Vector-Borne Diseases

​Overview


Key facts

• Vector-borne diseases account for more than 17% of all infectious diseases, causing more than 1 million deaths annually.
• More than 2.5 billion people in over 100 countries are at risk of contracting dengue alone.
• Malaria causes more than 600 000 deaths every year globally, most of them children under 5 years of age.
• Other diseases such as Chagas disease, leishmaniasis and schistosomiasis affect hundreds of millions of people worldwide.
• Many of these diseases are preventable through informed protective measures.

Main vectors and diseases they transmit

Vectors are living organisms that can transmit infectious diseases between humans or from animals to humans. Many of these vectors are bloodsucking insects, which ingest disease-producing microorganisms during a blood meal from an infected host (human or animal) and later inject it into a new host during their subsequent blood meal.
​
Mosquitoes are the best known disease vector. Others include ticks, flies, sandflies, fleas, triatomine bugs and some freshwater aquatic snails.

Deadly Mosquito-Borne Diseases
​

Mosquitoes cause thousands of deaths every year

Mosquitoes are one of the deadliest animals in the world. Their ability to carry and spread disease to humans causes thousands of deaths every year. In 2015 malaria alone caused 438 000 deaths. The worldwide incidence of dengue has risen 30-fold in the past 30 years, and more countries are reporting their first outbreaks of the disease. Zika, dengue, chikungunya, and yellow fever are all transmitted to humans by the Aedes aegypti mosquito. More than half of the world’s population lives in areas where this mosquito species is present. Sustained mosquito control efforts are important to prevent outbreaks from these diseases. There are several different types of mosquitoes and some have the ability to carry many different diseases. Click below to see which diseases are transmitted by the Aedes, Culex, and Anopheles mosquitoes.

Chikungunya
​

​Areas at risk:

The female Aedes aegypti and Aedes albopictus are found in the Americas, Europe, Africa, Asia and the Indian subcontinent. Chikungunya is in over 60 countries in Asia, Africa, Europe and the Americas. Mosquito breeding sites that are close to homes are a significant risk factor for chikungunya. In recent decades, these mosquitoes have spread to Europe and the Americas. In 2007, a localized outbreak in north-eastern Italy, and outbreaks in France and Croatia, were recorded subsequently.

Transmission:

The virus is transmitted by the bite of an infected female mosquito. Aedes bites during daylight hours, but peaks in activity at dawn and dusk. Both species are found biting outdoors, but Aedes aegypti will also feed indoors.

Symptoms:

Symptoms include fever and severe joint and muscle pain, headache, nausea, fatigue and rash. Joint pain is often debilitating and can vary in duration. The onset of chikungunya, after a mosquito bite, usually occurs between 4 and 8 days, but can also range from 2 to 12 days. Often symptoms are mild and the infection may go unrecognized, or misdiagnosed in areas where dengue also occurs. Most patients recover fully, but in some cases joint pain may persist. Rare cases of eye, neurological and heart complications have been reported, as well as gastrointestinal complaints. Serious complications are not common, but in older people, the disease can lead to arthritic pains of longer duration and may cause death.

Protect your health:

There is no specific antiviral drug treatment for chikungunya. Treatment is directed primarily at relieving the symptoms, including the joint pain using anti-pyretics, optimal analgesics and fluids. There is no commercial chikungunya vaccine. Personal protection with repellents, clothing that covers exposed skin and use of nets when resting during the day; window screens and destruction of mosquitoe breeding sites in and around houses and place of work is advised.

​West Nile virus

​Areas at risk:

The female Culex is 1 of 3 most common mosquitoes to be found worldwide, except for the extreme northern parts of the temperature zone. West Nile virus is found in Africa, Europe, the Middle East, North America and West Asia. The Culex also transmits Japanese encephalitis.

Transmission:

These mosquitoes feed on infected birds and transmit the disease to humans and horses. They bite from dusk until dawn.

Symptoms:

West Nile virus can cause a fatal neurological disease. Approximately 80% of those infected will not show any symptoms, but after 3-14 days, 20% of people infected may develop symptoms of severe disease including headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. People over 50-years- old, and those who are immunocompromised, are at highest risk.

Protect your health:

No vaccination exists for humans. Recommended personal protection and mosquito control includes repellents, clothing that cover exposed skin, window screens, destruction of breeding sites, especially polluted water bodies. Treatment for patients with neuro-invasive West Nile virus includes hospitalization, intravenous fluids, respiratory support, and prevention of secondary infections.

​Malaria

​Areas at risk:

In 2015, more than 3.2 billion people were at risk, and ongoing malaria transmission was found in 95 countries and territories. Sub-Saharan Africa carries a disproportionately higher share of the global malaria burden with 88% of cases and 90% of global malaria deaths. The female Anopheles transmits malaria parasites; P.falciparum is the most prevalent parasite in Africa and is responsible for most malaria deaths globally. The female Anopheles mosquitoe is an efficient vector that likes tropical, rural, and urban conditions. Anopheles that carry P. vivax are found mostly outside Africa.

Transmission:

Anopheles mosquitoes are the primary vector of malaria. These mosquitoes bite mainly at night, from dusk to dawn. Children and pregnant women are at a higher risk of disease in countries with higher rates of malaria transmission.

Symptoms:

In a non-immune individual, symptoms appear 7 days or more (usually 10-15 days) after the bite of an infected mosquito. The first symptoms are fever, headache, chills and vomiting. These symptoms may be mild and difficult to recognize as malaria. P. Falciparum malaria can progress rapidly to severe forms of the disease, especially in people with no or low immunity. Severe falciparum malaria is almost always fatal without treatment. Access to early diagnosis and prompt, effective treatment within 24-48 hrs of the onset of malaria symptoms is critical. Infants, children under 5 years of age, pregnant women and patients with HIV/AIDS, as well as non-immune migrants, mobile populations and travellers are especially vulnerable.

Protect your health:

Antimalarial medicines can also be used to prevent malaria. For travellers, malaria can be prevented through chemoprophylaxis. People living in malaria endemic areas develop partial immunity, reducing the risk of severe disease. National control programmes support the widespread use of long-lasting insecticidal nets and indoor residual spraying in targeted areas to control mosquitoes. Intermittent preventive treatment for pregnant women and infants are also recommended. Personal and household protection includes repellents, clothing that covers exposed skin, window screens, etc.

​Yellow fever

​Areas at risk:

The female Aedes also transmits yellow fever found in Africa, Latin America, in urban, jungle, forest, semi-humid conditions, and around houses. Yellow fever is mainly transmitted through the bite of the Aedes aegypti mosquito, but other "tiger mosquitoes" -Aedes albopictus also transmit yellow fever. In May 2016 a WHO Emergency Committee called for intensified national action and international support for yellow fever outbreaks in Angola and Democratic Republic of the Congo.

Haemagogus species occurs in Central and South America, Trinidad, Brazil, and Argentina, and carry the sylvan or "jungle" yellow fever, which is carried by monkeys.

Transmission:

Mosquitoes infect monkeys and then go on to transmit the disease from monkeys to humans, and from human to human. It bites mostly during the day.

Symptoms:

After 3-6 days symptoms include fever, muscle pains, backache, headache, shivers, loss of appetite, nausea or vomiting. Roughly 15% of patients enter a second, more toxic phase within 24 hours. Symptoms of this phase may include high fever, jaundice, and abdominal pain with vomiting. Bleeding can occur from the mouth, nose, eyes or stomach and blood appears in the vomit and faeces, and kidney function may deteriorate. Half of the patients who enter the toxic phase die within 10-14 days, the rest recover without significant organ damage.

Yellow fever can be difficult to diagnose and is confused with severe malaria, dengue, leptospirosis, viral hepatitis or other haemorrhagic fevers, such as West Nile virus and Zika, as well as poisoning.

Protect your health:

Vaccination is available for humans. A single dose of yellow fever vaccine can provide sustained immunity and life-long protection against yellow fever disease. One fifth of a regular dose may be used in emergencies and is found to provide up to 12 months of protection. Vector control including destruction of breeding sites and personal protection with repellents, clothing that covers exposed skin and the use of nets and window screens, is recommended.

Dengue

Areas at risk:

Female Aedes aegypti, and albopictus are found in Latin America, the United States of the America, Europe, Africa and Asia. Dengue is widespread throughout the tropics, in rural and urban areas. Recently, cases were reported in Florida, the USA and Yunnan province of China and Japan. Dengue is also common in South American countries, Costa Rica, Honduras and Mexico. In Asia, Singaporeand Lao People's Democratic Republic have reported a recent increase in cases, as well as China, the Cook Islands, Fiji, Malaysia and Vanuatu.

Transmission:

Dengue is endemic in more than 128 countries, with 3.9 billion people at risk.The Aedes aegypti mosquito is the primary vector of dengue. The virus is transmitted to humans through the bite of an infected female mosquito. Aedes aegypti bites mostly during the day.

Symptoms:

Flu-like symptoms occur 4-10 days after the bite of an infected mosquito; high fever accompanied by severe headache, pain behind the eyes, muscle and joint pains, nausea, vomitting, swollen glands or rash may occur. The disease can develop into severe dengue which is a leading cause of serious illness and death among children in some Asian and South American countries. Symptoms of severe dengue include decrease in temperature, severe abdominal pain, persistent vomiting, rapid breathing, bleeding gums, fatigue, restlessness and blood in vomit. Medical care is critical for the next 24-48 hours after symptoms are recognized to avoid complications and risk of death.

Protect your health:

There is no specific treatment for dengue and severe dengue. Early detection and access to proper medical care lowers fatality rates to below 1%. A dengue vaccine has been licensed in a few countries by some National Regulatory Authorities for people 9-45 years of age, living in endemic settings.

Zika virus

Areas at risk:

Outbreaks of Zika virus disease have been recorded in Africa, the Americas, Asia and the Pacific. Female Aedes aegypti, and Aedes albopictus are found in over 130 countries. There has been a steep rise in local Zika transmission in the Americas; since 2015, 62 countries and territories reported mosquito transmitted Zika virus. WHO announced a Public Health Emergency of International Concern on 1 February 2016.

Transmission:

Mosquitoes infect humans and people can infect each other through sexual transmission. Zika has been detected in blood, saliva, semen, spinal and other body fluids. Mother to child transmission in early pregnancy has also been reported. Aedes bites mostly during the day.

Symptoms:

Sympoms are usually mild and can include mild fever, skin rash, inflammation of the eyes (conjunctivitis), muscle and joint pain, malaise or headache. Symptoms normally last for 2 - 7 days. Zika infection during pregnancy causes microcephaly, babies born with small heads, and other fetal brain malformations. Zika is also a cause of Guillain-Barré Syndrome - a neurological condition that can lead to paralysis and death.

Protect your health:

The best form of prevention is protection against mosquito bites. Personal protection with repellents, clothing that covers exposed skin and use of nets when resting during the day; window screens and destruction of breeding sites are recommended. There is no specific treatment or vaccine currently available.

To reduce the risk of sexual transmission and potential pregnancy complications related to Zika virus infection people living in/travelling to/or returning from affected areas should practice safer sex, including wearing condoms.

​

Source:  World Health Organization