Leptospirosis is a major public health problem in the islands of the Indian Ocean. Recent studies have revealed distinct transmission cycles across the various islands in the region, particularly in Madagascar and the French islands of Mayotte and Réunion. From an evolutionary perspective, these studies demonstrate the presence of different bacterial lineages on the various islands. Indeed, Madagascar’s wildlife harbors a diversity of pathogenic leptospires, including an endemic species that was introduced to the neighboring island of Mayotte, where it accounts for a significant proportion of acute infections in humans. In contrast, on Réunion, human leptospirosis is caused by leptospires whose genetic diversity is clearly more limited, as they were likely introduced recently. From an operational standpoint, data generated on the French islands show that rats are not the only animals involved in transmission to humans; rather, they suggest the involvement of other introduced animals, particularly dogs, which warrant special attention. These studies, conducted within a "One Health" framework, demonstrate that investigating the environmental compartment of a zoonosis sheds light on the transmission cycles at work in each ecosystem and, furthermore, helps guide control strategies by adapting them to each specific environment.