The most dangerous mosquito-borne diseases

Malaria, dengue, zika—mosquitoes are not just annoying when they buzz around your ear at night, they can also infect us with countless illnesses.

Mosquitoes can infect us with malaria and other illnesses. | Photo Credit: CDC/BSIP/picture alliance

It has been more than 125 years since the role of mosquitoes as the transmitters of malaria was discovered on 20 August 1897 by British physician Ronald Ross.

Ross was awarded a Nobel Prize in Medicine for his work, which has helped educate people across the planet about the risks posed by the mosquito-borne diseases injected into the human bloodstream when the flying insects “bite”. The date of his discovery now serves as the annual World Mosquito Day.

Over the course of the 20th century, other diseases transmitted by mosquitoes have been discovered on every continent except Antarctica. Taken together, these illnesses have symptoms in common that include fever, headaches, fatigue, muscle, or joint pain. Correct diagnosis usually requires on careful assessment of these and other specific symptoms

Suitable treatments for mosquito-borne diseases are rare and medical attention is usually advised, though some antivirals and vaccines have been developed. Prevention remains the best tactic in the battle against them, with the use of mosquito nets, body-covering clothing, insect repellents, and actions to reduce mosquito reproduction (such as draining still or stagnant water sources) at the top of the list.

Anopheles mosquitoes and malaria—the ‘original’ infection

Malaria is caused by single-celled parasites from a group called Plasmodium. There were more than 249 million cases globally in 2022, up five million from the year before, and nearly 20 million more than a baseline set in 2015. Almost 95 per cent of all cases occur in Africa. The disease remains one of the biggest causes of child deaths globally.

It is transmitted by “night biting” female mosquitoes belonging to the genus Anopheles. Infection progresses in fever episodes, with symptoms including headaches and pains in the limbs, sweating and chills, but also diarrhoea. Over the course of the illness, severe damage to the nervous system can occur. Vaccines like Mosquirix are beginning to emerge onto market and anti-malarial drugs can lower the risk of contracting the disease.

Aedes and Culex mosquitoes – the viral carriers

Mosquitoes from the Aedes group (particularly Aedes aegypti and Aedes albopictus) are the common carriers of viruses that cause dengue, yellow fever, Zika and Chikungunya.

Culex mosquitoes transmit pathogens that cause West Nile fever and Japanese encephalitis, and can also carry Chikungunya. These are “day biting” mosquitoes, and prevention can be helped by wearing body-covering clothing, the use of mosquito nets when sleeping during the day, insect repellents, window screens, and control of standing-water environments that mosquitoes use for breeding.

Dengue fever

Dengue is the most common mosquito-borne disease, with more than 500 million people infected each year. Symptoms range from muscle and joint pains to headaches, nausea and vomiting, and fever. Severe dengue is characterized by fatigue, dehydration, blood in the stool, vomiting, and severe pain in the nose, mouth and abdomen. Repeat infections leave a sufferer more prone to severe symptoms.

There is now a vaccine recommended by the WHO for children aged 6-16, but so far only in regions where the disease is common, as even the vaccine can trigger deadly reactions in the body. Though not endemic to Europe, it can be “imported” by travellers returning from dengue-prone regions.

Yellow fever

This is a vaccine-preventable illness now endemic in 34 African and 13 South and Central American countries.

Not everyone infected with the virus that causes yellow fever develops symptoms, but those that do may experience fever, muscle pains, backaches, headaches, nausea, and vomiting. Recovery is usually within 3-4 days, but some people develop a more toxic second phase. It can cause liver and kidney problems, abnormal bleeding and—without treatment—can even lead to death.

Both the yellow fever vaccine and surviving a prior infection provide lifelong immunity. Survival rates for those infected with the disease can be improved through prompt medical care.

Zika virus

Zika is an often symptomless disease, but one that poses risks during pregnancy. A Public Health Emergency of International Concern due to Zika outbreaks in South America was declared in 2016. Case numbers have been at low levels since 2018.

When present, symptoms include rash, fever, aches, and pains that are usually mild and last about a week. The greatest danger posed by Zika is infection during pregnancy, where the infection can be passed onto the developing fetus and may lead to microcephaly (a reduction in the size of the skull) and problems with limb, eye, and hearing development. Pregnant women with symptoms are advised to seek medical attention if they live in regions known for the disease.

West Nile virus

Like dengue, yellow fever, and Zika, WNV is a potentially fatal flavivirus. Signs of infection in humans are like those caused by many other mosquito-transmitted diseases: Shaking, cold chills, fever, headaches, and dizziness, followed by a skin rash. Not every infected person shows these symptoms. They only appear in about one in five cases.

There is no vaccine against the disease, nor are there antiviral drugs suitable for treating it. Medical attention is recommended if symptoms present.

Chikungunya virus

Like many mosquito-borne viruses, Chikungunya may not cause symptoms in everyone. When present, they usually only last for about three days. They are generally followed by a period of severe joint pain lasting one to two weeks before subsiding.

Some infected patients develop itchy rashes as well as lesions on the mucous membranes (such as inside the nose or mouth). Infections usually cause no permanent damage. Once the disease has been overcome, patients are immune for the rest of their lives. In February 2024, the US Food and Drug Administration approved the Valneva vaccine for people aged 18 years and older.

Japanese encephalitis (JE) virus

JE is the most common cause of viral encephalitis across Asia, and though symptomatic cases are rare, people with severe cases experience rapid onset of disorientation, coma, and seizures. One in three symptomatic cases are fatal. Fortunately, there are vaccines available to prevent JE.