MAP. Lyme disease: here are the areas of France where ticks proliferate the most

Although very small, the tick if it is infected by a bacterium of the spirochete family can be dangerous and trigger Lyme disease in humans. (©Sebastian Kaulitzki? (©Chalabala /Adobestock)

According to the latest data available, probably more than 60,000 people contract the disease each year. Lyme borreliosis Where Lyme disease.

Generally, hospitalizations for this pathology linked to a tick bite take place between June and October and children aged 5 to 9 and adults aged 70 to 79 would be the most affected.

While the incidence rate tends to increase, and if not diagnosed in time, this disease can cause lasting joint pain or even partial paralysis of the limbs, the authorities have decided to distribute a map, and to repeat advice, common sense.

These tick zones in France

A team comprising researchers from the National Research Institute for Agriculture, Food and the Environment (Inrae), the National Institute for Higher Education and Research VetAgro Sup (merger of the National Veterinary School of Lyon, the National School of Agricultural Works Engineers of Clermont and the National School of Veterinary Services), ANSES (the National Agency for Food, Environmental and Occupational Health Safety) and CIRAD (Centre for International Cooperation in Agronomic Research for Development) have just released a map showing the areas where “Ixodes ricinus” ticks are most frequently encountered.

What do we see? The eastern and central regions (Alsace, Lorraine, Limousin in particular) have high annual incidence rates (more than 100 cases per 100,000 inhabitants), the western and south-eastern Mediterranean regions have lower annual incidence rates (less than 50 per 100,000 population).

As for the Mediterranean and high mountain regions, they are the least favorable to the proliferation of ticks, and appear in green on the map.

Lyme borreliosis or Lyme disease is an infectious disease caused by bacteria of the Borrelia burgdorferi sensu lato complex transmitted to humans by the bites of infected ticks.
Areas where ticks are most numerous (©INRAE)

In deciduous forests, undergrowth, pastures and meadows…

If the disease is present throughout the metropolitan territory, there are significant geographical disparities. These “Ixodes ricinus” ticks live in deciduous forests, undergrowth, pastures/meadows, they are infrequent in coniferous forests.

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They can also be found in peri-urban wooded areas and in city parks and private gardens. The “Ixodes ricinus” tick has never been detected in the overseas departments and regions, due to a climate not suited to their development, but caution should be exercised throughout the year in case of mild and humid climate as in Brittany.

Lyme borreliosis, or Lyme disease, what is it?

Lyme borreliosis or Lyme disease is an infectious disease caused by a bacterium of the Borrelia burgdorferi sensu lato complex transmitted to humans by bites from infected ticks, explains Public Health France. Lyme borreliosis is the most common tick-borne disease in France.

How do these ticks behave?

Ticks are mites taking “their blood meal” on animals, and humans are accidental hosts, underlines Public Health France. The search for a host for their blood meal is carried out when the environmental conditions are optimal (humidity, temperature).

The development cycle of the tick “Ixodes ricinus” takes place in several stages and the length of this cycle varies from two to six years, the researchers determined. From the egg is born a larva transforming into a nymph (2 mm) then into an adult (3-4 mm). A blood meal on a host is required for the egg laying of the adult female tick and at every stage of its development.

The “Ixodes ricinus” tick then becomes infected by feeding on the blood of hosts contaminated with “Borrelia burgdorferi sensu lato” bacteria.

The “blood meal” lasts three to seven days depending on the stage. At the end of the “meal”, the tick detaches itself from its host and falls into the vegetation.

Public health France

The main reservoir hosts of Borrelia burgdorferi sensu lato? They are small wild mammals (voles, field mice, squirrels, etc.). Certain species of birds or reptiles are also reservoirs. Large mammals such as deer are hosts of adult ticks, but incompetent reservoirs of Borrelia burgdorferi, ie incapable of transmitting the bacterium to an uninfected tick.

The “blood meal” lasts three to seven days depending on the stage. At the end of the “meal”, the tick detaches itself from its host and falls into the vegetation. It needs a minimum of humidity to survive (80%) and can wait several months to move on to the next stage of the cycle or lay its eggs if it is a female tick.

How is Lyme disease transmitted to humans?

“Borrelia burgdorferi” is transmitted to humans by the bite of an infected tick of the “Ixodes ricinus” complex. During the “blood meal” following the bite, bacteria from the tick’s gut pass to its salivary glands.

Transmission by saliva depends on the contact time and the rate of infestation of the tick.

In American studies, the risk of transmission appears low for durations of attachment of less than 72 hours. In Europe, experimental and clinical data have shown that this period would be shorter with an increased risk after 24 hours of attachment, details Public Health France.

Humans can be bitten by a tick at any stage of its development (larva, nymph, adult), but nymphs seem to be responsible for most transmissions. Lyme borreliosis is not transmitted from person to person, neither by direct contact with animals, nor through food, nor by the bite of other insects.

What are the symptoms of Lyme disease?

How do you know if you are sick? Most commonly, you will experience a skin rash “called erythema migrans”. The symptoms of Lyme borreliosis depend on the stage of the disease. The first occurs 3 to 30 days after the tick bite. It is characterized by this typical cutaneous manifestation. It is “an erythematous spot, at the site of the tick bite, painless and of annular and centrifugal growth”, details Public Health France.

“Erythema migrans” is the most common manifestation (60 to 90% of cases) and the most suggestive of Lyme borreliosis.

Neurological manifestations are also possible (facial paralysis, isolated meningitis, acute myelitis). And more rarely joint manifestations (arthritis with notion of effusion of a large joint such as the knee), cutaneous (borrelian lymphocytoma), cardiac or ophthalmological. Symptoms that can occur several months or even years after the tick bite. It is therefore advisable to do a serological test to look for specific antibodies against Borrelia burgdorferi sensu lato.

How to treat Lyme disease?

To overcome Lyme borreliosis, antibiotic treatment is sometimes necessary. At the early localized stage, antibiotic therapy allows the healing of erythema migrans but also to prevent the progression of the disease.

There is currently no vaccine available against Lyme borreliosis or against tick bites. However, a vaccine is available against tick-borne encephalitis (but does not protect against other tick-borne infections such as Lyme Borreliosis).

How to protect yourself from ticks?

It is advisable to walk on well-marked paths, without dense vegetation and tall grass, to wear covering clothing, possibly impregnated with repellents. Wearing light-colored clothing also makes it easier to spot ticks that have not yet attached to the skin.

If you think you have encountered a tick, a careful examination (because the tick, at the nymph stage measures only 1 to 3 mm) of the body after exposure to the risk of bites is necessary. Particular attention should be paid to “the skin folds, the back of the knees, the armpits, the genital areas, the navel and, in particular in young children, the scalp, the neck, the back of the ears”, list the authorities.

It may be useful to do a new inspection the next day because the tick, partly engorged with blood, will be more visible.

Another important advice is to remove the tick as quickly as possible to avoid transmission of Borrelia because the risk of transmission increases with the time of attachment. Use a tick hook (available in pharmacies) if possible, otherwise fine tweezers or, failing that, tweezers (do not pull the tick out with your fingers) and disinfect the bite site after removing the tick.

Then monitor the sting area for four weeks. In the event of the appearance of erythema migrans (red and round plaque) or general signs (fever, malaise, body aches) you should consult your doctor. Several infections can be transmitted by ticks in France apart from Lyme Borreliosis, but they are much rarer.

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