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Updated on 06/01/2021

Vectors and the diseases they transmit

Keywords : Vectors

Some arthropods (a family that includes insects and arachnids) such as ticks and mosquitoes can transmit pathogens that cause diseases in animals, plants and/or humans. They acquire the pathogen by feeding on an infected host, before transmitting it to another one. In order to combat the diseases caused by these pathogens, it is essential to know how their vectors live and spread. ANSES provides scientific expertise on vectors and their control. It also conducts research on vectors and vector-borne diseases in its various laboratories. What is a vector, what are vector-borne diseases? How does ANSES contribute to the fight against vector-related risks? Here are the answers to a few questions on the topic. 

What is a pathogen vector?

A vector is an arthropod – a member of a group including insects and arachnids – that transmits a pathogen (a virus, bacterium or parasite). It acquires this pathogen by feeding on an infected host, and then transmits it to other individuals. 

According to the World Health Organization (WHO), vector-borne diseases account for more than 17% of infectious diseases worldwide and cause more than one million deaths each year

Vectors of human and animal pathogens are haematophagous, i.e. they feed on blood. They include some mosquitoes, ticks, Culicoides, sandflies, fleas and horseflies. Vector-borne diseases can be strictly human (malaria, dengue, chikungunya, Zika, etc.) or strictly animal (African swine fever, bluetongue, etc.). They can also be transmitted from animals to humans and vice versa. In this case, they are known as zoonoses (Lyme borreliosis, West Nile virus infection, etc.).

With plants, the vectors are phloem-feeding insects such as leafhoppers or aphids, or woodborers. Vector-borne plant pathogens include the bacterium Xylella fastidiosa. Originally from America, it has spread to Europe and is transmitted by several species of insects. It can infect more than 300 different species of plants and is lethal. Another example is the pinewood nematode, a worm carried by beetles. The bacterium responsible for Huanglongbing disease, which threatens citrus crops in southern Europe, is also transmitted by vectors.

Did you know?     
The pathogen lifecycle often involves more than one host animal species. Some of these hosts are essential to the lifecycle of the pathogen, which can remain in them for a sustained period of time, multiply and then spread to other animals. These are known as reservoir hosts. Other species are infected by the pathogen more occasionally, and do not allow it to multiply and infect other animals. 
 

How do vectors transmit pathogens?  

With vertebrate hosts, vectors become infected when they acquire a pathogen from an infected host during a blood meal, a practice known as haematophagy. The pathogen can then be transmitted to a new host in a variety of different ways, the most common being:

  • Injection of saliva during a new bite: the viruses transmitted by certain mosquitoes or the parasite responsible for sleeping sickness (transmitted by the tsetse fly) are injected during a bite, 
  • Faeces: for example, the parasite responsible for Chagas disease is transmitted by the faeces of an infected triatomine bug,
  • Regurgitation: the bacterium responsible for the plague (Yersinia pestis) is transmitted by fleas during regurgitation,
  • Active migration of the parasite during the bite (filariasis).

What does ANSES's expert appraisal mission involve?

ANSES has been providing scientific expertise on vectors and their control since 2018. This was previously the responsibility of the National Centre of Expertise on Vectors. It aims to support the public authorities to better prevent and control risks associated with vector-borne pathogens. Among other things, this involves scientific monitoring.

Johanna Fite, Project Officer for Vectors, provides some explanations. 

Studying the "vector-pathogen" pair
ANSES's expertise is primarily based on better understanding vectors, how they function, spread and transmit pathogens. To assess the risks of transmission of pathogens responsible for vector-borne diseases, the vector-pathogen pair must first be studied. Vector competence, which refers to an arthropod's intrinsic ability to transmit a pathogen, results from genetic and biological factors that allow the pathogen to multiply in the vector and then be transmitted to a host. However, transmission of the infectious agent is only possible if this competence is accompanied by favourable ecological conditions. The vector has to be abundant, have sufficient longevity, and maintain close contact with reservoir hosts and receptive vertebrates. Vectorial capacity expresses the degree of co-adaptation of the "vector-infectious agent" pair in a given ecosystem.

Understanding the ecosystems where the vectors live and assessing the risks associated with the vectors
It is also important to study the ecosystems in which the vectors live and the conditions under which they develop. For example, water plays a key role in the lifecycle of mosquitoes, which have an aquatic life stage. Temperature is also an important factor in determining the geographical range of arthropod vectors. With climate change and increased global trade and transport, the range of vectors is changing rapidly. This means that in our health risk assessments, we may be required to make projections and develop scenarios based on different factors (environmental, etc.) in order to assess the probability of introduction and spread of certain vectors, as well as the risks of emergence of vector-borne diseases.

Assessing vector control strategies
ANSES also evaluates vector control (VC) strategies, their effectiveness and impacts, and recommends prevention and control measures adapted to the context. 
As part of the marketing authorisation (MA) procedures for products containing biocides used in VC (adulticides, larvicides, etc.), ANSES assesses their efficacy and the risks associated with their use, as well as the vectors' resistance to the insecticides or acaricides. 
The Agency is also working on the development of a systemic assessment guide. It will use various criteria such as effectiveness, relevance and acceptability to assess all aspects of VC strategies: community mobilisation and health education, integrated surveillance, intra- and inter-sectoral cooperation, treatments, etc. 

 
Collective expert appraisal and scientific support
To respond to the various calls for expert appraisals, ANSES has established a permanent working group (WG) on vectors, and regularly sets up ad hoc WGs depending on the topics to be dealt with (formal requests from outside the Agency or internal requests on its own initiative). It also participates in or organises events that help coordinate the community of scientific and technical experts working on vectors and vector control. Lastly, ANSES is responsible for implementing some of the actions in the National Plan for the control of Lyme disease and tick-borne diseases led by the Ministry of Health, and manages the Signalement-Moustique reporting website.

What research on vectors is undertaken at ANSES?

The Agency develops research activities by funding projects dedicated to furthering knowledge of vectors or vector control, via the National Research Programme for Environmental and Occupational Health

Its laboratories also study vector-borne diseases transmitted to animals and the ability of vectors to transmit these pathogens.