From 1965 until today, mosquito control operations in the coastal areas (Languedoc-Roussillon and Bouches-du-Rhône) have been perfected and have become more and more “specific”. They have, however, used two common pesticides belonging to the organophosphorus family, fenitrothion (anti-adult or imagocide) and temephos (larvicide). One advantage of these contact insecticides is that they rapidly decompose in the ecosystem; however, their toxicity is not selective, i.e., they affect vertebrates as well as invertebrates. Whereas mammals are relatively insensitive to them, this is not at all true of birds for which fenitrothion and temephos are “highly toxic”. A small quantity of these products is enough to kill a bird either by inhalation of particles, or through dermal contact with the insecticide spray or contaminated vegetation, or by ingestion of contaminated food or residues when birds groom their feathers. A third organic insecticide, which is being more and more used by the EID, is Bti (Bacillus thuringiensis israelensis). Bti is a bacterium that is naturally found in the soil and kills mosquito larvae by ingestion. More selective and less toxic than the previous products, Bti is not widely used due to its higher cost and lower effectiveness. Its impact on the non-targeted fauna concerns mainly aquatic invertebrates, which are consumed by fauna in the wetland zones.
As is the case for many invertebrates, mosquitoes can transmit parasites, bacteria, and viruses. This includes the “West Nile” virus, identified in the south of France in 1962, it struck in September 2000. 53 horses contracted the disease, but no humans were affected. In the United States, the virus appeared in 1999 in the state of New York, and spread rapidly to most of the other states, as well as Canada, infecting several thousand people and causing several hundred deaths. But it is important to note that the strain present in France is much less virulent than the one that struck in the United States, rarely causes anything more than a slight fever for a few days, and sometimes has no effect at all.
Birds are reservoirs for viruses, because they are likely to host and propagate a virus if they are bitten by an insect carrying it. Mammals, like the horse or man, are “epidemiological dead ends”, that is they can neither multiply nor transmit a disease, but can be affected by it. The return of the West Nile virus is frequently cited as a public health issue in order to justify mosquito control in the Camargue. However, this argument does not stand up to the facts: most of the sources of the virus identified until now are located in regions that have already had mosquito control operations (Hérault and Gard departments) or are far from zones with a high concentration of mosquitoes (the centre of the Var department). In addition, mosquito control “for reasons of comfort” only concerns mosquitoes from the Ochlerotatus (Aedes) genera, of which the mass emergence results when the eggs are washed into the water or flooded zone, whereas it is the mosquitoes of the Culex genera, which are not a big nuisance, that are the best vectors for transmitting the West Nile virus. Rather than massive (and ineffective) “sanitary” mosquito control, which is practiced in the United States, France has opted to set up a monitoring system. In this way, the epidemiological cycle can be understood and any circulation of the virus can be detected before any clinical signs appear among equines or human beings.
The Tour du Valat is part of a network of 16 sites that are participating in this surveillance programme by monitoring “sentinel” domestic birds in the Camargue. It is also carrying out campaigns to capture wild birds in order to evaluate the seropositivity of resident species and migratory species as soon as they arrive in the spring. The 2003-2004 results show very few seroconversions among the birds.
No West Nile case was observed among people or horses in the departments involved in this monitoring in 2003 (Bouches-du-Rhône, Gard, Hérault, Aude, and Pyrenees orientales). On the other hand, seven human clinical cases and four equine cases confirmed the presence of the virus in the Var, resulting in the extension of the monitoring to the entire Provence-Alpes Côte d’Azur area in 2004.
In the first nine months of 2005, no cases of seroconversion (among birds, horses or humans) were detected. Recent developments in the elaboration of a vaccine against the West Nile virus may provide the best long term solution for the human populations exposed to the virus.