amounts of protein and white blood cells with normal glucose, though in a significant percentage of patients, the cerebrospinal fluid may be normal. CT scan often is not helpful, as cerebral abscess is uncommon. Cerebral abscess is more common in patients with meningitis than encephalitis. Bleeding is also uncommon except in patients with herpes simplex type 1 encephalitis. Magnetic resonance imaging offers better resolution. In patients with herpes simplex encephalitis, electroencephalograph may show sharp waves in one or both of the temporal lobes. Lumbar puncture procedure is performed only after the possibility of prominent brain swelling is excluded by a CT scan examination. Diagnosis is often made with detection of antibodies in the cerebrospinal fluid against a specific viral agent (such as herpes simplex virus) or by polymerase chain reaction that amplifies the RNA or DNA of the virus responsible (such as varicella zoster virus).¹⁹

Treatment:

Treatment is usually symptomatic. Reliably tested specific antiviral agents are available only for a few viral agents (e.g. acyclovir for herpes simplex virus) and are used with limited success for most infection except herpes simplex encephalitis. In patients who are very sick, supportive treatment, such as mechanical ventilation, is equally important. Corticosteroids (e.g. methylprednisolone) are used to reduce brain swelling and inflammation. Sedatives may be needed for irritability or restlessness.²⁰

Encephalitis lethargica:

Encephalitis lethargica is an atypical form of encephalitis which caused an epidemic from 1918 to 1930. Those who survived sank into a semi-conscious state that lasted for decades until the Parkinson's drug L-DOPA was used to revive those still alive in the late 1960s by Oliver Sacks.

There have been only a small number of isolated cases in the years since, though in recent years a few patients have shown very similar symptoms. The cause is now thought to be either a bacterial agent or an autoimmune response following infection.²¹

Limbic system encephalitis:

In a large number of cases, called limbic encephalitis, the pathogens responsible for encephalitis attack primarily the limbic system (a collection of structures at the base of the brain responsible for emotions and many other basic functions).

Arboviral Encephilitiedes:

While the term Arboviral Encephilitiedes may sound ridiculous, any time you hear the phrase Arboviral Encephilitides mosquito borne illness you should be concerned. Encephilitides are viral diseases transferred by mosquitoes that can cause inflammation of the brain. In extreme cases, this inflammation eventually can lead the infected person to experience brain damage, fall into a coma, or even meet death.²²

Brain damage and death is the total extreme when it comes to these diseases. Most infected persons end up with less minor flu-like symptoms which include:

· Headaches

· Muscle Aches

· Fever

· Malaise

For most, symptoms never even surface. For some, the symptoms are much more extreme than flu-like.

· High Fever

· Confusion

· Stiff Back

· Stiff Neck

· Sensitivity to Light

· Vomiting

There are a few varieties of Encephalitides found in the United States. These varieties include:

· Eastern Equine Encephalitis

· Western Equine Encephalitis

· St. Louis Encephalitis

· La Crosse Encephalitis (LAC)

· West Nile Encephalitis

Out of all these varieties of mosquito illnesses, the Eastern Equine is viewed as the most deadly. Even with only an average of four deaths a year in the United States, it is one of the mosquito illnesses that could someday affect a much larger portion of the population.^23-26

Minimizing Mosquito Bites:

There various ways to minimize mosquito bites:

· Apply anti-mosquito lotions and sprays. Tests by consumer magazines show that many popular ones do not actually last long. The best are icaridin and DEET. DEET is most commonly sold despite criticisms of toxicity, but it does not significantly affect the vast majority of people. (It does not kill or actually repel mosquitoes, either; it just stops mosquitoes from recognizing you as prey by blocking their senses.) Apply it to clothes and unbroken skin, but avoid ingestion and don't use it on infants. Mosquitoes are immensely worse than DEET. The next best alternatives seem to be eucalyptus (eucalyptol) and citronella oil.

· Cover your body with clothes, including thick socks, even if you wear sandals

· It's said that mosquitoes are more attracted to dark colors, so wear white, maybe even thin nightwear

· Do not apply perfumes and colognes

· At restaurants, request mosquito coils and places them upwind.

· At home, cover any standing water (such as the urns), deal with any places that hold rainwater, and for any ponds with lotus flowers and the like you should add some small fish that eat mosquito larvae

· Get a mosquito net for travelling with

· Notably, tests show that electronic mosquito killers and repellers do not kill a significant fraction of the mosquito population.

N,N-Diethyl-meta-toluamide Permethrin

Allathrine Icaridin

Structures of the Mosquito Repellents:

The best ways to kill mosquitoes in your home are with your hands, insecticide spray, or electronic rackets sold at superstores. Insecticide spray can have side effects such as headaches, especially for children. Most mosquitoes are most active around sunrise and sunset, which is why you see a peak in activity around these times, but some mosquitoes are active all times of the day and night. Some species seek shelter during the heat of the day. Mosquitoes eat flower nectar and some other things. Only female mosquitoes bite humans, as they need the protein to make their eggs.^27-30

CONCLUSION:

At the last, mosquitoes are one of the prime vect0r agents which spreads the number of diseases to death through viruses and parasites by withdrawing immune system of a person, so prevention must be taken and full treatment must be provided until complete recovery has been achieved.

ACKNOWLEDGEMENT:

All the authors are thankful to the search engine of library of SSPC, Mehsana for data collection of various aspects of mosquito borne diseases.

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Received on 10.03.2011

Modified on 23.03.2011

Accepted on 11.04.2011

Research J. Science and Tech. 3(3): May-June. 2011: 119-126