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.

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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?

References

• Centers for Disease Control and Prevention (CDC): Parasites – Insects.
• Ellis, B. R., & Wilcox, B. A. (2009). The ecological dimensions of vector-borne disease research and control. Cadernos de Saúde Pública, 25, S155-S167.
• European Centre for Disease Prevention and Control. Disease Vectors.
• Lemon, S. M., Sparling, P. F., Hamburg, M. A., Relman, D. A., Choffnes, E. R., Mack, A., & Sparling, F. (2008). Vector-borne diseases: understanding the environmental, human health, and ecological connections. Workshop summary. In Vector-borne diseases: understanding the environmental, human health, and ecological connections. Workshop summary. National Academies Press. Link aquí.
• Organización Mundial de la Salud (OMS): Enfermedades transmitidas por vectores.
• Vector-Borne Disease – Primary Examples. MosquitoZone® Corporation.

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

‘Danger mosquito’ icon: Ivlichev Viktor Petrovich