The pesky mosquito is a nuisance to most of us but it can in fact be deadly. Mosquitoes cause more deaths globally than any other insect. However, this has to be put into context because it is not the mosquito itself that is so deadly but rather the parasites that it may spread through its bites. While many of these mosquito-borne diseases were rare in North America, global travel has introduced some of these diseases to the United States.
Mosquito Parasites and Infections
There are three main organisms that may be responsible for causing disease in mosquito-borne infections:
Some of these parasites require the mosquito to complete part of its life cycle. Others are simply carried by the mosquito and spread to humans. Mosquitoes are therefore vectors in that it plays a role in transmitting the disease, and are not the actual cause of infection. One of the concerns with some of these mosquito-borne diseases is the ability of the insect to feed on an infected person and then infect the next person it feeds on. It can occur with certain infectious agents. However, contrary to some myths HIV and the ebola virus cannot be transmitted by mosquitoes.
- SPREAD: Caused by protozoa (Plasmodium species) which is in the mosquitoes saliva. It is injected into the human bloodstream when the mosquito feeds. It is specifically spread by the Anopheles mosquito.
- LOCATIONS: Was a problem in southern United States until it was eradicated. Endemic areas include parts of South America, Africa and Asia. Transmission occurs only in some areas within these regions where as it occurs throughout in other regions.
- RECOVERY: The prognosis is good if uncomplicated malaria is treated promptly. Marked improvement is seen within 48 hours and fevers resolve by 96 hours after starting treatment. Death is possible with certain Plasmodium species if its left untreated.
Read more on malaria prevention for travelers.
- SPREAD: Caused by different serotypes of the dengue virus (DENV-1 to DENV-4) which is in the Aedes mosquito’s saliva. The mosquito acquires the virus when it feeds on an infected person, and then infects the next person upon subsequent feedings.
- LOCATIONS: The southeastern United States has previously experienced a dengue epidemic but it is mainly travelers that are at risk. The disease is endemic in over 100 countries. Regions include Latin America, the Carribean, Pacific Islands, tropical Asia and sub-Saharan Africa.
- RECOVERY: Dengue is a self-limiting disease but severe dengue (dengue hemorrhagic fever) can be deadly in 50% of cases if left untreated. The prognosis is very good in almost all mild cases and treated severe cases.
- SPREAD: Caused by the an alphavirus (CHIKV) which the mosquito acquires when feeding on an infected person. It then spreads the virus upon feeding on subsequent people. As with dengue, chikungunya is spread by the Aedes species of mosquitoes.
- LOCATIONS: The endemic regions for chikungunya in Asia and Africa are similar to dengue. The similarities of both diseases in terms of transmission and initial presentation often leads to confusion among those infected.
- RECOVERY: The prognosis for chikungunya is very good. Total recovery occurs in most cases and serious complications are rare.
Read more on chikungunya and dengue similarities.
Eastern Equine Encephalitis (EEE)
- SPREAD: Caused by a virus that is spread through the bites of the Culex and Culiseta mosquito species, and sometimes through other mosquito types like the Aedes species.
- LOCATIONS: EEE transmission occurs in the United States and is also prevalent in other Gulf coastal areas, including Mexico, parts of South America and the Caribbean.
- RECOVERY: The prognosis for EEE is poor. Up to 70% of infected people will die and most survivors have a mild to severe impairment for life. Only 10% of those infected will fully recover.
Read more on viral encephalitis that is caused by viruses such as EEE virus.
- SPREAD: Caused by a virus that is mainly spread by the Culex species of mosquitoes, although the Aedes species has also been implicated. Domesticated pigs and birds are the main hosts and the mosquitoes may then spread it to humans.
- LOCATIONS: It is a seasonal disease that is mainly found in South and East Asia, not only in Japan. Cases in the United States are seen among expatriates and military personnel residing in these endemic areas for long periods.
- RECOVERY: Very few people who are infected will develop symptomatic disease. The prognosis is poor in patients with symptomatic infections, with either long term complications or death rates as high as 35% in rural areas.
St. Louis Encephalitis (SLE)
- SPREAD: Caused by an virus that is spread by mosquitoes from wild birds to humans. It is the Culex species of mosquitoes that are the main vectors.
- LOCATIONS: SLE is widespread in the Americas, from Canada to Argentina. Almost all human cases have occurred in the United States and in the eastern and central states in particular.
- RECOVERY: The risk of death is as high as 30% of those infected and older patients in particular are at risk. The recovery is fair with supportive measures but there may be some long term complications in about 20% of patients.
La Crosse Encephalitis (LAC)
- SPREAD: Caused by an virus, the La Crosse Virus (LACV), which is spread by the Aedes triseriatus mosquito (tree-hole mosquito). Transmission occurs between small mammals and humans.
- LOCATIONS: Named after La Crosse in Wisconsin where it was discovered. Mainly found in the Appalachian and Midwestern regions of the United States.
- RECOVERY: The prognosis is good although there is no specific treatment available for the infection. Very young children are at greater risk of death, although fatalities are rare.
Western Equine Encephalitis (WEE)
- SPREAD: Caused by an alphavirus that is spread by the Culex tarsalis mosquito, sometimes other mosquitoes like the Aedes species and occasionally small wild animals.
- LOCATIONS: It is mainly found in the United States and America, usually in regions west of the Mississippi River and west of the Rocky Mountains.
- RECOVERY: The prognosis is excellent in patients who have not developed neurological symptoms. The prognosis declines significantly in infected children who develop neurological symptoms. Fatality rate are very low, usually around 3% to 4%.
West Nile Virus (WNV)
- SPREAD: Caused by a flavivirus that is spread by the Aedes, Culex or Anopheles species of mosquitoes. The virus is transmitted by mosquitoes from animals to humans.
- LOCATIONS: Infection with the West Nile virus has occurred Asia, Africa, Europe, the Middle East and North America, including the United States and Canada.
- RECOVERY: The prognosis is very good with less than 1% of infected patients developing severe disease. The elderly with multiple medical conditions are at greater risk of severe disease and death.
- SPREAD: Caused by a flavivirus that is spread by the Haemagogus and Aedes species of mosquitoes. The virus is spread from monkeys to humans or from human to human.
- LOCATIONS: Yellow fever is mainly seen in specific regions in Africa and less commonly in South America. It is rare in the United States, even among travelers returning from endemic areas.
- RECOVERY: The prognosis depends on the severity of the infection. It can vary from a mild self-limiting disease to a severe life-threatening disease. Death can be as high as 50% in patients with the toxic stage of yellow fever.