Mosquito Borne Diseases

List of Diseases Spread by Mosquitoes

Focus on the Facts
Insects are wonderful things, with respect to their beauty and the reliance all of nature has on insects for the proper flow of natural environments. However, there is a dark side to insects too, and we absolutely MUST understand the ability many of them have to cause us harm. Only by educating ourselves with the facts can we make the proper decisions on how to proceed with avoiding the problems. There are many “urban legends” out there that we read about, and wonder whether or not they are true. If we are swayed to believe things that are not true we may be led to take action by demanding a response that is not warranted.

Mosquitoes Can Carry Many Deadly Diseases

Of all the insects that are destructive to humans none is worse than the mosquito.

Among the various diseases spread by mosquitoes is Malaria, which kills two to three million people each year in the world at this time.

Yellow Fever and Dengue are two more devastating mosquito-borne diseases.

Yet another disease spread by these critters is Encephalitis, actually a compilation of around 7 or 8 strains of viruses that cause similar effects in infected people. Encephalitis is closely tied to birds, as it is in birds that the encephalitis virus must live for a period of its life cycle, and when mosquitoes then feed on infected birds they acquire the virus, which then can be passed to a person who is bitten.

mosquito borne diseases
mosquito diseases by state


The United States is not immune to the horrors of mosquitoes and their diseases, and many decades ago these diseases were quite common in this country.

Managing the Spread
It has been through the diligent efforts of Mosquito and Vector Control Agencies and the pest control industry that we have beaten back the disease. We certainly have plenty of the species of mosquitoes that are capable of spreading any of the mosquito-borne diseases, but for the most part the disease agents themselves – the bacteria, viruses, or Plasmodium – have been kept out of the mosquito populations. Encephalitis is the exception, but careful monitoring around populated areas allows the mosquito control agencies to determine when infected mosquitoes are getting “too close”, and they can proceed with intense mosquito control programs.

West Nile Virus

Now, however, there is a new player in the country, and this disease is called West Nile Virus, also referred to as West Nile Fever.

This virus is believed to have originated in Africa, where it likely has existed for millions of years. For many years now it has caused infections and death in people in many African and Middle East countries, and in the past few years has been reported in Europe as well.

First Appearance
Until 1999 it had never been reported in the Western Hemisphere (North America and South America), but that year it occurred in New York, causing the death of 7 people in the summer of 1999.

How did West Nile Virus get here?
As with anything new and frightening there already are extraordinary claims emerging about West Nile Virus, including that it is a government conspiracy or that it is a product of bio-terrorism. These kinds of scenarios are at best outrageous, and at their worst are criminal. There simply is no evidence whatsoever of such things, but it makes a lot of people feel better to be able to blame someone.

The most reasonable explanation for the sudden emergence of this problem in the United States is the same reason we have so many other imported, exotic pests in this country – the world is getting “smaller”. Human travel takes us everywhere in a very casual style, and the likelihood that we may bring things back in our suitcases or boxes of souvenirs is very high.

small asian tiger mosquito pic
Asian Tiger Mosquito

Asian Tiger Mosquito
About 10 years ago a new mosquito species entered the U.S., called the Asian Tiger Mosquito. While it is not known with absolute certainty, it is believed it hitchhiked here in pools of water in tires that were brought here from Asia for recycling. West Nile Virus most likely came with infected mosquitoes that hitchhiked on airplanes or ships coming from Africa or another country where the disease already existed. Once in the population of people in the United States it was picked up by some of our native mosquito species, and now appears to be a permanent resident of this country.

Where has West Nile Virus been found in the U.S.?
Like other encephalitis-like viruses birds are an integral player in the life of the West Nile Virus. Mosquitoes that are infected with the virus feed on a bird, transferring the virus to the bird. Other mosquitoes that feed on the blood of that bird can then be infected, and so on.


The Bird Connection
To date crows seem to be the most sensitive birds, and many hundreds of them have been killed by the disease. However, up to 20 other kinds of birds have tested positive for the virus, including other migrating birds such as mallards and robins. Several species of hawks as well as the Bald Eagle have tested positive as well.

Range of the Disease
It is possible that it is within these infected migrating birds that the virus spreads. While initially found in New York City in 1999, the known range of infected birds by October 2000 was as far south as North Carolina and as far north as Maryland and Washington DC. Wherever infected birds fly it is possible for the disease to be spread, and at the time of this article’s creation in summer 2004 all of the contiguous states of the United States are known to have infected birds. The summer of 2004 found the first human cases in California, so West Nile Virus now is here to stay throughout our country.

More Information – CDC: West Nile Cases by State

How is West Nile Virus spread?
It is only by the bite of an infected mosquito that West Nile Virus is passed to people. An infected person cannot spread the disease to another person, and it is unlikely that it can be passed directly from infected birds to people.

Contact with Dead Animals
It must be injected into the blood stream, and this is the mechanism mosquitoes use when they feed. It is recommended, however, that you wear gloves or place your hands within a plastic bag whenever you need to dispose of a dead bird. This is good advice when handling any dead animal, and if there are cuts or other openings on your skin it is always possible that pathogens could find their way in.

Warmer Months & Transmission
Several species of mosquitoes appear capable of spreading West Nile Virus, including some that tend to fly in daylight hours as well as those that are primarily night fliers. These mosquitoes cannot survive the cold months as biting adults, but instead pass through the winter in the egg stage that has been placed in some protected environment by the adults the summer before. Obviously, then, transmission of the disease is only likely during those months when mosquito adults are active.

How Dangerous is this Disease?
West Nile Virus is considered to be a rather mild strain of encephalitis to humans. While many people have been killed by it, and that is a terrible thing, the vast majority of people who become infected with this virus may feel either no effect at all or very mild symptoms.

Typical Symptoms
They may experience a mild headache and fever and then recover completely.

While it is possible that anyone might be seriously affected by the disease, those who are at the highest risk are the elderly, the very young, and those with weakened immune systems.

west nile chart
Incidence by Age

No Vaccine
There currently is no vaccine for this virus for humans, and treatment of the disease for those severely affected is to treat the symptoms until the disease runs its course.

If there is a positive aspect to WNV it is that those who receive a mild infection appear to acquire immunity to the disease from then on. The numbers of cases of the disease in the eastern states, where WNV appeared years earlier, are dramatically lower now than in previous years. The highest numbers of cases are in the states in which WNV is just emerging, and no immunity is present in the residents of those areas.

In 1999, in the New York City area, seven people were killed and 62 were seriously ill from West Nile Virus. In 2000 there was a single death and only 17 confirmed cases of seriously ill people. In other countries in Europe and the Middle East it has been responsible for dozens of deaths in several major outbreaks in the past 3 or 4 years, so the potential for major problems clearly exists. As the disease reached the middle states in the U.S. in 2002 and 2003 there were huge numbers of human cases, and Colorado alone accounted for the majority of these cases.

Post Infection
While deaths to WNV are only a very low percentage of the numbers of people who become infected, this does not mean that those who survive are necessarily back to normal. Remember, that West Nile Virus causes an inflammation of the tissues surrounding the brain, and it commonly causes serious and lasting repercussions as it damages brain function. It is very important not to trivialize this disease simply because it does not kill vast numbers of people.

eastern equine encephalitis virus (EEE) and West Nile virus (WNV)

West Nile Virus is also a serious disease of horses, and many thousands of these valued companions died in the first few years that WNV hit the United States. There now is an effective vaccine for horses, and it appears to very successfully stave off the problems in horses.

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Anopheles mosquito

This disease in humans results from infection with a protozoan blood parasite transmitted by a species of the mosquito genus Anopheles. The human clinical condition known as malaria is caused by infection with one of four species of the genus Plasmodium: Plasmodium falciparum, P. vivax, P. malariae, and P. ovale. The first two are the most common and most important, and P. falciparum infection often can be fatal in the absence of treatment.

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Natural History
The Plasmodium species are blood parasites, although some also invade liver cells where they lie dormant until later release brings a relapse with fevers associated with the destruction of red blood cells. The vector mosquitoes imbibe the parasites with the bloodmeal, and the sexual stages unite in the mosquito gut to create a stage which invades the gut wall and forms a cyst, which in turn releases many infective stages (sporozoites) which invade the salivary glands, and are injected into a new host when the mosquito feeds. The sporozoites invade liver cells and later developmental stages of the parasite invade red blood cells which they disrupt (causing fever), form sexual stages and the cycle is completed.

Malaria is characterised clinically by fever (usually periodic), varying degrees of anaemia and splenic enlargement, and a range of syndromes resulting from the physiological and pathological involvement of certain organs, including the brain, liver and the kidneys.

Mosquito Vectors
Mosquito vectors of human malaria vary with global region and sometimes even locality, but all belong to the genus Anopheles. However, not all Anopheles can transmit the parasites; relatively few species are important primary vectors, with others of secondary importance, and many of little or no consequence.

Malaria is one of the most widespread of all human parasitic diseases, and in the early part of the last half century more than two-thirds of the world’s population lived in endemic areas. By about 1950 it was estimated that the annual incidence of the disease was approximately 250 million cases, with about 2.5 million deaths per year, and significant interference in the agricultural and industrial development of tropical countries. From the 1950s, the World Health Organisation instigated and co-ordinated efforts towards worldwide eradication of the disease.

By the early 1970’s the population freed from the risk of malaria transmission had increased from 400 million to over 1200 million, with the disease being eradicated from the whole of Europe, most of North America, most of the Caribbean and large parts of South America, and Singapore, Japan, Korea, Taiwan and Australia.

Return of the Disease
However in recent years the position has deteriorated in some countries where control efforts were making an impact, and the disease has returned to others from which it had been regarded as eradicated. In Africa south of the Sahara, where little eradication has been attempted, the distribution of malaria has remained essentially unchanged. In 1988, the global incidence of malaria was estimated to be of the order of 110 million cases annually with some 270 million people infected. At the present time, the momentum for global malaria eradication has declined, and few tropical countries maintain the enthusiastic and often efficient vector control efforts that existed in the 1960’s.

Prevention of malaria in many countries has been heavily dependent on anti-malarial drugs and residual insecticides since the 1950s, but this has broken down in a lot of places for various reasons, including development of drug resistance by the parasites, insecticide resistance by the mosquitoes, and failures in administrative and logistical systems at central, regional and local government levels. Thus, the importance of self protection for local inhabitants and visiting travellers in “at-risk” areas has become more and more emphasised in recent years.

This involves the use of:

  • protective clothing where possible
  • chemical repellents
  • bed nets when sleeping in unscreened or otherwise unprotected situations

Drugs for protection against infection are still useful in certain areas, but expert advice should be sought with respect to the regions and localities being visited, the time to be spent there, and other factors likely to relate to exposure to infection.


Mostly present in the developing world, Chikungunya causes an estimated 3 million infections per year. The virus is a pathogen transmitted by Asian Tiger and Yellow Fever mosquitoes. Genetically, it appears that the viral strain currently spreading throughout the Americas is more easily transmitted by the Yellow Fever mosquitoes.

chikungunya graphic

Both species, though, lay their eggs in containers and other items that hold water close to where humans live, and as a result, the only effective method of controlling the disease is by removing water bearing containers from human habitation areas.

Although it’s largely confined itself to the Caribbean, it has resulted in two locally transmitted cases in Florida during July 2014, and 497 travel related cases have been identified in 35 states, Puerto Rico, and the US Virgin Islands. Although it’s rarely fatal with a fatality rate of 1 in 1000, its symptoms can be debilitating and may be prolonged over a period of several weeks. Currently, there is no vaccine against the disease and the primary treatment is limited to pain medication.

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Dog Heartworm

The dog heartworm, Dirofilaria immitis, is an increasing major veterinary health problem for dogs. Although heartworm in Australia has in the past been considered to be mainly a problem of tropical and subtropical coastal regions, in recent decades it has become increasingly prevalent in more southern areas.
dog heartworm mosquitoes

Adult female worms in the dog’s heart produce microscopic embryonic microfilariae which circulate in the peripheral blood. After ingestion, the microfilariae migrate from the mosquito’s midgut, undergo development involving two moults, and after a period of approximately two weeks the infective third stage larvae migrate towards the mosquito’s head. These larvae then transfer to the skin of the final host during feeding of the infected mosquito. Following their penetration into the final host, the third stage larvae migrate through body tissue where they develop into fourth stage larvae, then immature adults which finally enter the blood stream and reach the heart via the venous system. Adults mature in about 6 months and are subsequently found in the heart and major vessels. The adults mate, and the female produces millions of microfilariae to circulate in the peripheral blood to renew the cycle. The dog can be severely affected, with major vessels becoming blocked with chronic infections; death usually results in such cases from heart failure.

Mosquito vectors in all parts of Australia have not been adequately identified, although in southeastern Australia, Aedes notoscriptus and Culex annulirostris are prime suspects because they have been shown to be infected in field studies and are very good laboratory vectors. The former species is an excellent laboratory vector, has close contact with dogs in domestic and peri-domestic urban, and rural situations, and is thus suspected to be a very important vector. In certain circumstances Ae. vigilax, Ae. camptorhynchus, Cx. quinquefasciatus and An. annulipes, and possibly other species may need to be considered as local vectors.

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Arthropod borne viruses or arboviruses are the most diverse, numerous, and serious diseases capable of being transmitted to susceptible vertebrae hosts by mosquitoes. These viruses have a complex lifecycle involving a nonhuman primary vertebrate host, and a primary arthropod vector or carrier.

Though the virus cycles usually stay undetected, the virus commonly escapes when humans enter a natural habitat of carrier mammals, and, as a result, become infected. Because humans don’t contribute to the transmission cycle, they are what is known as “dead-end hosts”, but they can still develop clinical illness from the virus. There are several virus agents of encephalitis in the United States that’s caused by arboviruses, all of which are transmitted through mosquitoes.


Although Dengue is rarely fatal and has a low mortality risk, it has extremely uncomfortable symptoms and has become more serious in recent years. It’s a serious arboviral disease that is mainly confined to the Americas, Asia, and Africa.

Dengue in the US
During the past 16 years it has become more common. For example, in 1999, 55 cases were reported in Texas, and more recently, Hawaii recorded 85 cases in 2001 and Florida recorded 20 cases in 2010. Likewise, Venezuela recorded over 11,600 cases in 2004 and an outbreak in Indonesia caused over 600 deaths in more than 54,000 cases.

In 2010, Puerto Rico experienced an outbreak during which 21,000 people were infected, and Florida reported the first cases of local transmission in 2009.

The carriers of the disease, the Aedes aegypti and Ae. Albopictus mosquitoes, prefer to lay their eggs in areas close to human habitations. Because they can’t be effectively controlled by normal spraying techniques, the only effective solution to prevent disease is to remove these containers from human habitation as part of an overall mosquito control strategy.

Eastern Equine Encephalitis (EEE)

Eastern equine encephalitis (EEE), which can be spread to horses and humans by infected mosquitoes is one of the most serious and devastating mosquito borne viruses. It can affect the central nervous system, and cause severe complications and even death in humans. It has a complex lifecycle involving birds and a specific type of mosquito to enable transmission. These mosquitoes feed on the infected birds and then become carriers of the disease, which can then transmit it to humans, horses, and other mammals when they feed. As stated, because humans are dead-end hosts, the disease cannot be transmitted from humans or other mammals.

The symptoms of Eastern equine encephalitis range from having no symptoms to mild flu symptoms including fever, headache, and a sore throat. More serious infections and disease can cause more severe fever, headaches, followed by seizures and even coma.

Because of its seriousness, about half of the patients that have serious infections die from the disease. Of those people that survive, many suffer permanent damage and the require lifetime care. As at date, there is no specific treatment for the disease and there is no vaccine available.

St. Louis Encephalitis (SLE)

St. Louis encephalitis is a disease that is transmitted from birds to humans and other mammals by infected mosquitoes. It’s typically found across the United States, and it most often occurs along the Gulf of Mexico and especially in Florida, which has seen various epidemics before.

Symptoms are similar to those found in Eastern equine encephalitis and includes fever, headaches, and a sore throat. Similar to Eastern equine encephalitis, there is currently no vaccine that can protect humans against the disease.

Lacrosse Encephalitis (LAC)

La Crosse encephalitis is much less common than Eastern equine encephalitis, but there are still about 90 cases per year, which occur in all 13 states east of the Mississippi.

The carrier of the disease is thought to be a specific type of woodland mosquito and the disease was first reported in 1963. Like Eastern equine encephalitis and St. Louis encephalitis, there is currently no vaccine for La Crosse encephalitis.

It’s not transmissible from human to human, and infrequent deaths occur in children younger than 16.

Western Equine Encephalitis (WEE)

First recorded in 1950, Weston equine encephalitis is found west of the Mississippi, including parts of Canada and Mexico. The primary carrier of the disease is Culex Australis, and birds are often the most common vertebrate hosts for the virus. Since 1964, fewer than 1000 cases have been reported. Like Eastern equine encephalitis, a vaccine is not currently available to protect humans against the disease.

Zika Virus

Zika virus, a Flavivirus virus related to West Nile virus, Yellow Fever, St. Louis encephalitis, and Eastern equine encephalitis was first discovered in Macaque monkeys in 1947 in Uganda.

Since its discovery, cases have been reported in 35 countries in the Americas. As at date there has not been any local transmission of the disease within the continental United States, but there have been 426 reported cases of the disease due to people traveling to endemic areas.

The disease is typically transmitted through the bite of an infected a decent Aedes agypti or Aedes albopictus mosquito.

These mosquitoes are peri domestic and prefer to lay their eggs above the water line of containers, tree holes, creases in tarpaulins and other vessels that may hold water. It is therefore vital, to remove any of these potential habitats from human habitation to decrease the risk.

Generally, the illness is quite mild, and symptoms include a fever, rash, conjunctivitis, and joint pain, which can last anything from a few days to several weeks or months.

Often, patients are not even sick enough to seek medical treatment, which leads to many cases not being reported.

Despite this, cases of microcephaly, a congenital defect of cranium and brain size which results in neurological defects in newborns have been identified as being caused by a Zika infection.


What Should I Do to Avoid Becoming Infected?

As with any other “vector” borne disease, the best defense is to keep from being bitten if at all possible. Keeping mosquitoes out of the house by maintaining screened windows and doors is very important. Avoiding outdoor activities in the early evenings, if possible, also will help you to avoid the times of high mosquito activity.

Since keeping kids indoors on summer evenings is next to impossible, they should wear long-sleeved shirts and pants if at all possible, and any exposed skin should be coated with mosquito repellent. Repellents with an active ingredient called DEET have been shown to be the most effective.

Several studies have been conducted by universities researchers, and the consistent conclusion is that DEET products are capable of providing several hours of protection, while many of the “home remedy” repellents may be effective for only a few minutes. DEET can be used safely and provides the best protection.

On Your Property
You also should take a critical look around your own property, and determine what sites you are allowing that could breed mosquitoes.

Remove Standing Water
Since mosquito larvae need to live in water you should empty any containers that do not need to have water in them:

  • Cans, buckets, yard decorations, dog water buckets, barrels, garbage cans
  • Discarded tires!! – these are a huge source of mosquitoes, since they are so hard to empty. If they cannot be removed from the property and recycled properly, then try to drill holes in them to allow the water to drain out.
  • Clogged roof gutters, potted plant containers, etc.
  • “Tree Holes” – some of the most common sources of mosquito larvae, and thus adults, is holes in trees where a branch broke off or trunks come together. These holes need to be emptied of water, filled with sand, or drained by drilling a hole from below.

If you have ponds or other permanent water bodies on your property you can call the local Mosquito Control Agency and see if they can provide mosquito fish for your pond, or treat the water with a biological material that will kill the mosquito larvae. This agency is variously referred to as the Mosquito and Vector Control District, Mosquito Abatement District, or simply Mosquito Control Agency. Your local County Health Department also may have “vector” specialists in their Environmental Health office.

West Nile Virus is a problem, and not one to take lightly. It rapidly is spreading its area of concern as birds spread their territories, carrying the virus with them. However, compared with many other risks in our lives the risk from severe problems from this virus is small. Understanding the disease and its biology and ecology can be important in helping to prevent your risk from it.

Prevention Through Pest Control

One of the important protections you have available is the Professional Pest Control Industry in your area. These are licensed, educated individuals who have access to equipment and materials that may be applied to reduce mosquito populations on your property. A call to them also could provide you with an experienced person who can inspect your property to offer suggestions on steps you can take to reduce the possibility of raising mosquitoes on your own property. West Nile Virus is an important problem, and these technicians are likely to have had some training on the topic at their professional meetings and conferences.

For some very thorough internet sites that cover virtually every aspect of West Nile Virus you might click on the following Internet address:

This is the Centers for Disease Control website, and this agency will offer the most up-to-date information available, and present it to you in a manner that will be factual and trustworthy. It is important to understand a problem in order to be able to avoid it.