Leptospirosis is a zoonosis caused by Leptospira spp, and the contact with the bacteria can lead to a severe condition, mainly characterized by hepatitis and/or glomerulonephritis. The rodents have a significant importance in the maintenance of the disease in the environment and transmission of the leptospiras to dogs and humans. The dogs also contribute to the transmission of this disease to humans. Ilha Grande dos Marinheiros is an island located on Guaíba river in Porto Alegre, Brazil. The environment at Ilha dos Marinheiros may contributes to the dissemination of leptospirosis, because this is a flood area, without sanitation, with high population of dogs and rodents and no garbage proper handling. We selected 116 adult dogs, both gender, and history of daily contact with rodents. All the animals weren't vaccinated and the most part were in poor body condition. Blood and urine samples were obtained for further laboratory analyses. Biochemistry analyses were performed on blood samples and also the serum samples were sent to Instituto de Pesquisas Veterinárias Desidério Finamor (IPVDF) for serologic tests.

The serovars tested were L. australis, L. autumnalis, L. bratislava, L. canicola, L. copenhageni, L. grippotyphosa, L. hardjo, L. hebdomadis, L. icterohaemorrhagiae, L. pomona, L. pyrogenes, L. wolffi and L. tarassovi with dilution from 1:100. The urine samples were submitted to DNA extraction, and a polymerase chain reaction (PCR) test was performed to detect pathogenic leptospira's DNA. All the creatinine and alanine aminotransferase (ALT) results were normal. The reagent serovars were L. australis, L. autumnalis, L. bratislava, L. canicola, L. copenhageni, L. hardjo, L. hebdomadis, L. icterohaemorrhagiae, L. pomona, L. pyrogenes, L. wolffi and L. tarassovi. At the serologic evaluation, 42 (36.2%) samples were positive for at least one serovar, and only 3 (2.58%) samples presented titers higher than 1:100. Only one urine sample was positive at the PCR test, and the dog also had a high serologic title (1:1600) for L. copenhageni. This dog was apparently healthy. The frequency of positive serology and the variety of serovars found in dogs without vaccination demonstrate the importance of leptospirosis in this community.