The agent responsible for leptospirosis is a spirochete of the species Leptospira. The 3 main species in Europe are: Leptospira interrogans, Leptospira kirschneri and Leptospira borgpetersenii. These species may belong to different serovars according to their surface antigens: Australis, Canicola, Grippotyphosa, Icterohaemorrhagiae, etc.
The rodents but also small mammalian are to be considered as the main reservoir of the disease. They are a healthy carrier of leptospirosis that is excreted in their urine.
The bacterium is found in the environment where it can survive several months with favourable conditions (warm and humid).
Dogs become infected when their mucous membranes or injured skin meet the pathogen. The incubation period is about 7 days. Leptospirosis can occur in several forms: superacute, acute, subacute, and chronic. Usually, the first clinical signs are not very specific: anorexia, abatement, fever. Subsequently, the animal frequently develops acute renal and/or hepatic failure, which can be fatal.

Haemorrhagic gastroenteritis can also be observed. The superacute form causes rapid death of the animal (within a few hours).
It is often necessary to hospitalise the animal to stabilise it (fluid therapy, management of complications due to renal failure, management of gastrointestinal disorders, etc.). Although life-threatening, treatment is available as leptospires are sensitive to antibiotics. The earlier the antibiotic treatment is started, the better the chances of survival of the animal.
Several vaccines are available, however, none of them are instead of is 100% effective in preventing infection and the onset of clinical signs.