IRSEA project: Ticks, lyme disease and pesticides

Ticks are hematophagous mites that feed on the blood of vertebrates. Their adult size varies from 3 to 6 mm on average depending on the species (up to 3 cm for some tropical species), making them the largest representatives of the order of mites.

There are about 900 species of ticks classified in 2 main groups:

- The Ixodidaes: commonly called hard ticks because of the presence of a hard plate on the dorsal side.

- The Argasidaes: these are soft ticks with a soft tegument without a dorsal patch.

Note that there is a group with only one species, the Nuttalliellidaes, which is an "intermediate" group with features belonging to the other two.

About forty species are present in France, but 3 species of ticks are largely dominant : Dermacentor reticulatus, Ixodes ricinus and Rhipicephalus sanguineus.

The life cycle of a tick can last from a few months to several years. Ticks develop through different evolutionary stages: egg, larva, nymph and finally adult (the Argasidaes have several stages). These transformations require, for most species, a blood meal.

In hard ticks, the female makes a single meal that will last several days, she will then lay several thousand eggs before dying. In soft ticks, the female makes episodic meals lasting 15 to 20 minutes and each meal allows the female to lay 20 to 150 eggs.

In most species, individuals spend most of their lives on the ground and only go in search of a host to feed.

There are 2 strategies, depending on the species: the hunting strategy (looking for a host while moving) and the ambush strategy (waiting for a host to pass by, lying in wait on the vegetation).

Moreover, ticks have a very particular organ, Haller's organ. It is, among other things, sensitive to hygrometry, CO2 and pheromones. It allows the tick to locate its host. This organ is located on the first pair of legs, which is why, when ticks are on the lookout, they extend and move their legs.

Once on the host, the rostrum is the part of the tick that penetrates the skin. The tick attaches itself with its chelicerae, before secreting saliva that pre-digests the host's tissues. Then, its hypostome penetrates the tissue, and the rostrum is fixed. This fixation is reinforced by a cementum (glue produced by the saliva).

About 100 infections can be associated with 116 species of ticks (32 species of Argasidae and 84 of Ixodidae). Ticks are among the most important disease vectors in the world. With diverse hosts and exceptional longevity, the tick is both a good vector and an excellent tank.

These diseases can be caused by :

- Bacterial (Lyme disease, rickettsiosis, tularemia, bartonellosis),

- Viral (tick-borne encephalitis, tick-borne and haemorrhagic fevers, sheep wolf-ill),

- Parasites (canine piroplasmosis, bovine babesiosis).

The most important tick-transmitted disease in Europe and the United States is Lyme disease (also called Lyme borreliosis). It is an infectious disease caused by a bacterium (Borrelia, of which there are several species). Lyme disease was first described in 1975, although it has already been observed for many years in Europe. The first known human to be infected with Borrelia burgdorferi was Ötzi, the 5,300-year-old ice man. Estimates suggest that there are more than 300 000 new cases each year in the United States and 65 000 in Europe. In France, it would be 27,000 cases per year. It is the inhabitants of the regions surrounding Alsace, Lorraine, Limousin, Rhône-Alpes and Auvergne who are the most exposed to Lyme disease, as the climatic conditions are favourable to development. Lyme borreliosis is transmitted by the bite of a tick of the Ixodes family. This tick must carry the bacterium in order to transmit the infection, but for some European countries, 5 to 20% of these insects carry this pathogenic germ. These figures are greatly exceeded in certain American regions, as they can reach 100%.

In recent decades, global warming, changes in land use and altered human activities and habits have caused significant changes in the spatial distribution and abundance of ticks, increasing the exposure of the human and animal population to their attacks and the diseases they can transmit.

To combat this scourge and to compensate for the use of pesticides (which are harmful to the environment and favour the emergence of resistance), IRSEA has developed an analogue of a semiochemical that inhibits tick attack. The aim is to create confusion in the parasite by camouflaging the host usually attacked, thanks to a substance emitted by an individual of another species that naturally repels the parasite. Tests have been carried out on humans with Ixodes ricinus and Rhipicephalus sanguineus and promising results should be published soon. Tests on dogs should also take place this spring.

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