• About Aedes-transmitted diseases

Over the last few decades, diseases transmitted by the Aedes mosquito have spread rapidly. Population growth, urbanization, international travel and geographic expansion of mosquitoes have driven epidemics of Aedes-transmitted viruses, including dengue, Zika, chikungunya and yellow fever. Dengue, in particular, is now endemic in more than 120 countries with over half the world’s population at risk of infection.

The mosquitos that transmit these diseases, Aedes aegypti and Aedes albopictus, feed throughout daylight hours, with peaks of activity in the early morning and late afternoon. They have adapted to breeding in urban environments, which increases the magnitude and impact of outbreaks.

The reemergence of these Aedes-transmitted viruses has been a cause of global concern over the past decade. Zika and chikungunya have caused explosive epidemics in the Americas after recent introductions from Asia. Neurological complications from Zika virus infection led the World Health Organization (WHO) to declare a public health emergency of international concern in February 2016. Yellow fever outbreaks in urban centers, in Africa in 2016 and in Brazil in 2016-2018, highlight the continued risk of epidemic yellow fever to global public health. Co-circulation of different Aedes-transmitted viruses has potential consequences yet to be fully understood. There are a number of other arboviruses that can be transmitted by Aedes (Stegomyia) aegypti mosquitoes should they emerge from their natural, enzoonotic cycle into the urban environment as dengue, yellow fever, Zika and chikungunya have done. The potential economic impact of these Aedes-transmitted diseases is an important issue.

High quality disease and vector surveillance data, reliable diagnostics, trained staff and coordinated application will be required at a local, national and international level to reverse this worrying trend.

Dengue

Dengue is the most common vector-borne viral disease, with an estimated 400 million people infected each year.

The disease is caused by 4 distinct virus serotypes (DENV-1, DENV-2, DENV-3, DENV-4). Each serotype confers lifelong protective immunity only to itself, and infection with one serotype does not give long-term protection against the others. As a result, a single individual may be infected up to four times during in their lifetime. A second infection with a heterologous serotype puts people at greater risk for dengue hemorrhagic fever and dengue shock syndrome, the severe and potentially lethal forms of the disease. An estimated 500,000 cases of dengue hemorrhagic fever and 20,000 deaths occur annually, mostly among children.

Major epidemics in large tropical urban centers result in significant morbidity and mortality, especially in resource-poor countries where epidemics often create chaos and a break down in primary healthcare as hospitals and clinics become overloaded. The disease thus imposes significant economic and public health costs in endemic countries. Despite public health efforts to control the disease in the past 40 years, dengue epidemics have increased in frequency and magnitude as both the mosquito vectors and the viruses have continued to expand their geographic distributions.

Zika

Zika, previously thought to occur only in an enzootic cycle in the rainforests of Africa and Asia, dramatically emerged in the Asia-Pacific region and rapidly spread around the world because of its transmission by Aedes aegypti mosquitos.

Zika virus is mainly transmitted by Aedes mosquitos but it can also be transmitted from the mother to the fetus during pregnancy, and through sexual intercourse. This is of particular concern due to the increased risk of giving birth to infants with microcephaly and other severe congenital malformations if women are exposed to the virus during pregnancy. Pregnant women are therefore advised to avoid areas with Zika virus transmission.

Outbreaks of Zika virus disease have occurred in Africa, the Americas, Asia and the Pacific. In 2007, the first outbreak was reported on the Island of Yap (Federated States of Micronesia). A large outbreak of Zika virus infection followed in French Polynesia in 2013 and other countries and territories in the Pacific and Asia. From early 2015 until late 2017, Brazil experienced a major Zika outbreak associated with adverse fetal outcomes and neurological complications, including microcephaly and Guillain-Barré syndrome.

A total of 86 countries and territories have reported evidence of mosquito-transmitted Zika infection.

WHO Zika virus fact sheet

Global response to Zika virus

Chikungunya

Known as epidemic polyarthritis, this mosquito-borne viral disease can cause debilitating joint pain. The symptoms of chikungunya in infected individuals are often mild and confused with dengue, allowing he infection to go unrecognized or be misdiagnosed in areas where other arboviruses occur.

The first chikungunya outbreak was described in Tanzania in 1952. The virus caused widespread epidemics in Asian countries in the 1950s-1980s, then disappeared from that region. It reemerged in East Africa in 2004 and rapidly spread around the world. The disease has since been identified in over 60 countries in Asia, Africa, Europe and the Americas. A small chikungunya outbreak occurred in Europe (north-eastern Italy) for the first time in 2007, and sporadic local transmission has also been reported in the United States.

WHO Chikungunya fact sheet

Yellow fever

Yellow fever is one of the original emerging infectious diseases, causing major epidemics with high fatality rates in South, Central and North America and Europe in the 17th-20th centuries.

In a small percentage of patients, this viral hemorrhagic disease evolves into a toxic phase, which results in death for up to half of them, depending on availability of good clinical management.

The disease is enzootic in tropical regions of 34 countries in Africa and 9 countries in Central and South America.

There is an effective yellow fever vaccine, which is the main mode of prevention. To avoid transmission in a region with a yellow fever outbreak, at least 80% of the population must be vaccinated.

Major outbreaks occur when infected individuals or monkeys introduce the virus into an urban setting where there is a low vaccination coverage, and mosquitos able to transmit the virus.

WHO Yellow fever fact sheet