An underestimated, potentially serious infectious disease

What is Leptospirosis?

Leptospirosis is a bacterial infection, referred to as a “zoonosis” due to the fact that it is transmitted to humans by animals.
Mammals can be carriers of this disease, particularly rodents and rats. It is also called Weil’s disease.
You may recently have heard about this infection in the media, following the proliferation of rodents in some regions.

Leptospirosis, an emerging public health issue(1)

Worldwide, more than one million severe cases of human leptospirosis are estimated to occur each year, 60,000 of which are fatal. In Europe, leptospirosis is an emerging zoonosis.
These figures are likely an under-estimation, for the following reasons (among others):

  • not all countries are obliged to report instances of this disease
  • diagnosis is often a complex and slow-moving process.

leptospirose zoom


Head of the National Leptospirosis Center at the Pasteur Institute.

Leptospirosis: not just a tropical infection

An average of 700 cases of leptospirosis are observed in French overseas departments and territories each year. Leptospirosis is re-emerging in Europe(2). It is an overlooked, and very probably underestimated, disease(1). In the United Kingdom, for example, 2017 saw the highest incidence for 10 years(3,4). In metropolitan France, the number of cases has doubled in recent years, rising from 300 to more than 600 identified cases annually, i.e. more than one case a day since 2014. In 2014, 628 cases of leptospirosis were identified in mainland France, the highest incidence for 80 years(5). This growth in the number of leptospirosis cases is linked to climate change, but also to the effects of increased urbanization and the presence of rats.

A potentially serious infectious illness

Leptospirosis is a potentially serious, sometimes fatal, infectious disease caused by bacteria from the Spirochete group (Leptospira, Treponema, Lyme disease and Syphilis): Leptospira. The Leptospira interrogans species causes the pathogenic forms, and there are more than 250 serovars spread over 24 serogroups(6).


Dr. Jean-Michel ESTAVOYER
Former Head of the Infectious Diseases Dept., Besançon University Hospital

The Icterohaemorrhagiae serogroup is the most lethal worldwide, with a 13.6% mortality rate in untreated cases(7).

In metropolitan France, the Icterohaemorrhagiae serogroup is the most commonly implicated form of the bacteria in the most serious cases in humans(1). The Icterohaemorrhagiae form is implicated in a third of leptospirosis cases, and has been identified in two thirds of serious cases requiring hospitalization(8).

In the United Kingdom, icterohemorrhagic leptospirosis accounts for approximately half of identified cases(9), and two thirds of identified cases in the Netherlands(10).

In French overseas departments and territories, the Icterohaemorrhagiae serogroup is dominant in the Antilles (Guadeloupe and Martinique), French Guyana, Réunion Island, French Polynesia and New Caledonia, with the sole exception of Mayotte, where this serogroup is not present(1).This serogroup has also been identified in serious cases, as in New Caledonia(11), and in up to 91% of serious cases requiring hospitalization in the Antilles(12,13).

(1) Filleul L. et al. ; Santé Publique France – La leptospirose dans les régions et départements français d’outre-mer ; Bulletin Epidémiologique Hebdomadaire ; 4 avril 2017 ; numéro 8-9.
(2) Dupouey J et al. Human leptospirosis : an emerging risk in Europe? Comparative Immunology, Microbiology and Infectious Diseases 37 (2014) 77– 83.
(3) Public Health England. Zoonoses Overview Report UK 2016. HM Government ; 2017. 50.
(4) Public Health England. Common animal associated infections quarterly report (England and Wales) : fourth quarter 2017. Health Protection Report. Volume 12 Number 5. 9 february 2018. 7.
(5) Picardeau M., Rapport annuel d’activité du CNRL 2017 pour l’année d’exercice 2016.
(6) Adler B. (Ed) Leptospira and Leptospirosis, Current Topics in Microbiology and Immunology Sringer-Verlag Berlin Heidelberg 2015; 387: 1-293. DOI 10.1007/978-3-662-45059-8.
(7) Andrew J. Taylor. «A Systematic Review of the Mortality from Untreated Leptospirosis. PLoS Negl Trop Dis.» ; Juin 2015.
(8) Estavoyer JM, « Leptospirose en Franche-Comté : données cliniques, biologiques et thérapeutiques. Médecine et maladies infectieuses » ; Septembre 2013: pages 379-385.
(9) A.E. FORBES, W.J. ZOCHOWSKI, S.W. DUBREY and V. SIVAPRAKASAM. Leptospirosis and Weil’s disease in the UK. QJM. 2012 ; 105 : 1151-1162.
(10) Goris, M.G.A. (2016, April 6). Leptospirosis: epidemiology, clinical aspects and diagnosis. Erasmus University Rotterdam. Retrieved from
(11) Tubiana S., «Risk Factors and Predictors of Severe Leptospirosis in New Caledonia» PLoS Negl Trop Dis. 2013 Jan; 7(1): e1991.
(12) Hochedez P., «Factors Associated with Severe Leptospirosis», Martinique 2010–2013», Emerg Infect Dis. 2015 Dec; 21(12): 2221–2224.
(13) Herrmann-Storck C. ar al. Severe Leptospirosis in Hospitalized Patients, Guadeloupe. Emerging Infectious Diseases 2010; 16(2): 331-334. doi: 10,3201/eid1602,030139.