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La lutte antivectorielle en France

 | 
Didier Fontenille
, 
Christophe Lagneau
, 
Sylvie Lecollinet
, 
et al.

English Version

Introduction

Texte intégral

  • 1 See the overview of human and animal infections (vectors, epidemiology, transmission, surveillance (...)

1Each year dengue virus infects around 50 to 100 million people in the world. Malaria kills nearly one million per year. To this tally should be added the morbidity linked to other diseases transmitted by organisms such as mosquitoes (like chikungunya, Rift Valley fever, yellow fever, West Nile virus), ticks (such as Lyme’s disease, rickettsioses, tick-borne encephalitis) or lice (exanthematic typhus, borreliosis, bartonellosis). Animal health is affected, with bluetongue in ovines which recently expanded into southern, then northern Europe, plus tick-borne diseases (babesiosis, anaplasmosis, rickettsiosis and so on), or by fly vectors (tsetse fly for sleeping sickness, trypanosomiasis1). Optimism that reigned in the late 1960s had led to slackening of vigilance on the pretext that most vector-borne illnesses (apart from malaria) did not pose a threat. However, the past 20 years have seen their resurgence, re-emergence or emergence in areas previously spared. This changed the situation radically: with events such as chikungunya outbreaks in Italy in 2007, West Nile virus in Italy, Romania, Hungary and Austria in 2008.

2Vector-borne diseases have returned to the forefront. Their resurgence is all the more significant in that their spread is favoured by such factors as intense circulation of people and goods, human activity-linked environmental changes plus climatic factors. Global climate change will foreseeably have a significant impact on the dynamics of these infections in the coming decades, as diseases respect no borders. Such shifts are already suspected of modifying the distribution of vectors, their transmission ability and their contact with parasites and hosts.

3France is faced with a number of vector-borne diseases (see Maps in Appendix of this Introduction), especially at the heart of its tropical territories: malaria in French Guiana and Mayotte; dengue in the French Départements of the Americas (FDA: Martinique, Guadeloupe, French Guiana), Reunion

  • 2 D. Fontenille, Écosystèmes, entomologie et lutte antivectorielle, Annales des Mines. Responsabilité (...)

What is a vector?
A vector is a haematophage (blood feeding) arthropod that ensures active biological transmission of a pathogen (virus, bacterium, parasite) from one vertebrate to another.
However, “the vector is not just a ‘transporter’ of pathogens. It becomes infected itself by taking up virus, bacterium, rickettsia, protozoan, helminth from an animal carrier, during a blood meal. At the end of a period of extrinsic development, generally about 5 to 15 days, during which the pathogen proliferates or undergoes changes, the vector transmits it to a new vertebrate host. The vectors will therefore only transmit blood or skin parasites. Only a limited number of invertebrate families among the haematophage insects and acarians are concerned. However, transmission modes are varied, the most frequent being through bites (malaria, dengue, bluetongue in ovines, chikungunya), excrement (Chagas disease, rickettsiosis) or by regurgitation (plague). These vectors form the core study focus of medical and veterinary entomology.”
2

4Island and the Pacific; chikungunya in the Indian Ocean; Bancroft’s filariasis in Polynesia, Wallis-and-Futuna and Mayotte; Rift Valley fever in Mayotte; Chagas disease in French Guiana; and Lyme disease and tick-borne encephalitis in metropolitan France. In animal health, bluetongue in ovines or babesioses generate enormous economic losses. Moreover, new vectors are spreading into French territory (for example, Aedes albopictus), creating a risk of chikungunya and dengue epidemics.

5Today, control of vector-borne disease by attacking the vectors is therefore a major public issue for human and animal health, for countries of both North and South. Such control is evidently exercised by implementing legislative and/or regulatory tools, readily available or needing improvement, and through well-defined systems of governance (see “From legal framework to governance”). It also requires careful consideration of the strategies for vector control and for communication and winning-over of communities exposed to such risk (see “Implementation) and also their assessment in biological, technical, economic and environmental terms (see “Assessment of vector control”). Vector control is a truly multidisciplinary field, yet success can be achieved only by dint of reliable evaluation of epidemic risks, settingup and provision of appropriate training, regional and/or international cooperation schemes as often as possible. And through real support for both fundamental and applied research (see “Tools for risk management and forecasting”), including work on human and social sciences aspects.

What is vector control?
In its broadest sense, vector control includes control of and protection against haematophage arthropods (insects and acarians), vectors of human and vertebrate animal pathogens, and their surveillance. It includes control of insect pests when these are potential vectors or when the nuisance becomes a public or animal health problem.
Vector control is founded on a variety of methods depending on the vector organisms involved and the prevailing epidemiological and socio-economic situations. It includes biocide-based, biological and genetic control, individual protection, action on the environment, health education, social awarenessraising and rallying and continuous assessment of all these methods.
The objective is to work in conjunction with other public health actions to reduce the risks of endemization or epidemization to a minimum, decrease pathogen transmission by vectors, and contribute to management of vectorborne disease epidemics, all within a planned strategic framework.

VECTOR SYSTEMS: A PERPETUAL STATE OF CHANGE

6A vector system involves populations of vectors, pathogens and vertebrates in a given environment. A population consists of a set of individuals of the same species located at the same place at the same moment, reproducing randomly between them (by panmixia). The success of a system, here meaning the transmission of a pathogen (virus, bacterium, protozoan, nematode), results from the encounter and compatibility between the different partners of the cycle.

7Such an encounter depends on the ability of individuals of a species to live in a given ecosystem, characterized by a set of biotic and abiotic components, including climatic factors. For example: the encounter between the protozoan Trypanosoma cruzi, responsible for Chagas disease in French Guiana, and its vector, a triatom, can only take place in the Americas, as the triatom vectors only occur on that continent. Meeting also depends on the behaviour of each of these actors: it is not fixed within one species of vector, pathogen, or vertebrate host, but is usually specific to a population. Thus, in Polynesia, the agent of lymphatic filiariasis or elephantiasis (the thread-like filiarial worm Wuchereria bancrofti) circulates in the surface capillaries of humans during the day, its vector being the diurnal mosquito Aedes polynesiensis, yet in Mayotte, where its vector (Anophele gambiae) is nocturnal, this parasite circulates at night in the cutaneous blood vessels.

8Compatibility comes from a complex mechanism which can involve responses of the all-or-nothing type or indeed graded ones. For example, human Plasmodium are only transmitted by mosquitoes of the genus Anopheles (15 % of mosquitoes); the genera Aedes, Culex, Culiseta, Mansonia, Haemagogus represent blockages for these pathogens. However, only about 60 out of more than 450 described anopheles species on Earth are really effective vectors, even though experimentally none of them seem completely inactive in this domain. Within a particular species, the levels of vector competence are highly variable and depend, among other things, on the adaptation of the vector-parasite or vector-virus couple. This adaptation is the result of co-evolution between populations of pathogens, vectors and vertebrates. Thus, anopheline populations of metropolitan France, which were probably good vectors of European Plasmodium in the xxth century, show very low competence for African Plasmodium that are currently being introduced.

9Vector systems are in any case far from being fixed: they are in perpetual evolution whereas the three populations of actors involved (vectors, pathogens, vertebrates) themselves respond to the changes that occur. Typically, a genetic change in chikungunya virus populations was observed when it was spread to Reunion by Aedes albopictus, whereas previously, in the Comoro Islands and East Africa, it had been transmitted by Aedes aegypti (see Box). The selection of insecticide resistance mechanisms by the vectors (or of resistance to medicines by the pathogens) is none other than an adaptation of the system to a new environment. A change of any sort whatsoever (in components of the vector system, or in their biotic or abiotic environment) inevitably alters the risk of transmission. One of the objectives of vector control stems from this: act on the vector system in order to lower the probability of transmission.

Particular case of the Aedes albopictus vector
Aedes albopictus has long been known in Reunion Island. It recently colonized Mayotte, where it was first reported in 2001. It has been present in mainland France since 2005, in the départements of Alpes-Maritimes (2005), Var (2008), Haute-Corse (2005) and Corse-du-Sud (2008), coming from Italy where it was introduced in the 1990s. Aedes albopictus was responsible for the chikungunya epidemic in Reunion and Mayotte. The mosquito is capable of transmitting a range of viruses, including dengue. It therefore has vector status in the Indian Ocean, but nuisance status in metropolitan France which raises the question of how control systems can be organized (see chapter “Legislative and regulatory framework for vector control” and “How is governance organized?”). Several entomological indicators concern this mosquito in particular, but they must be validated in the perspective of vector control assessment (see “Practices and methods” and “From vector risk assessment to epidemic risk assessment”). Models also exist for predicting its distribution (see “From vector risk assessment to epidemic risk assessment), even if it is always wise to handle modelling with prudence. The effectiveness of control methods, notably the efficacy of biocides on the vector’s density, is still in need of assessment (see “Practices and methods”) and research is conducted on new control strategies (see “Research: a key to successful vector control”). Concerning the cost for society of vector-borne diseases and vector control, economic assessment exercises have been carried out for a certain number of pathologies, in particular for chikungunya (see “Economic assessment: France lags behind”). As for the acceptability of vector control methods by communities concerned and the social cost of vector presence (in terms of image of an area and its governing authorities), these questions should be given particular attention in future studies (see “Perceptions of risk in relation to vector control communication”, especially its recommendations).

TRANSMISSION OF VECTOR-BORNE INFECTIONS

10One of the major scientific challenges is therefore to understand the mechanisms behind this transmission, from animal to animal, animal to humans and humans to humans. The difficulty lies in the number of players in the system (several vectors, several hosts, and the presence of a vertebrate or invertebrate reservoir). As with any infectious disease, the probability of transmission of a vector-borne disease will depend on the duration of the host’s infectivity (length of time during which the infectious agent is present in the host): the longer it is, the greater the probability that the individual affected will be bitten by a vector and that the latter becomes infected, which increases the probability of transmission to other individuals.

11Immunity evidently plays a highly significant role in transmission dynamics. For example, for malaria in an endemic zone, a balance is slowly established between the parasite and the organism. However, immunity is only partially effective, even after several years, and it rapidly disappears if the organism is no longer infected. The protection acquired coexists with the presence of a low parasitaemia, which in theory allows transmission towards the vector to continue. In such a case there is a state of premunition or semiimmunity. In places hit by malaria throughout the year, semi-immunity is a protection against severe forms of the disease. In such regions, it is infants, pregnant women or new arrivals who are the most severely hit. Where malaria is seasonal, with low transmission level (as in the Sahel), this premunition is low because the protection acquired during the transmission period is quickly lost. This paradox raises questions about the situations that require vector control and the strategies that should be implemented to reduce malaria transmission without lowering immunity.

VECTOR RISK: FROM WARNING SIGNS TO EPIDEMIC

12At what point does a situation become cause for concern? Any given vector system needs the presence of a competent vector, but the simple presence of such a vector capable of transmitting a pathogen in a specific environment does not necessarily signal a risk of epidemic or even a situation of alert: a particular set of conditions must coincide. The pathogen must be present. Suitable contacts for effective transmission must be established between the vector and the reservoir of infectious agents, but also between the vector and the possible amplifying host. Lastly, contacts must be possible between the vector and the susceptible hosts.

13Passage to an effective phase of transmission itself depends on a large number of factors: ability of the vector to become established durably after its introduction, its vector competence and capacity, the host population’s relative susceptibility or immunity. Any arthropod early warning system must therefore be based at once on parameters relative to the vector, the environment and the host, particularly its level of immunity and its ability to protect itself.

14It remains that the suitability of using entomological parameters to warn about epidemic risk differs depending on the vector-borne disease’s transmission cycle and, for the same disease, the endemo-epidemiological situation and the ecosystem. For example, the presence and expansion of a dengue virus vector in metropolitan France, an area unaffected until recent years, are justified as a criterion for strengthening surveillance and devising action plans taking into account, among other factors, the incoming of viraemic subjects from epidemic areas. In the French départements of America (FDAs) (areas of endemization of this disease), surveillance is also based on epidemiological aspects (suspected cases, confirmed cases).

VECTOR CONTROL IN FRANCE

15More generally, vector risk assessment essentially entails analysis of indicators and characteristics of the “vector system” (including hosts, pathogens, environment, society). The procedure aims to gain understanding of the dynamics of transmission and the effects of changes to one or more components of the vector system and/or to its interactions. In a complementary way, vector control demands study not only of the positive consequences but also of potential negative effects that could be set off well before consequences for health come into play (anticipation), or during a warning situation, even an epidemic (analysis of management options).

16In France, as elsewhere, vector control therefore does not just boil down to implementing assumedly universally applicable formulae. The anophelines in Mayotte cannot be controlled in the same ways as Aedes albopictus in Corsica. Moreover, “people’s claims for a world without risk, the aspiration for reducing use of pesticides in the context of the [French national environmental agreement] ‘Grenelle de l’environment’ and for sustainable development must lead to the development of research on the risks, early warning systems and new control strategies against different types of vector”(see note 2, p. 294). The challenges are taking on even more complexity with the considerable influence global changes (climatic, environmental, societal) exert on vector systems.

Table 1 Main vector borne infections in France*

Table 1 Main vector borne infections in France*

Note * The main vector-borne infections that occur in French territory with their principal characteristics (vector; reservoir; geographical distribution; epidemiological status). Itssignificance for public health has been treated qualitatively in terms of morbidity (severity of the clinical picture: high, moderate and limited, with a more detailed commentary for certain diseases); incidence (high, moderate, low or to be determined); fatality (high, low or zero) and trends (expansion; regression and, where applicable, theplace or population concerned by this trend). The diseases have been classified into three groups (human diseases, zoonoses, animal diseases).

Table (continuation) – Main vector-borne infections in France

Table (continuation) – Main vector-borne infections in France

Table (continuation) – Main vector-borne infections in France

Table (continuation) – Main vector-borne infections in France

Figure 1. Martinique, Guadeloupe, French Guiana

Figure 2. Mayotte, Reunion Island

Figure 3. Metropolitan France

Figure 4. New Caledonia, Walis-and-Futuna

Notes

1 See the overview of human and animal infections (vectors, epidemiology, transmission, surveillance and so on) on the CD-ROM: “The context of vector control in France”, edited by J.-C. Desenclos, S. Lecollinet and T. Balenghien and elaborated by a team of some 30 specialists.

2 D. Fontenille, Écosystèmes, entomologie et lutte antivectorielle, Annales des Mines. Responsabilité & Environnement, 58 : 55-60, 2008.

Table des illustrations

Titre Table 1 Main vector borne infections in France*
Légende Note * The main vector-borne infections that occur in French territory with their principal characteristics (vector; reservoir; geographical distribution; epidemiological status). Itssignificance for public health has been treated qualitatively in terms of morbidity (severity of the clinical picture: high, moderate and limited, with a more detailed commentary for certain diseases); incidence (high, moderate, low or to be determined); fatality (high, low or zero) and trends (expansion; regression and, where applicable, theplace or population concerned by this trend). The diseases have been classified into three groups (human diseases, zoonoses, animal diseases).
URL http://books.openedition.org/irdeditions/docannexe/image/1264/img-1.jpg
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Titre Table (continuation) – Main vector-borne infections in France
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Titre Table (continuation) – Main vector-borne infections in France
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Légende Figure 1. Martinique, Guadeloupe, French Guiana
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Légende Figure 2. Mayotte, Reunion Island
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Légende Figure 3. Metropolitan France
URL http://books.openedition.org/irdeditions/docannexe/image/1264/img-6.jpg
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Légende Figure 4. New Caledonia, Walis-and-Futuna
URL http://books.openedition.org/irdeditions/docannexe/image/1264/img-7.jpg
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