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Mosquito-Borne Illnesses

Mosquitoes are responsible for more human suffering than any other creature, causing over a million deaths globally each year due to the diseases they transmit. In addition to their impact on human health, mosquitoes spread illnesses that affect animals like dogs and horses, including heartworm, West Nile virus (WNV), and Eastern equine encephalitis (EEE). Besides transmitting diseases, mosquito bites can cause skin irritation and itching as a result of an allergic reaction to the mosquito's saliva.

Mosquito-borne diseases come in several forms, including:

Protozoan diseases such as malaria
Filarial diseases like dog heartworm
Viral diseases including dengue, encephalitis, and yellow fever

For those traveling to regions where mosquito-borne illnesses are more common, the CDC Travelers’ Health resource provides valuable information to help travelers stay informed and protected.

Malaria

Malaria is an ancient disease likely originating in Africa, with records as far back as 2700 BC in Chinese medical texts and 1700 BC in Sumerian sources. The disease is caused by Plasmodium parasites, which are transmitted by female Anopheles mosquitoes. The term “malaria” comes from the Italian mal-aria (“bad air”), a reference to the old belief that it spread through swamp and river air.

Global Impact

Population at Risk: Approximately 40% of the global population, primarily in tropical and subtropical regions, is at risk of malaria. Each year, the disease results in over one million deaths and 300–500 million cases, with one child reportedly dying every 40 seconds.

Control and Eradication Efforts

Historical Research and Control: Scientific study of malaria intensified in the late 19th century. With the introduction of DDT and other insecticides in the 20th century, malaria was largely eliminated in temperate regions. Public health advancements, including air conditioning and screened windows, have since limited transmission to tropical areas, though sporadic cases are still seen in northern Europe.

Malaria in the United States

Early Colonization to Present: Malaria impacted early colonial settlements along the eastern U.S. and wasn’t effectively controlled until the 1940s with dedicated Anopheles mosquito management. The Vietnam War saw a resurgence due to returning military personnel, though any modern cases in the U.S. are managed swiftly with aggressive control measures.
Future Risks: Factors like travel, immigration, and tourism may contribute to occasional, localized outbreaks.

Advances in Treatment and Prevention

Antimalarial Drugs: Available for over 50 years, these drugs help prevent and treat malaria. Recent scientific breakthroughs, including decoding the malaria parasite genome, may aid future eradication efforts. For now, case detection and mosquito control remain essential.

Chikungunya

Chikungunya is a mosquito-borne virus that established itself in the Caribbean in late 2013, with approximately 350,000 suspected cases reported across the Western Hemisphere. By July 2014, Florida had confirmed two locally-transmitted cases, and 497 travel-related cases were reported across 35 U.S. states, Puerto Rico, and the U.S. Virgin Islands. This spread has raised concerns among public health officials, who fear the virus may establish itself in areas bordering the Caribbean.

Symptoms and Transmission

Name Origin: “Chikungunya” comes from the Kimakonde word meaning “that which bends up,” referring to the severe joint pain that causes those infected to stoop in pain. While rarely fatal, symptoms are debilitating and can last for several weeks.
Treatment: No vaccine exists for Chikungunya, and treatment primarily involves pain management.

Mosquito Vectors

Primary Carriers: The virus is transmitted by the Asian Tiger Mosquito (Aedes albopictus) and the Yellow Fever Mosquito (Aedes aegypti). Studies suggest that the strain spreading in the Americas is more readily transmitted by Aedes aegypti.
Breeding Habits: These mosquitoes lay eggs in man-made containers like cans, discarded tires, and other items that collect water, typically close to human dwellings. While Aedes albopictus is more widely distributed, Aedes aegypti is mainly found in the southeastern U.S.

Control Measures

Challenges with Spraying: Traditional control methods like truck-mounted and aerial sprays are largely ineffective against Aedes mosquitoes. Effective prevention focuses on removing water-bearing containers and maintaining clean environments to reduce breeding sites.

Dog Heartworm (Dirofilaria Immitis)

Dog heartworm (Dirofilaria immitis) is a potentially life-threatening disease in dogs, caused by a parasitic roundworm. Occasionally, cats, foxes, and raccoons may also be infected, acquiring the parasite through mosquito bites.

Transmission and Life Cycle

Mosquito Dependency: The heartworm relies on both a mammalian host and mosquitoes to complete its life cycle.
Infection Process: Young heartworms, called microfilariae, circulate in the blood of an infected dog. When a mosquito bites the dog, it ingests these microfilariae, which develop within the mosquito over 2–3 weeks. The mature larvae then migrate to the mosquito’s mouthparts, ready to infect another animal.
Infection in Dogs: Upon the mosquito’s next bite, larvae are deposited on the animal's skin, entering through the bite wound. They migrate under the skin and continue developing for 3–4 months, eventually reaching the dog’s heart and lungs.

Risk and Prevention

Treatment Risks: Heartworm can be controlled or prevented with medications, but some risk is associated with treatment if a dog is already infected. Prevention through regular medication is typically safer.
Mosquito Control: Effective mosquito control also reduces the likelihood of transmission.
Spread: Originally confined to the southeastern and eastern U.S., dog heartworm is now found in all 50 states and parts of Canada.

Dengue Fever

Dengue is a serious mosquito-borne viral disease prevalent across the Americas, Asia, and Africa. Although it has a relatively low mortality rate, its symptoms are highly uncomfortable, and the disease has increased in both frequency and severity over recent years.

Transmission and Spread

Vectors: Dengue is transmitted primarily by Aedes aegypti and Aedes albopictus mosquitoes, which lay eggs in water-holding containers near human habitations. These mosquitoes are challenging to control with standard spraying methods.
Global Spread: The global spread of dengue is largely due to the adaptability and spread of its mosquito vectors. Over the past two decades, dengue cases have risen dramatically worldwide.

Notable Outbreaks

United States: South Texas saw an outbreak in 1999 with 55 cases and one death. In 2010, Florida experienced its first locally transmitted cases in 75 years, with 67 cases reported in Key West and surrounding areas by the year’s end.
Caribbean: Puerto Rico experienced its largest dengue outbreak in 2010, reporting 21,000 cases.
Global Examples:
- In 2004, Venezuela recorded over 11,600 cases of classic dengue fever and 700 cases of dengue hemorrhagic fever (DHF).
- Indonesia faced a major outbreak with 54,000 cases and over 600 deaths.

Yellow Fever

Yellow fever is a viral disease with a history spanning over 400 years, now found primarily in tropical regions of Africa and the Americas. It has two transmission cycles:

Urban Cycle: Transmitted between humans via mosquitoes in populated areas.
Jungle Cycle: Involves transmission among forest-dwelling animals, with occasional spillover to humans.

Prevalence and Prevention

Yellow fever is rare among travelers due to vaccination requirements enforced by many countries. Nevertheless, around 200,000 cases occur each year, with approximately 30,000 deaths across 33 endemic countries in Africa and South America. Importantly, yellow fever does not occur in Asia.

Recent Incidents

United States: In 2002, a fatal case was reported in an unvaccinated traveler returning from the Amazon.
Africa: In May 2003, an outbreak in southern Sudan resulted in 178 cases and 27 deaths.
South America: In 2003, 226 jungle yellow fever cases were reported, with 99 fatalities.

Vaccination is the primary safeguard against yellow fever, supported by mosquito control efforts in affected areas. For the latest information on yellow fever and vaccination requirements, visit the CDC's Yellow Fever information page.

Eastern Equine Encephalitis (EEE)

Eastern Equine Encephalitis (EEE) is a rare but severe mosquito-borne virus that primarily affects the central nervous system and can lead to serious complications or even death. It is transmitted to humans and horses by mosquitoes that have fed on infected birds, particularly in freshwater hardwood swamps along the Atlantic and Gulf Coast states, as well as in parts of Central and South America and the Caribbean.

Transmission and Hosts

EEE’s life cycle involves birds and specific mosquito species, such as Culiseta melanura and certain Culex species. These mosquitoes become infected from birds and can then transmit the virus to humans, horses, and other mammals. However, since humans and other mammals do not spread the virus further, they are referred to as “dead-end hosts.”

Symptoms and Outcomes

Mild cases may present with flu-like symptoms, including fever, headache, and sore throat.
Severe cases can involve sudden high fever, severe headache, seizures, and coma, potentially resulting in permanent brain damage or death.

Approximately half of those with severe infections do not survive. Survivors often face long-term neurological effects. Currently, there is no specific treatment or human vaccine available, though a vaccine exists for horses.

St. Louis Encephalitis (SLE)

St. Louis Encephalitis (SLE) is a mosquito-borne virus primarily transmitted from infected birds to humans and other mammals through certain Culex mosquito species. SLE occurs throughout the United States, especially along the Gulf of Mexico, with Florida experiencing several major outbreaks in the past. The elderly and very young are most at risk of severe illness.

Key Facts and Transmission

Transmission: SLE spreads from birds to humans via Culex mosquitoes.
Regions: Found across the U.S., with frequent cases in Gulf states.
Epidemics: Significant outbreaks occurred in Florida in 1959, 1961, 1962, 1977, and 1990.

Symptoms and Vulnerability

Symptoms of SLE resemble those of Eastern Equine Encephalitis (EEE) and may include flu-like signs. Severe cases can lead to inflammation of the brain, which can be life-threatening, particularly for infants and the elderly.

Prevention and Treatment

There is no vaccine or specific treatment for SLE. Prevention relies on mosquito control and avoiding bites, especially in regions with known outbreaks.

LaCrosse Encephalitis (LAC)

LaCrosse Encephalitis (LAC) is a relatively rare mosquito-borne virus, with about 90 cases reported annually, primarily in the Appalachian region and states east of the Mississippi. The virus was first identified in 1963 in LaCrosse, Wisconsin. It is transmitted by the Aedes triseriatus, or tree-hole mosquito, and small mammals typically serve as its main hosts.

Key Facts and Transmission

Primary Vector: Aedes triseriatus (tree-hole mosquito)
Regions: Primarily found in Appalachian areas and states east of the Mississippi
At-Risk Population: Children under 16 are most susceptible to severe cases.

Symptoms and Risks

Severe cases are rare but can lead to encephalitis, particularly in younger children. Human-to-human transmission does not occur.

Prevention and Treatment

There is currently no vaccine or specific treatment for LAC, making mosquito control and prevention measures essential in affected areas.

Western Equine Encephalitis (WEE)

Western Equine Encephalitis (WEE) was first identified in 1930 in a horse in California and is primarily found west of the Mississippi River, including parts of Canada and Mexico. The main vector for WEE is Culex tarsalis, a species of mosquito, and birds are the primary vertebrate hosts, with small mammals playing a secondary role.

Key Facts and Transmission

Primary Vector: Culex tarsalis
Hosts: Birds are the most important vertebrate hosts; small mammals play a minor role.
Regions: Found west of the Mississippi, including parts of Canada and Mexico.

Symptoms and Risks

While human cases are rare, WEE can cause symptoms such as fever, headache, and occasionally encephalitis. Since 1964, fewer than 1,000 cases have been reported.

Prevention and Treatment

A vaccine is available for horses but not for humans. In Arizona, three counties have reported sentinel chicken flocks testing positive for WEE.

West Nile Virus (WNV)

West Nile Virus (WNV) originated in Africa in 1937 and has since spread across Europe, the Middle East, Asia, and the Americas. It is a member of the Flavivirus family, which includes more than 70 known viruses. WNV is closely related to viruses that cause diseases like Japanese encephalitis and St. Louis encephalitis. The virus cycles between birds and mosquitoes and is transmitted to humans and mammals, including horses, through mosquito bites.

Types of WNV Illness:

West Nile Fever: The mildest form, with symptoms like fever, headache, body aches, and sometimes a rash. This usually lasts a few days to several weeks.
Neuroinvasive Disease: This severe form affects the nervous system, causing:
- West Nile Encephalitis: Inflammation of the brain.
- West Nile Meningitis: Inflammation of the membranes around the brain and spinal cord.

History and Spread

WNV first appeared in North America in 1999, when it caused 62 confirmed cases and 7 deaths in New York. Since then, it has spread across the United States, Canada, and the Caribbean, infecting both humans and animals.

2001: 66 human cases, 10 deaths, and over 700 horse cases.
2002: More than 3,800 human cases and 232 deaths across 39 states.
2003: Nearly 10,000 cases in the U.S., with over 4,200 horse infections and 5,000+ infected birds.

Transmission and Symptoms

WNV is transmitted by mosquitoes, particularly Culex pipiens. While most people infected with WNV do not show symptoms, approximately 20% develop West Nile Fever. Around 1 in 150 people will develop neuroinvasive diseases such as encephalitis or meningitis, which can be fatal or lead to long-term complications.

Fatality Rate: About 10% for those with neuroinvasive disease.
Asymptomatic Cases: Roughly 82% of infections are asymptomatic.

Geographic Spread

United States: WNV has been reported in all 50 states, with the highest case numbers in states like Florida, Colorado, and California.
Canada: Over 1,200 confirmed human cases and more than 400 horse cases.
Mexico: WNV was reported in 2003, with over 2,400 horse cases and 117 infected birds.

Prevention and Treatment

There is no specific treatment for WNV. Preventive measures include:

Mosquito control: Use of insect repellent, wearing protective clothing, and eliminating standing water.
Horse vaccine: Available for horses, but not for humans.

Global Surveillance

WNV has been found in various animals, including alligators in Georgia.
The UK and other nations monitor for potential human cases, although no widespread outbreaks have been reported.

As of 2014, 36,437 cases of WNV have been reported in the U.S., with 15,774 resulting in meningitis/encephalitis, and 1,538 fatalities. The CDC estimates that actual cases may be much higher due to underreporting, as WNV resembles other viral infections.

Zika Virus

Zika virus is a Flavivirus, related to West Nile, Yellow Fever, and St. Louis encephalitis viruses. First discovered in macaque monkeys in Uganda's Zika Forest in 1947, it has since spread rapidly to the South Pacific and the western hemisphere, particularly affecting the Americas.

Geographic Spread

Since 2014, Zika cases have been reported in 35 countries in the Americas, particularly in South America and the Caribbean.
As of April 28, 2016, there have been 426 travel-related cases in the U.S. However, local transmission in the continental U.S. has not been reported. In U.S. territories, such as Puerto Rico and the U.S. Virgin Islands, over 590 cases have been locally acquired.

Transmission

Zika is mainly transmitted by Aedes mosquitoes (Aedes aegypti and Aedes albopictus), which bite during the day and night. Zika can also be spread through sexual contact in rare cases. The mosquito vectors typically lay eggs in water-filled containers or treeholes near human dwellings.

Symptoms

Zika infection is usually mild and includes:

Fever
Rash
Conjunctivitis (red eyes)
Joint pain These symptoms usually last a few days to several weeks. Many infected individuals do not seek medical attention, so many cases go unreported. Most individuals gain immunity after an infection.

Serious Health Effects

While Zika itself is typically mild, it has been linked to severe outcomes:

Microcephaly: A birth defect that causes small head size and severe neurological damage, usually resulting in death in newborns. Zika infection during pregnancy has been positively linked to microcephaly.
Guillain-Barré Syndrome (GBS): An autoimmune disorder that can cause muscle weakness, paralysis, and in rare cases, death.

Prevention and Control

Mosquito Control: Aedes mosquitoes are difficult to control with traditional outdoor spraying because of their cryptic feeding habits. Removing water-holding containers and cleaning them regularly is the most effective way to reduce mosquito breeding sites.
Personal Protection: Use EPA-registered insect repellents, wear long sleeves and pants, and sleep under mosquito nets, especially in areas where Zika is endemic.
Travel Recommendations: For those traveling to Zika-affected areas, staying in air-conditioned hotels with screens and sleeping under nets is advised.