Dynamics of Leptospirosis Transmission Within Urban Norway Rat (Rattus norvegicus) Populations in Densely Populated French Areas: Implications for Public Health

Leptospirosis, a bacterial zoonosis with a worldwide distribution, represents a major public health challenge. It is caused by the spirochete Leptospira, whose main reservoir in urban environments is the brown rat (Rattus norvegicus). Understanding the transmission dynamics of this disease within a rat population is essential for controlling the risk of human infection. In this study, an original capture method was used to analyze variations in carriage and bacterial load according to age in two distinct populations of brown rats, to provide a better understanding of the transmission routes of Leptospira within a population. A total of 508 rats were captured from all age categories, from newborns to very young rats (representing 18% of the animals) to very old rats (representing 21% of the animals). The overall prevalence of leptospirosis was between 30% and 50%, depending on the population. A single strain was identified in both studied populations: Leptospira interrogans belonging to the Icterohaemorrhagiae serogroup and the Icterohaemorrhagiae serovar. Surprisingly, in both populations, our study reveals a sudden change in the prevalence at 300/400g, jumping from 20 to 30% to over 75%. Moreover, none of the 98 fetuses collected from 13 pregnant females infected with Leptospira was detected as infected. This sudden change and the absence of infected fetus demonstrate the major role of horizontal transmission in the dynamics of leptospirosis infections and minimize the importance of vertical transmission.