Dengue fever is an infectious viral disease. It is transmitted by mosquitoes in tropical and sub-tropical…
What are mosquito-borne illnesses?
The mosquito has been called 'the deadliest animal in the world', due to mosquito-borne illnesses. Mosquito-borne illnesses place a heavy toll on global health systems, causing a significant portion of human illness and death from infectious disease - more than one million people die of mosquito-borne illnesses each year   . A disproportionate share of this burden falls on poorer countries, where climate conditions are often more suited to mosquitoes.
'Mosquito-borne illnesses' is an umbrella term for several different diseases that share the same mode of transmission - they are all passed between humans (and animals) via mosquito bites.
Malaria is an infectious blood disease transmitted between people via the bites of infected female Anopheles mosquitoes. Malaria is the most common and most deadly mosquito-borne-illness in the world, affecting hundreds of millions of people and killing hundreds of thousands every year, particularly children. It is common in warm, equatorial regions, including parts of Africa, South-East Asia and Latin America. Cases of malaria outside these regions are most often due to travellers returning from countries where malaria is prevalent.
Malaria destroys the red blood cells in the body and interferes with the body's ability to make new red blood cells, starving the body's vital organs of oxygen, which can be fatal. Malaria parasites can also block the tiny blood vessels in the brain. This condition, known as cerebral malaria, can result in coma, seizures, permanent disability or death.
A person infected with malaria will feel feverish and weak. The symptoms usually take some time to appear, typically developing within 1-4 weeks following the infection. The severity of the symptoms is less in people who were previously exposed to malaria, or are taking certain medications.
If you have been infected, you are likely to experience the symptom most associated with a malaria infection, known as the malaria paroxysm. This is due to the life cycle of the malaria parasite. The paroxysm commonly lasts 6-10 hours and goes through three stages. The first is the cold stage, in which you suddenly feel very cold and experience chills and shivers. This is followed by the hot stage, when you feel feverish and restless. The third stage is the sweating stage, characterised by profuse sweating.
The paroxysm can appear every day in the first phase of the disease. After several weeks, it usually stabilises and can appear only once every 2-3 days.
If you have been diagnosed with malaria, you will receive antimalarial medication. Various medications are prescribed either separately or as a combination. Different antimalarial medications target different parasites and the different life-stages of each parasite type. Specific courses of treatment take many factors into account, such as geographical region, the age and immune status of the patient and drug resistance.
Malaria is a treatable and preventable disease. Uncomplicated cases of malaria, if treated correctly, eventually subside. Medication can be prescribed by your doctor to try and reduce your chances of becoming infected with malaria if you are travelling to a malaria endemic area. There is currently no vaccine against malaria, though much research is being conducted on the subject.
Viral mosquito-borne illnesses
Dozens, if not hundreds, of different viruses are spread by mosquitoes all over the globe. Many of them are restricted to specific regions (such as the Murray Valley encephalitis virus in Australia, or the Saint Louis encephalitis virus in the United States), but some are spread out in many parts of the globe. The viral illnesses briefly described here are some of the more significant ones affecting humanity today.
For all the illnesses described below, there is no specific treatment. If you are suffering from one of these illnesses, treatment will focus on rest, replacing fluids, pain and fever relief and management of other symptoms of the illness.
Yellow fever is an infectious viral disease transmitted by mosquitoes in tropical and sub-tropical climates. It is a serious health concern, particularly in African and South American countries. A yellow fever vaccine is available and is routinely given to people who live in, or are intending to travel to, countries where the disease is common.
Yellow fever is a common disease in Africa and South America. Each year around 200,000 cases of yellow fever occur, claiming around 30,000 lives.
Many people who become infected with the virus show no symptoms at all. Symptoms can include fever, headache, back pain, muscle and joint pain, jaundice, nausea and vomiting, loss of appetite, weakness, restlessness and irritability. In those people where the disease has progressed to its severe stage, internal bleeding from multiple organs can cause numerous symptoms and complications, including death.
Dengue fever is an infectious viral disease. It is transmitted by mosquitoes in tropical and sub-tropical climates. Dengue fever is among the fastest-spreading infectious diseases in the world. Over the past 30 years, dengue fever has been expanding, spreading across many areas of the world, including Asia, Africa and the Americas. The number of countries experiencing dengue fever epidemics has grown from nine in the 1970s to currently more than 100.
Reported cases of dengue fever number in the millions each year, but as most cases of dengue fever do not lead to any serious symptoms, the number of actual infections is undoubtedly much higher. Estimates range between 50-390 million cases annually. Each year, about 500,000 people are hospitalised with severe dengue fever, and tens of thousands die of the disease.
Most people infected with dengue fever will not experience any symptoms, or only mild ones. These symptoms tend to be temporary, eventually resulting in a full recovery. However, complications of dengue fever can have serious and potentially life-threatening outcomes.
Early symptoms of dengue fever include high fever, severe headache, severe pain behind the eyes, bone, muscle and joint pain, rashes, swollen lymph nodes, nausea and vomiting, bleeding gums and bruising of skin. Dengue fever can also result in complications such as damage to the circulatory system which causes internal bleeding from multiple organs. This is known as severe dengue fever, occurring in 1-5% of people who will then require hospitalisation for treatment. Severe dengue fever is most common in people - especially children - who have been infected with a different type of dengue fever virus in the past. With good supportive treatment, the mortality rate can be lower than 1%, compared to about 20% without treatment.
Currently, there is no vaccine against dengue fever.
This is a viral illness caused by the chikungunya virus. It was first identified in Kenya in the 1950s and has since re-emerged in the African continent periodically. In 2004, a new strain of the virus emerged that spread throughout many areas of the world, including Asia, Central and South America, the Caribbean islands, the Pacific and Indian Ocean regions and Europe. The first case of locally-acquired chikungunya in the United States was reported in Florida on July 17, 2014. Millions of people have so far been affected by epidemic outbreaks of the illness.
The symptoms of chikungunya closely resemble those of dengue fever. However, unlike dengue, its effect on adults is stronger than on children. The virus is not known to cause death by itself, but it appears to increase the risk of death from other causes.
There is no vaccine against the chikungunya virus, but a promising candidate, the virus-like particle (VLP) vaccine, is currently in trials.
Ross River virus / Barmah Forest virus
These are two related, fever-causing viruses originally identified in Australia (Ross River virus was later also found in other locations in the Pacific region). Together, these illnesses affect about 6000-7000 people a year in Australia   . Mosquitoes transmit these viruses from an animal (kangaroos or rodents) to humans. The viruses do not pass from human to human and do not pass via direct animal-to-human or human-to-human contact.
Most people who contract the viruses experience no symptoms at all. A minority of infected people will experience symptoms such as:
- Joint and muscle pain and swelling and stiffness, which are most noticeable in the wrists, knees, ankles, fingers and toes;
- A rash, and;
- Flu-like symptoms (fever, chills, headache, fatigue and sensitivity to light).
The symptoms can last for several weeks, and then gradually disappear. Rarely, the symptoms can persist for up to a year. Rest is the recommended treatment and no complications are expected. In the later stages of the illness, it is important to stay moderately mobile and active to prevent the development of stiffness in the joints.
Ross River and Barmah Forest fevers are diagnosed by blood tests. It is important to diagnose them correctly in order to rule out more serious diseases with similar symptoms, such as rheumatoid arthritis.
West Nile virus
The various types of the West Nile virus are transmitted from birds to humans via the bites of several kinds of mosquitoes. This virus was originally identified in 1937 in the West Nile region of Uganda and is widespread throughout Africa, the Middle East, Europe, Australia and parts of West Asia. It emerged in 1999 in North America and has been causing periodic outbreaks there since.
The virus usually appears in outbreaks. Of people infected with the virus, 80% show no symptoms  . Common symptoms of infection include a rash, fever and pain in the joints. In one out of every 150 people, particularly those with weak immune systems, severe disease can appear as encephalitis and meningitis. These complications can be life-threatening. Treatment of the illness is supportive and no vaccine is available.
Japanese encephalitis virus
Members of the Japanese encephalitis virus family are spread across the globe. The best-known variant of the family is found mostly across southern Asia (east as well as west), Papua New Guinea and northern Australia. It is most common in rural areas with large bodies of standing water, such as rice fields.
The main danger of the virus is encephalitis: an estimated 50,000 - 70,000 cases occur every year, causing about 10,000 deaths  . Like the West Nile virus, it is transmitted via mosquito bites from birds to humans.
Rift Valley fever
Mostly a disease of livestock, Rift Valley fever can also affect humans. It is caused by the Rift Valley fever virus. It is found mostly in Africa, but has recently been identified in the Middle East. The virus is transmitted via mosquito bites, but humans can also be commonly infected through contact with infected animals, especially via blood and exposed tissues of slaughtered animals.
The disease normally disappears without treatment after a few days of symptoms which can include fever, fatigue, back pain and dizziness, sometimes accompanied by vision problems. However, about one in 100 people infected with the virus will develop serious complications, such as encephalitis and severe internal bleeding, and are at serious risk of death   .
Lymphatic filariasis (elephantiasis)
Filariae are tiny worm parasites. Three types of filarial parasites (W. bancrofti, B. malayi and B. timori) are transmitted to humans through mosquito bites. The parasites move from the infected person's skin to the lymphatic system and settle there, forming 'nets' of worms and blocking lymphatic tubes. The parasites then move to the bloodstream where they are taken up by mosquitoes again and the cycle continues.
An estimated 120 million people worldwide are infected with filariae  . A large number of people infected with filariae experience no symptoms and might not know they are infected. Typical symptoms of filariae infection include considerable swelling, as a result of the blocked lymphatic system, general bodily pain, fever, genital problems (inflammation of the scrotum) and kidney damage.
- Specific anti-filarial medication;
- Hygiene practices to minimise the risk of further infections, and;
- Measures designed to unblock the lymphatic vessels.
Bite prevention methods
A good way of reducing the chance of mosquito-borne illnesses is to prevent being bitten by mosquitoes.
Anopheles and Culex mosquito bite prevention
Anopheles mosquitoes, responsible for transmitting malaria, and Culex mosquitoes, responsible for transmitting filariasis, are active mostly during the night-time. You can protect yourself by taking the following measures:
- Staying indoors around dusk, night-time and dawn, when mosquitoes are most active;
- Sleeping under mosquito nets treated with insect repellent;
- Using insect repellent, insecticides and mosquito-control devices (coils, zappers);
- Fitting flyscreens in windows, doors and gutters of buildings;
- Avoiding staying near freshwater puddles, pools and lakes;
- Wearing long clothes in light colours to cover as much skin as possible, and;
- Avoiding perfumes and other artificial body scents.
Aedes mosquito bite prevention
The above measures are also good for preventing the bites of Aedes mosquitoes, the ones mainly responsible for transmitting dengue fever, yellow fever, West Nile virus, chikungunya and many encephalitis-causing viruses. However, these mosquitoes are mostly active during the day. Therefore, bed nets and other night-time bite prevention strategies are less useful against them.
Vaccinations are the best way to prevent illness - if they are available. As noted above, for some of these illnesses, vaccinations are not yet available.
Mosquito control methods
Water source reduction
An effective way of controlling mosquito numbers is to prevent their access to standing water, which they use to lay eggs. This requires governments and citizens to be aware of sources of standing water in their environment (containers, puddles, old tyres, pots, etc.) and drain or seal them.
Biological and chemical control of mosquitoes
Insecticides can be used on sources of mosquito populations.
Mosquitoes are a source of food for fish, dragonflies and aquatic turtles, which can be introduced into the infested area to minimise mosquito numbers.
Bacteria are also being trialled as a means of mosquito control - an infection of mosquitoes with bacteria of the Wolbachia family may prevent mosquitoes from becoming infected with a variety of viruses that cause dengue, malaria, yellow fever, chikungunya and West Nile  .
Finally, genetic approaches that generate modified mosquitoes, such as the flightless female mosquitoes or sterile male mosquitoes, may interfere with mosquito reproduction and reduce their numbers.