Mosquito-borne diseases or mosquito-borne illnesses are diseases caused by bacteria, viruses or parasites transmitted by mosquitoes. Nearly 700 million people contract mosquito-borne illnesses each year, resulting in over a million deaths.

Diseases transmitted by mosquitoes include malaria, dengue, West Nile virus, chikungunya, yellow fever, filariasis, tularemia, dirofilariasis, Japanese encephalitis, Saint Louis encephalitis, Western equine encephalitis, Eastern equine encephalitis, Venezuelan equine encephalitis, Ross River fever, Barmah Forest fever, La Crosse encephalitis, and Zika fever, as well as newly detected Keystone virus and Rift Valley fever. A preprint by Australian research group argues that Mycobacterium ulcerans, the causative pathogen of Buruli ulcer is also transmitted by mosquitoes.

There is no evidence that COVID-19 can be transmitted by mosquitoes, and it is extremely unlikely this could occur.

Types

Protozoa The female mosquito of the genus Anopheles may carry the malaria parasite. Five different species of Plasmodium cause malaria in humans: Plasmodium falciparum, Plasmodium malariae, Plasmodium ovale, Plasmodium knowlesi and Plasmodium vivax (see Plasmodium). Worldwide, malaria is a leading cause of premature mortality, particularly in children under the age of five, with an estimated 207 million cases and more than half a million deaths in 2012, according to the World Malaria Report 2013 published by the World Health Organization (WHO). The death toll increased to one million as of 2018 according to the American Mosquito Control Association.

Bacterial In January 2024, a publication by an Australian research group demonstrated significant genetic similarity between Mycobacterium ulcerans in humans and possums, compared to PCR screening of M. ulcerans from trapped Aedes notoscriptus mosquitoes, and concluded that Mycobacterium ulcerans, the causative pathogen of Buruli ulcer, is transmitted by mosquitos.

Myiasis

Botflies are known to parasitize humans or other mammalians, causing myiasis, and to use mosquitoes as intermediate vector agents to deposit eggs on a host. The human botfly Dermatobia hominis attaches its eggs to the underside of a mosquito, and when the mosquito takes a blood meal from a human or an animal, the body heat of the mammalian host induces hatching of the larvae.

Helminthiasis Some species of mosquito can carry the filariasis worm, a parasite that causes a disfiguring condition (often referred to as elephantiasis) characterized by a great swelling of several parts of the body; worldwide, around 120 million people are living with a filariasis disability.

Virus The viral diseases yellow fever, dengue fever, Zika fever and chikungunya are transmitted mostly by Aedes mosquito vectors such as Aedes aegypti and Aedes albopictus.

Other viral diseases like epidemic polyarthritis, Rift Valley fever, Ross River fever, St. Louis encephalitis, West Nile fever, Japanese encephalitis, La Crosse encephalitis and several other encephalitic diseases are carried by several different mosquitoes. Eastern equine encephalitis (EEE) and Western equine encephalitis (WEE) occur in the United States where they cause disease in humans, horses, and some bird species. Because of the high mortality rate, EEE and WEE are regarded as two of the most serious mosquito-borne diseases in the United States. Symptoms range from mild flu-like illness to encephalitis, coma, and death.

Viruses carried by arthropods such as mosquitoes or ticks are known collectively as arboviruses. The ones carried solely by mosquitoes are sometimes called moboviruses (mosquito-borne viruses). West Nile virus was accidentally introduced into the US in 1999 and by 2003 had spread to almost every state with over 3,000 cases in 2006.

Other species of Aedes as well as Culex and Culiseta are also involved in the transmission of disease.

Myxomatosis is spread by biting insects, including mosquitoes.

Transmission A mosquito's period of feeding is often undetected; the bite only becomes apparent because of the immune reaction it provokes. When a mosquito bites a human, it injects saliva and anti-coagulants. With the initial bite to an individual, there is no reaction, but with subsequent bites, the body's immune system develops antibodies. The bites become inflamed and itchy within 24 hours. This is the usual reaction in young children. With more bites, the sensitivity of the human immune system increases, and an itchy red hive appears in minutes where the immune response has broken capillary blood vessels and fluid has collected under the skin. This type of reaction is common in older children and adults. Some adults can become desensitized to mosquitoes and have little or no reaction to their bites, while others can become hyper-sensitive with bites causing blistering, bruising, and large inflammatory reactions, a response known as skeeter syndrome.

One study found Dengue virus and Zika virus altered the skin bacteria of rats in a way that caused their body odor to be more attractive to mosquitoes.

Signs and symptoms Symptoms of illness are specific to the type of viral infection and vary in severity, based on the individuals infected.

Zika virus Symptoms vary in severity, from mild unnoticeable symptoms to more common symptoms like fever, rash, headache, achy muscle and joints, and conjunctivitis. Symptoms can last several days to weeks, but death resulting from this infection is rare.

West Nile virus Most people infected with the West Nile virus usually do not develop symptoms. However, some individuals can develop cases of severe fatigue, weakness, headaches, body aches, joint and muscle pain, vomiting, diarrhea, and rash, which can last for weeks or months. More serious symptoms have a greater risk of appearing in people over 60 years of age, or those with cancer, diabetes, hypertension, and kidney disease.

Dengue fever Dengue fever is mostly characterized by high fever, headaches, joint pain, and rash. However, more severe instances can lead to hemorrhagic fever, internal bleeding, and breathing difficulty, which can be fatal.

Chikungunya People infected with this virus can develop sudden onset fever along with debilitating joint and muscle pain, rash, headache, nausea, and fatigue. Symptoms can last a few days or be prolonged to weeks and months. Although patients can recover completely, there have been cases in which joint pain has persisted for several months and can extend beyond that for years. Other people can develop heart complications, eye problems, and even neurological complications.

Malaria Early onset symptoms of malaria can start anywhere from 10–15 days after exposure and can present as fever, headache and chills. Symptom severity may very depend on age and previous exposures, however children under five, pregnant women and immune compromised will be at higher risk of more severe symptoms. Severe symptoms range from extreme lethargy, loss of consciousness, convulsion, difficulty breathing, bloody urine, jaundice, irregular bleeding, and death.

Mechanism Mosquitoes carrying such arboviruses are able to stay healthy due to their immune system being able to recognize the virions as foreign particles and "chop off" the virus' genetic coding, rendering it inert. A human is infected with a mosquito-borne virus when a female mosquito carrying the virus, along with its viral particles that have yet to be destroyed by the mosquito, bites a human by penetrating the skin and releasing the virus into the bloodstream. It is not completely known how mosquitoes handle eukaryotic parasites to carry them without being harmed. Data has shown that the malaria parasite Plasmodium falciparum alters the mosquito vector's feeding behavior by increasing frequency of biting in infected mosquitoes, thus increasing the chance of transmitting the parasite.

The mechanism of transmission of this disease starts with the injection of the parasite into the victim's blood when malaria-infected female Anopheles mosquitoes bite into a human being. The parasite uses human liver cells as hosts for maturation where it will continue to replicate and grow, moving into other areas of the body via the bloodstream. The spread of this infection cycle then continues when other mosquitoes bite the same individual. The result will cause that mosquito to ingest the parasite and allow it to transmit the Malaria disease into another person through the same mode of bite injection.

Flaviviridae viruses transmissible via vectors like mosquitoes include West Nile virus and yellow fever virus, which are single stranded, positive-sense RNA viruses enveloped in a protein coat. Once inside the host's body, the virus will attach itself to a cell's surface through receptor-mediated endocytosis. This essentially means that the proteins and DNA material of the virus are ingested into the host cell. The viral RNA material will undergo several changes and processes inside the host's cell so that it can release more viral RNA that can then be replicated and assembled to infect neighboring host cells. Mosquito-borne flaviviruses also encode viral antagonists to the innate immune system in order to cause persistent infection in mosquitoes and a broad spectrum of diseases in humans. The data on transmissibility via insect vectors of hepatitis C virus, also belonging to family Flaviviridae (as well as for hepatitis B virus, belonging to family Hepadnaviridae) are inconclusive. WHO states that "There is no insect vector or animal reservoir for HCV", while there are experimental data supporting at least the presence of [PCR]-detectable hepatitis C viral RNA in Culex mosquitoes for up to 13 days.

Currently, there are no specific vaccine therapies for West Nile virus approved for humans; however, vaccines are available and some show promise for animals, as a means to intervene with the mechanism of spreading such pathogens.

Diagnosis Doctors can typically identify a mosquito bite by sight.

A doctor will perform a physical examination and ask about medical history as well as any travel history. Be ready to give details on any international trips, including the dates you were traveling, the countries you visited and any contact you had with mosquitoes.

Dengue fever

Diagnosing dengue fever can be difficult, as its symptoms often overlap with many other diseases such as malaria and typhoid fever. Laboratory tests can detect evidence of the dengue viruses, however the results often come back too late to assist in directing treatment.

West Nile virus

Medical testing can confirm the presence of West Nile fever or a West Nile-related illness, such as meningitis or encephalitis. If infected, a blood test may show a rising level of antibodies to the West Nile virus. A lumbar puncture (spinal tap) is the most common way to diagnose meningitis, by analyzing the cerebrospinal fluid surrounding your brain and spinal cord. The fluid sample may show an elevated white cell count and antibodies to the West Nile virus if you were exposed. In some cases, an electroencephalography (EEG) or magnetic resonance imaging (MRI) scan can help detect brain inflammation.

Zika virus

A Zika virus infection might be suspected if symptoms are present and an individual has traveled to an area with known Zika virus transmission. Zika virus can only be confirmed by a laboratory test of body fluids, such as urine or saliva, or by blood test.

Chikungunya

Laboratory blood tests can identify evidence of chikungunya or other similar viruses such as dengue and Zika. Blood test may confirm the presence of IgM and IgG anti-chikungunya antibodies. IgM antibodies are highest 3 to 5 weeks after the beginning of symptoms and will continue be present for about 2 months.

Prevention