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Public Health

Vetted resources and information on current public health events.

What are Vector-Borne Diseases?

Vectors are living organisms that can transmit infectious pathogens 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 transmit it into a new host, after the pathogen has replicated. Often, once a vector becomes infectious, they are capable of transmitting the pathogen for the rest of their life during each subsequent bite/blood meal.

Vector-borne diseases are human illnesses caused by parasites, viruses and bacteria that are transmitted by vectors. Every year there are more than 700,000 deaths from diseases such as malaria, dengue, schistosomiasis, human African trypanosomiasis, leishmaniasis, Chagas disease, yellow fever, Japanese encephalitis and onchocerciasis.

The burden of these diseases is highest in tropical and subtropical areas, and they disproportionately affect the poorest populations. Since 2014, major outbreaks of dengue, malaria, chikungunya, yellow fever and Zika have afflicted populations, claimed lives, and overwhelmed health systems in many countries. Other diseases such as Chikungunya, leishmaniasis and lymphatic filariasis cause chronic suffering, life-long morbidity, disability and occasional stigmatization.

Distribution of vector-borne diseases is determined by a complex set of demographic, environmental and social factors (eg. Global travel and trade, unplanned urbanization).

Vector-borne disease transmission occurs when a living organism carries a disease pathogen and passes it to another living organism. The disease does not infect or cause harm to the vector, but rather it simply passes it on to another organism. Vectors are often insects that feed on blood (mosquitos, fleas, ticks, etc.), they can pass the pathogen through the blood stream of the host; these diseases are often referred to as "Arboviruses", which are literally any virus that is spread through an arthropod. Some vector-borne diseases are spread by mammals and other animals as well, such as Rabies.

List of vector-borne diseases, according to their vector

The following table is a non-exhaustive list of vector-borne disease, ordered according to the vector by which it is transmitted. The list also illustrates the type of pathogen that causes the disease in humans. 


Disease caused

Type of pathogen





Lymphatic filariasis

Rift Valley fever

Yellow Fever









Lymphatic filariasis





Japanese encephalitis

Lymphatic filariasis

West Nile fever




Aquatic snails

Schistosomiasis (bilharziasis)



Onchocerciasis (river blindness)



Plague (transmitted from rats to humans)






Louse-borne relapsing fever





Sandfly fever (phlebotomus fever)




Crimean-Congo haemorrhagic fever

Lyme disease

Relapsing fever (borreliosis)

Rickettsial diseases (eg: spotted fever and Q fever)

Tick-borne encephalitis








Triatome bugs

Chagas disease (American trypanosomiasis)


Tsetse flies

Sleeping sickness (African trypanosomiasis)



Malaria is an acute febrile illness caused by Plasmodium parasites, which are spread to people through the bites of infected female Anopheles mosquitoes. There are 5 parasite species that cause malaria in humans, and 2 of these species – P. falciparum and P. vivax – pose the greatest threat. P. falciparum is the deadliest malaria parasite and the most prevalent on the African continent. P. vivax is the dominant malaria parasite in most countries outside of sub-Saharan Africa.


The first symptoms – fever, headache and chills – usually appear 10–15 days after the infective mosquito bite and may be mild and difficult to recognize as malaria. Left untreated, P. falciparum malaria can progress to severe illness and death within a period of 24 hours.

Risk Factors

In 2020, nearly half of the world's population was at risk of malaria. Some population groups are at considerably higher risk of contracting malaria and developing severe disease: infants, children under 5 years of age, pregnant women and patients with HIV/AIDS, as well as people with low immunity moving to areas with intense malaria transmission such as migrant workers, mobile populations and travelers.

Dengue Fever

Dengue (break-bone fever) is a viral infection that spreads from mosquitoes to people. It is more common in tropical and subtropical climates. There are four types of dengue virus (DENV 1, DENV 2, DENV 3, DENV 4). Infection with one serotype provides long-term immunity to the homologous serotype, but not to other serotypes; sequential infections put people at greater risk of severe dengue.


Most people who get dengue won’t have symptoms. But for those that do, the most common symptoms are high fever, headache, body aches, nausea and rash. Most people with dengue have mild or no symptoms and will get better in 1–2 weeks. Rarely, dengue can be severe and lead to death.  

If symptoms occur, they usually begin 4–10 days after infection and last for 2–7 days. Symptoms may include:

  • high fever (40°C/104°F)
  • severe headache
  • pain behind the eyes
  • muscle and joint pains
  • nausea
  • vomiting
  • swollen glands
  • rash. 

Individuals who are infected for the second time are at greater risk of severe dengue.

Severe dengue symptoms often come after the fever has gone away:

  • severe abdominal pain
  • persistent vomiting
  • rapid breathing
  • bleeding gums or nose 
  • fatigue
  • restlessness
  • blood in vomit or stool
  • being very thirsty
  • pale and cold skin
  • feeling weak.

People with these severe symptoms should get care right away. 

After recovery, people who have had dengue may feel tired for several weeks.

Zika Virus

Zika virus was first discovered in 1947 and is named after the Zika Forest in Uganda. In 1952, the first human cases of Zika were detected and since then, outbreaks of Zika have been reported in tropical Africa, Southeast Asia, and the Pacific Islands. Zika outbreaks have probably occurred in many locations. Before 2007, at least 14 cases of Zika had been documented, although other cases were likely to have occurred and were not reported. Because the symptoms of Zika are similar to those of many other diseases, many cases may not have been recognized.


Zika is spread mostly by the bite of an infected Aedes species mosquito (Ae. aegypti and Ae. albopictus). These mosquitoes bite during the day and night. Zika can also be transmitted from human-to-human either through sex or from a pregnant woman to her fetus (vertical transmission).

Pregnancy Risk

Zika infection during pregnancy can cause a birth defect of the brain called microcephaly and other severe brain defects. It is also linked to other problems, such as miscarriage, stillbirth, and other birth defects. There have also been increased reports of Guillain-Barré syndrome, an uncommon sickness of the nervous system, in areas affected by Zika.


Chikungunya is a mosquito-borne viral disease caused by the chikungunya virus (CHIKV), an RNA virus in the alphavirus genus of the family Togaviridae. The name chikungunya derives from a word in the Kimakonde language, meaning “to become contorted”.

CHIKV was first identified in the United Republic of Tanzania in 1952 and subsequently  in other countries Africa and Asia (1). Urban outbreaks were first recorded in Thailand in 1967 and in India in the 1970s (2). Since 2004, outbreaks of CHIKV have become more frequent and widespread, caused partly due to viral adaptations allowing the virus to be spread more easily by Aedes albopictus mosquitoes. CHIKV has now been identified in over 110 countries in Asia, Africa, Europe and the Americas. Transmission has been interrupted on islands where a high proportion of the population is infected and then immune; however, transmission often persists in countries where large parts of the population have not yet been infected.

All regions with established populations of Aedes aegypti or Aedes albopictus mosquitoes have now experienced local mosquito-borne transmission.


Chikungunya virus is transmitted by mosquitoes, most commonly Aedes (Stegomyia) aegypti and Aedes (Stegomyia) albopictus, which can also transmit dengue and Zika viruses. These mosquitoes bite primarily during daylight hours. They lay eggs in containers with standing water. Both species feed outdoors, and Ae. aegypti also feeds indoors.

When an uninfected mosquito feeds on a person who has CHIKV circulating in their blood, the mosquito can ingest the virus. The virus then replicates in the mosquito over several days, gets to its salivary glands, and can be transmitted into a new human host when the mosquito bites them. The virus again begins to replicate in this newly infected person and reaches high concentrations in their blood, at which point they can further infect other mosquitoes and perpetuate the transmission cycle.


In symptomatic patients, CHIKV disease onset is typically 4–8 days (range 2–12 days) after the bite of an infected mosquito. It is characterized by an abrupt onset of fever, frequently accompanied by severe joint pain. The joint pain is often debilitating and usually lasts for a few days but may be prolonged, lasting for weeks, months or even years. Other common signs and symptoms include joint swelling, muscle pain, headache, nausea, fatigue and rash. Since these symptoms overlap with other infections, including those with dengue and Zika viruses, cases can be misdiagnosed. In the absence of significant joint pain, symptoms in infected individuals are usually mild and the infection may go unrecognized.

Most patients recover fully from the infection; however, occasional cases of eye, heart, and neurological complications have been reported with CHIKV infections. Patients at extremes of the age spectrum are at higher risk for severe disease. Newborns infected during delivery and older people with underlying medical conditions may become severely ill and CHIKV infection can increase the risk of death.

Once an individual is recovered, available evidence suggests they are likely to be immune from future infections.

Dengue and Chikungunya in the Americas

Geographical expansion of cases of dengue and chikungunya beyond the historical areas of transmission in the Region of the Americas

WHO Disease Outbreak News: 23 March 2023

Situation at a glance

The increase in the incidence and geographical distribution of arboviral diseases, including chikungunya and dengue, is a major public health problem in the Region of the Americas (1). Dengue accounts for the largest number of cases in the Region, with epidemics occurring every three to five years. Although dengue and chikungunya are endemic in most countries of Central America, South America, and the Caribbean, in the current summer season, increased transmission and expansion of chikungunya cases have been observed beyond historical areas of transmission. Furthermore, 2023 is showing intense dengue transmission. In addition, higher transmission rates are expected in the coming months in the southern hemisphere, due to weather conditions favourable for the proliferation of mosquitoes.

There have been 2.8 million dengue cases reported in the Americas in 2022, which represents over a two-fold increase when compared to the 1.2 million cases reported in 2021. The same increasing trend has been observed for chikungunya, with a high incidence of meningoencephalitis possibly associated to chikungunya reported by Paraguay, which is of further concern.

At the regional level, WHO is assessing the risk as high due to the widespread presence of vector mosquitoes, the continued risk of severe disease and even death, and the expansion outside of historical areas of transmission, where all the population, including risk groups and healthcare workers, may not be aware of clinical manifestations of the disease, including severe clinical manifestations; and where populations may be immunologically naïve

Regional overview

In 2022, a total of 3 123 752 cases (suspected and confirmed) of arboviral disease were reported in the Region of the Americas. Of these, 2 809 818 (90%) were dengue cases and 273 685 (9%) were chikungunya cases. This represents a proportional increase of approximately 119% compared to 2021. In 2022, both dengue and chikungunya peaked at epidemiological week (EW) 18 (week commencing 1 May 2022).


In 2022, a total of 2 809 818 cases of dengue, including 1290 deaths, representing a two-fold increase in cases and almost three-fold increase in deaths compared with the cases reported in 2021 (1 269 004 cases, including 437 deaths). During the same period, the highest cumulative incidence of dengue cases was reported in the following countries: Nicaragua with 1455.4 cases per 100 000 population, followed by Brazil with 1104.5 cases per 100 000 population, and Belize with 788.9 cases per 100 000 population (3).

Between 1 January 2023 and 4 March 2023, a total of 342 243 dengue cases including 86 deaths were reported in the Region of the Americas. During the same period, the highest cumulative incidence of dengue cases was reported in Bolivia, with 264.4 cases per 100 000 population, followed by Nicaragua with 196.8 cases per 100 000 population, and Belize with 145.6 cases per 100 000 population.


Between 1 January and 4 March 2023, a total of 113 447 cases of chikungunya were reported in the Region of the Americas, including 51 deaths, representing a four-fold increase in cases and deaths compared with the same period in 2022 (21 887 cases, including eight deaths). These counts also exceeded the average number of cases for the previous five years for EW1 through EW10. Of the cases reported in the Region, the highest cumulative incidence of chikungunya cases was reported in Paraguay with 1103.4 cases per 100 000 population, followed by Brazil with 14.2 cases per 100 000 population, and Belize with 10.4 cases per 100 000 population. Of the total deaths reported in 2023, all were reported from Paraguay (3).

In 2022, the number of cases exceeded the average for the previous four years (2018-2021), with a total of 273 685 cases including 87 deaths, representing a two-fold increase in cases and seven-fold increase in deaths compared with the cases reported in 2021 (137 025 cases, including 12 deaths). Of the total deaths reported in 2022, all were reported from Brazil.

WHO risk assessment

Dengue and chikungunya can have serious public health impacts. The viruses that cause these infections have been circulating in the Region of the Americas for decades due to the widespread spread of the Aedes spp. mosquitoes (mainly, Aedes aegypti). These arboviruses can be carried by infected travelers (imported cases) and may establish new areas of local transmission in the presence of vectors and a susceptible population. As they are arboviruses, all populations in areas where the mosquito vectors are present are at risk, however, the impact is greatest among the most vulnerable people, for which the arboviral disease programs do not have enough resources to respond to outbreaks.

Although dengue and chikungunya are endemic in most tropical and subtropical countries of the Americas and the Caribbean, increased transmission and expansion of chikungunya cases has been observed beyond historical areas of transmission. Furthermore, 2023 is showing intense dengue transmission.

The impact of the increased transmission in the Region will depend on several factors, including country capacities for a coordinated public health response and for clinical management; the early start of the arbovirus season in the southern cone; high mosquito densities due to interrupted vector control activities during the COVID-19 pandemic; and the large population susceptible to arbovirus infections, particularly in areas where these viruses are newly circulating. Competing disease priorities and risks may adversely affect disease control and proper clinical management, because of (i) misdiagnosis, given that chikungunya and dengue symptoms may be non-specific and resemble other infections, including Zika and measles, potentially leading to inadequate case management; (ii) overwhelmed healthcare facilities in some areas dealing with a high caseload and other concurrent outbreaks; and (iii) the effects of the COVID-19 pandemic on the decrease of resources available to arboviral disease programs and the need for capacity building and training of vector-control and healthcare workers, as well as maintenance and procurement of equipment and insecticides to perform vector control activities.

The apparent higher proportion of acute meningoencephalitis attributed to chikungunya in Paraguay is of concern. It is not yet known what is causing a higher rate of neurologic disease, which is considered an atypical clinical presentation. Sequencing has identified the East-Central-South-African (ECSA) lineage, which is expanding in geographic range within the region, having first been identified in Brazil in 2014. Introduction of chikungunya virus into new areas with immunologically naïve populations would promote further spread.

Aedes spp. mosquitoes are widely distributed in the Region of the Americas, therefore cross-border transmission of dengue and chikungunya is likely. Countries bordering areas with very high transmission of these diseases may be at higher risk,  e.g., those adjacent to Bolivia (dengue) and Paraguay (chikungunya). Additionally, the Southern Hemisphere summer, with high temperatures and high levels of humidity, affects vector dynamics and may increase the likelihood of arboviruses transmission.

Thus, the risk at the regional level is assessed as high, due to the widespread presence of the mosquito vector species (especially Aedes aegypti), the continued risk of severe disease and even death, and the expansion outside of historical areas of transmission, where all the population, including risk groups and healthcare workers, may not be aware of warning signs and may be immunologically naïve. Moreover, one country in the Region (Paraguay) is experiencing an unprecedented increase of chikungunya cases, and another country in the Region (Bolivia) is experiencing high incidence of dengue cases.

Other challenges reported by Member States in the Region include stockouts of several essential supplies for prevention and control, lack of reagents and consumables for laboratory diagnosis, and the need for re-training of field teams and health workers. In addition, higher transmission rates are expected in the coming months, due to weather conditions favourable for vector breeding in the first semester of the year in the southern hemisphere.