Arxiu d'etiquetes: dengue fever

Insect vectors: why are they spreading worldwide?

Last week, Maribel talked us about disease outbreaks, many of which are transmitted by insect vectors. In the last years, media has been reporting an increase of illnesses transmitted by insects and other arthropods. One of the latest cases was the outbreak of zika virus, which is transmitted by mosquitoes of the genus Aedes (the same one as the tiger or forest mosquito, Aedes albopictus). These insects, known as vectors, used to be distributed across tropical regions, but they have started to spread away from their native range.

In this article, you’ll learn what a vector is and why are they spreading around the world.

What is a vector? 

In epidemiology, a vector is defined as an agent (either a human, animal or microorganism) that carries and transmits a pathogen or any other infectious agent from an infected organism to another, either directly via the blood flow or indirectly via the food, water or any other element a susceptible organism may be in contact with.

Transmission cycle of vector transmitted diseases, also known as vector-borne diseases (Picture from Ellis et al. 2009).

The most remarkable and abundant vectors belong to Insecta class (although there are other arthropod vectors). Even though vectors could transmit infectious agents to either plants or animals, in this article we will focus on those agents related to animal diseases.

Insects as vectors can assume different roles according to the relation they establish with the infectious agent they carry:

  • Mechanical vectors: The only function of these vectors is to transport the infectious agents, which don’t really need vectors to complete their life cycle (that is, the insect isn’t a host of the pathogen). Some flies carry infectious agents that are the main cause of different intestinal infections, but these pathogens do not need flies to complete their life cycle; in fact, any other vector could transport them and there would be no difference.
Sarcophaga sp. eating rests of salmon (Picture by Ernie Cooper ®, 2013)
  • Obligate hosts (obligate host-parasite relation): in this case, vector is an essential element in the pathogen’s life cycle, which needs the insect (that is, the vector) to complete its development before being transmitted to another organism. Most of infectious agents travel inside insect’s hemolymph (the liquid equivalent to blood in insects). This is the case of the pathogen responsible of Malaria (a protozoan from the genus Plamsodium), which travels from an organism to another inside the hemolymph of different mosquitoes species from the genus Anopheles.
Mosquito of the species Anopheles stephensi, one of the Malaria vectors (Public domain, by CDC).

Even though some insects become hosts of different pathogens, they rarely get ill as a consequence of this relation because vectors are an essential element for pathogens to reach their final host, which is usually a vertebrate (as we humans). So, pathogens avoid to damage vectors because they need them to reach other organisms.

However, although infectious agents try to not damage their hosts, sometimes they induce some morphological changes in vector’s bodies that improve their transmission skills. For example, some mosquitoes undergo structural modifications of their mouthparts that limit the amount of blood they can suck at a time, so they bite more frequently in result.

Why are vectors so problematic?


Some infectious agents would have a limited spreading ability if there were not vectors that transport them. Most of vectors are hematophagous insects (i.e., that feed on blood), thus the transference of pathogens to a new organism takes place by a more direct way, e.g. through saliva or blood. Therefore, for some pathogens using vectors to pass from one host to another is an essential fact to assure their survival and, especially, to guarantee their dispersal through the space.

Therefore, we consider vector-borne diseases the most hazardous and unpredictable, which is mainly due to the reasons listed below:

  • They are the most difficult to prevent due to their enormous resilience in front of control activities and management. This is a consequence of the great adaptation of vectors to their ecosystems.
  • Vectors greatly increase the dispersal and the transmission range of infectious agents in relation with those strictly dependent to direct contact between two or more organisms.
  • Vectors act as a bridge between different species of animals and humans; without vectors, many illnesses would remain affecting specific organisms instead of spread through different species.
  • Moreover, vectors act as a reservoir of pathogens. This allows pathogens to survive to unfavorable infective periods (e.g., winter or dry seasons).
  • On the other side, the relationship between the pathogen and its vector tends to last until the end of vector’s life, so the vector will be always capable to infect.

Examples of different vectors and their impact

According to WHO, mosquitoes form the major group of vectors responsible of different vector-borne diseases: they’re responsible of illnesses like Malaria, Dengue or Yellow Fever, as well as other diseases less famous like Lymphatic filariasis.

As well as mosquitoes, there are a lot of insects and other arthropods (or even other invertebrates) that could act as vectors, e.g. ticks, flies, sand flies (Phlebotominae, a subfamily of dipterans that look like mosquitoes), triatomines (bugs from the family Reduviidae) or even some freshwater snails.

Sand fly (Phlebotomus sp.), a vector that transmits different diseases, e.g. leishmaniasis (Public domain, from CDC).
Triatoma infestans, one of the bugs from the family Reduviidae responsible of the transmission of Chagas disease (Specimen from the Zoologische Staatssamlung München. Location: Bolivia, Cochabamba, Leg. Zischka. Author: Bärbel Stock, CC).

If you are interested on learning more about each of these vectors and the diseases they transmit, you can visit the WHO’s website.

Which impact have vectors on public health?

  • According to data from WHO, there are more than 1000 million of cases and more than 1 million of deaths as a consequence of vector-borne disease outbreaks every year.
  • Almost 17% of the total amount of known infectious diseases are mediated by vectors.
  • Malaria (vector: mosquito from the genus Anopheles) causes more than 600.000 deaths every year, most of them of children under 5 years old. On the other side, more than 2500 millions of persons in more than 100 countries are in a major risk to contract Dengue (vector: mosquito from the genus Aedes).

Why are vector-borne diseases currently in expansion?

Many environmental and social factors determine the way vector-borne diseases are currently spreading worldwide.

Typically, most vectors are located in tropical regions; currently, variations in temperature and different climate patterns could be driving changes in their native range. The increase of annual mean temperatures and the introduction of disturbances in seasonal rhythms in latitudes located above tropics seem to be the major causes of the spreading of these organisms. Thus, in a few years the number of cases of vector-borne diseases will probably increase greatly in different non-tropical locations around the world (e.g. Dengue, Chikungunya fever or West Nile Fever transmitted by mosquitoes of the genera Aedes, Anopheles and Culex).

However, climate change is not the only possible cause of their expansion: global transport and commercial exchanges worldwide are an open door to the international transport of vectors. If there is also a climate matching (that is, a correlation or similarity in climatic conditions) between the native and the new range, it is way easier for vectors to settle in the new location.

There are also evidences that changes in some agricultural practices forced by changes in temperature and precipitations could be influencing on the spreading of vector-borne diseases (mainly due to an inappropriate use of water sources, since many insects go through an aquatic larval phase).

To sum up the current situation of diseases transmitted by mosquitoes in Europe, take a look at the infography below from the European Centre of Disease Prevention and Control (enter this link to download it in a better resolution). In this web, you can also consult other infographies and find more information about each vector.

vector-borne-disease.             .             .

Vector-borne diseases are a major concern for public health at a global scale. Although they are unpredictable and very hard to control, the implementation of good management practices in different economic, environmental and agricultural sectors could slow down their progress. In relation with the effects headed by climate change and globalization…Could it be too late? And you, what do you think about this issue?


Main picture: mosquito responsible of Chikungunya (from the Centre for Disease Control and Prevention).

‘Danger mosquito’ icon: Ivlichev Viktor Petrovich


Disease outbreaks, another effect of climate change?

We know that many infectious diseases depend on climatic factors such as temperature. So, can climate change cause an increase of the outbreaks? Let’s find out!


 According to some surveys conducted by the Pew Research center, 54% of respondents believe that climate change is a serious problem and their major concerns include drought, intense rainfall and heat. If you are interested in to learn more about this survey, you can find them in the following article.

These changes have a negative effect on human health. The World Health Organization (WHO) expected that between 2030 and 2050 climate change will cause some 250,000 additional deaths a year. The effects can be very varied: deaths by  heat, floods, increase in respiratory diseases, stress etc. One of the important health effects is an increase in the transmission of infectious diseases.

Graphic of impacts of climate change on human health (Photo: CDC)

Infectious diseases are closely related to  environment’s characteristics (such as temperature and humidity). In some cases, these diseases are transmitted by vectors (bats, arthropods, snails, rodents, ticks…). A  temperaturerising  will modified its geographical distribution, seasonality and population size. An example is  the presence of the mosquito Aedes albopictus, known as mosquito tigre, in Spain.

On the other hand, changes in the use of the soil, overcrowding of cities, poor hygienic habits and other socio-economic factors also have an effect in the transmission of certain diseases. For example, deforestation and poor hygiene of the population increases the breeding sites of the mosquitoes, causing an increase in the probability of malaria transmission.

Human activities may effect diseases transmission rate. (Photo: OMS)


Vector diseases are those that are transmitted through a vector animal (whether a mosquito, rodent, tick, snail, bat…). These diseases may be zoonotic (animal to human, as rabies) or antroponotic (among humans, such as malaria or dengue). If you want to know more about the effects of climate change on vector, feel free to access this article.

Sin título
Different types of vector diseases. (Photo: OMS)

There are many vector diseases which should be monitored in the coming years, as for example the malaria, dengue fever chikungunya, Boutonneuse etc. Let’s look at the two best known infectious diseases.


This disease is caused by parasites of the genus Plasmodium, which is transmitted by the bite of mosquitoes of the genus Anopheles. There are four different types of malaria, but the most deadly is that caused by the species Plasmodium falciparum.

Plasmodium falciparum gametocyte. (Photo: CDC)

The WHO estimates that in the year 2013, 198 million people were infected,  584,000 of which died. It is expected that these numbers will increase due to climate change. Temperature rise leads to an increase in the infective period of the mosquito and the modification of vector’s geographical distribution. Possibly in the next few years, if the trend does not change, there will be an increase in the spread of the disease in endemic areas  and will probably resurface in other areas (red areas on the map).

Estimation of the spread of malaria in 2050 (Photo: Randolph Rogers)

In Spain, the autochthonous malaria was eradicated in 1964. Currently, the spanish cases of malaria are imported from countries with indigenous malaria. Even so, note the geographic situation of our country, the rising temperatures, the presence of a competent vector and the presence of imported parasit, significantly increase the likelihood of disease’s transmission.


This is a viral disease (caused by viruses of the genus Flavivirus) that is transmitted by the bite of mosquitoes of the Aedes genus (including the Tiger mosquito). Dengue fever is a widespread disease in tropical countries, although its suffering geographical changes due to changes in temperature, precipitation and a demographic overcrowding of the cities.

Structure of dengue fever virus (Photo: César Cabezas)

Before 1970, only nine countries had experienced serious dengue epidemic episodes. In recent decades, the cases have increased sharply. According to WHO estimates, each yerar are produced about 390 million infections,  23%  of which are clinically manifested.

forecast of the spread of dengue fever in Europe during the twenty-first century. Expressed in nº of cases /100.000 habitants. (Photo: Moha Bouzid)

As in the case of malaria, current climatic variations alter the geographical distribution of the vector. As we can see in the previous map, the predictions for this century, if conditions do not change, are a significant dengue fever cases increase in Northern Europe (lighter areas are potential sites of infection). As we see in the case of Spain, the Mediterranean would be the region that would have more cases of dengue fever.


Climate change also affects the water cycle. The news about weather disasters (floods, strong drought, torrential rains, hurricanes…) never cease to appear in the media. These climatic variations affect those diseases that are spread by water, either by contamination of the flows, by human migration and low hygiene that exist in certain places of overcrowded cities.

The most known diseases associated with floods and droughts are infections of Cryptosporidium or cholera. Let’s look at this last example.


Vibrio cholerae is a bacilar bacteria that causes this disease. It is a diarrheal infection that suffer every year between 1.4 and 4.3 million people, 142,000 which end up dying. The transmission of this Bacillus is closely linked to environmental mismanagement. Heavy rains or flooding can cause water pollution, and extreme drought increases the bacterial charge of the existing flows.

vibrio colerae
Microphoto of Vibrio cholerae (Photo: Louisa Howard).

During the 19th century, cholera spread across the world from Ganges (India). The last cholera epidemic began, as we can see in the map, in the South of Asia in 1961. Now cholera has been distributed worldwide due above all to human migrations (bacillus carriers), the agglomeration of people in suburban areas without hygiene habits and climate disasters. The WHO estimates that by 2030 there will be 10% more cases due to climate change.

evolution of last epidemic of cholera (1961-2004). (Photo: IPCC)

It may not be possible to quantify in that measure climate change can affect the transmission of these diseases, since these depend on many other factors (demographic dynamics, immunization, etc.). Is worth mentioning, that the provisions set out in this article are assumptions obtained from current data. That means, that if the mechanisms for the reduction of global climate change works and environmental conditions improve, these data would no longer have any statistical value


Remember that it is better to be safe than sorry!

Cares for the environment: the Earth is your home.