Leptospirosis: why late summer is the highest-risk period

Leptospirosis is the disease people instinctively associate with rats — and rightly so. Every year, 600 to 700 cases are recorded in mainland France, and incidence peaks in late summer. Swimming in fresh water, gardening after a thunderstorm, cleaning a damp storeroom: the most ordinary July and August activities are also the most exposed. Here is what the official health sources say, and why rodent control is the first line of defence.

A bacterium rats shed continuously

Leptospirosis is an infection caused by bacteria of the genus Leptospira. Rodents — and rats in particular — are its "main reservoir": they carry it without being ill and shed the bacteria in their urine throughout their lives, contaminating their environment for the long term.

In France, the main source of contamination is contact with fresh water soiled by the urine of wild rodents (rats, coypu). Other animals can contribute: livestock (pigs, cattle), pets (dogs, horses, pet rodents) and wildlife (wild boar, hedgehogs, shrews).

Contamination is most often indirect: water from a pond, mud, bedding or a soiled surface. The bacterium enters through a wound, even a tiny one, or through the mucous membranes (eyes, mouth, nose). A simple graze is enough — which is also why the risk rises sharply when heavy rain washes over contaminated ground.

Why cases cluster in late summer

Three factors coincide:

1. Rat populations are at their peak after a full breeding season.
2. High-risk activities are in full swing: freshwater swimming and sports, canoeing, fishing, gardening, outdoor work.
3. Summer storms leach the soil and help Leptospira survive in water and mud.

The result: the annual incidence peak is observed in late summer. The disease has been a notifiable condition since August 2023 — any doctor or laboratory diagnosing a case must report it, which allows outbreaks to be identified quickly.

Symptoms you should not shrug off

In most cases, leptospirosis presents as a flu-like illness: fever, headache, muscle pain. That is precisely the trap — in the middle of summer, it gets blamed on tiredness.

• Incubation: usually 5 to 14 days after exposure, and up to 21 days.
• Severe forms: kidney or neurological damage, haemorrhagic manifestations. These can be fatal.
• Treatment: antibiotics — oral in mild forms, with hospital admission in severe forms.

The reflex that counts: if you develop fever, aches or headaches within 21 days of swimming in a river, being exposed to standing water or coming into contact with a soiled environment, see a doctor promptly and mention that exposure. Early treatment considerably reduces the risk of complications.

Who is most exposed

Some occupations involve frequent contact with water and soiled environments: refuse collectors, sewer workers, wastewater network staff, farmers, livestock breeders. For them, protective equipment is essential: waterproof gloves, boots, safety goggles, overalls. A vaccine is available for people exposed on a daily basis.

For households, exposure comes mainly from freshwater leisure activities — especially with open skin wounds — from gardening, and from cellars, basements, garages or damp premises frequented by rats.

Prevention that actually works

As an individual:

• Do not swim in fresh water if you have a wound or broken skin, however small.
• After contact with water or soil, rinse with drinking water and disinfect any wounds.
• Wear gloves when handling soil, waste, bedding or items stored where rodents roam.
• Cover any wound with a waterproof dressing before a high-risk activity.

Collectively — and this is where it is decided:

Prevention relies on rodent control, drainage of flooded areas and water monitoring. In other words: fewer rats, fewer bacteria in the environment. A badly closed compost bin, open dustbins, a woodpile against a wall, an unsealed duct — every access point sustains a population that then contaminates the soil and water around your home.

For the signs that betray a rat presence and the structural measures to put in place, read our guide rats spreading through cities: understand and react. And to work out where they get in, our article mice in the house in winter: prevention and action details the sealing points, which apply to all rodents.

Two common mistakes

Assuming a rat seen in daylight is an isolated case. Rats are nocturnal: a daytime sighting usually signals an already dense population competing for food. That is a warning sign, not an anecdote.

Putting down a few baits and stopping there. Without removing food sources, sealing access points and following up, the population rebuilds within weeks — and soil contamination persists. Effective rodent control is a protocol: survey, treatment, physical exclusion, monitoring.

Calling in a professional

Rats in a cellar, a plant room, a garden, a shared building, a restaurant or a farm? A professional survey identifies runs, burrows and entry points, applies a treatment compliant with biocide regulations and, above all, seals the access points for good.

Discover our rodent control services and our protection solutions and traps. You can also check our prices and our service areas.

Rats in your home or business premises? Contact our experts for a survey — and in a critical situation (food premises, nursery, advanced infestation), our emergency service responds the same day.

In short — 600 to 700 cases a year, a peak in late summer, a bacterium shed continuously by rats: the best protection against leptospirosis is not a product, it is the absence of rats around you. And if you develop a fever within 21 days of exposure, see a doctor without delay.

Sources: French regional health agencies (ARS), Santé publique France — leptospirosis has been a notifiable disease since August 2023.