Zoonosis: Leptospirosis on the Rise in France and Around the World

Nearly a century after its discovery, leptospirosis remains a neglected, poorly understood, and underestimated disease, particularly with regard to its modes of transmission, risk factors, and clinical strains. Primarily linked to water, heat, and rodents, this emerging zoonosis has been spreading in recent years in France, where its incidence is rising sharply, as well as worldwide. With one million cases and approximately 60,000 deaths per year, its global incidence is now similar to, or even higher than, that of other neglected diseases such as dengue or leishmaniasis. According to the special issue of the BEH* dedicated to Leptospira in France, there is an urgent need to better understand the epidemiology of the disease and to raise awareness among public health and animal health professionals regarding its diagnosis.   Leptospirosis is the most widespread zoonotic disease globally due to the many mammalian reservoirs—both wild and domestic—capable of transmitting the pathogenic bacteria to humans, who are occasional hosts. It represents an emerging public health problem, particularly in Europe, linked to climate change (warming, flooding), the growth of recreational activities, and urbanization (increase in high-risk activities, rodent populations, unsanitary areas, etc.). Furthermore, it remains largely underestimated due to nonspecific symptoms, which can be confused with the clinical signs of other diseases such as influenza, malaria, chikungunya, or dengue fever. According to the latest estimates, more than one million severe cases of leptospirosis are reported worldwide each year, with a mortality rate ranging from 5 to 20 percent. In France, the incidence among humans has doubled since 2011, reaching one case per 100,000 inhabitants in 2014–2015 (more than 600 cases recorded during that period), a record high since 1920. France is one of the industrialized countries with the highest number of new cases. In the overseas departments and territories, where the disease is endemic, the incidence is up to fifty times higher than in metropolitan France. Although leptospirosis has not been a notifiable disease in France since 1986, it remains under passive surveillance. Diagnostic testing is primarily carried out by the National Reference Laboratory, which analyzes approximately 4,000 samples per year, and by private laboratories, which perform two-thirds of the biological tests. Animal health professionals also play an active role. Through partnerships between the National Reference Center (CNR) and veterinarians, the latter actively contribute to the surveillance of this zoonosis, particularly by conducting prevalence studies of Leptospira in wildlife, rodents, and Livestock.  

In Animals

All mammals can potentially harbor pathogenic leptospires. The infection, whether acute or chronic, presents a wide range of clinical signs in its hosts: while rodents are primarily asymptomatic carriers of the bacteria, ruminants are particularly susceptible (abortions, loss of milk production), as are dogs (severe forms are often fatal). Certain serovars are host-specific, such as L. canicola in dogs, L. icterohaemorrhagiae in rats, L. ballum in house mice, etc. In total, there are 22 species of Leptospira , 10 of which are pathogenic, and more than 300 serovars grouped into some 24 serogroups... Infection occurs primarily through contact with contaminated water or the urine of an infected animal, but other routes of infection (in utero , sexual transmission, breast milk) cannot be ruled out. In a favorable environment (water or wet soil), leptospires can survive for several weeks before entering the body through skin lesions or via the mucous membranes (eyes, mouth, nose).  

In humans

After an incubation period of about ten days, human leptospirosis presents a clinical picture that is equally variable, ranging from the anicteric febrile form observed in most cases to potentially fatal multiorgan failure, characterized by renal failure, internal bleeding, and jaundice. Antibiotic therapy must be initiated as early as possible to prevent severe forms of the disease. In temperate regions, leptospirosis most commonly affects people in high-risk occupations (veterinarians, farmers, livestock breeders, sewer workers, fish farmers) and those who participate in water sports (divers) or outdoor recreational activities (swimming, fishing, rafting, canyoning) through contact with freshwater contaminated by the urine of infected animals. Recently, several cases of leptospirosis have been reported among people who keep pet rats. Considered a neglected disease, leptospirosis is undoubtedly vastly underestimated worldwide. According to Éric Bertherat (WHO), it is “undoubtedly an emerging public health problem of international significance,” and it is high time to bring it “out of the media obscurity surrounding bacterial diseases.” This is because “many questions regarding disease control remain unanswered, particularly during epidemics.”     * Leptospirosis in France, Weekly Epidemiological Bulletin No. 8-9 – April 4, 2017, http://invs.santepubliquefrance.fr/beh/2017/8-9/index.html