Ixodes ticks, vectors of B. burgdorferi, carry the spirochaetes to a number of vertebrates; however in natural conditions only the species from outside the forest biotope display the clinical form of borreliosis, mainly humans and dogs. In dogs, B. burgdorferi s.l. has been implicated as a cause of various disorders often resembling those observed in human borreliosis, including polyarthropathy, anorexia, malaise, and neurological dysfunction. In areas infested with I. ricinus, and I. dammini in the USA and known to be endemic for Lyme disease, veterinarians may suspect borreliosis in dogs with limb/joint disorders. In serological diagnosis, it should be considered that the occurrence of even specific antibodies does not necessarily mean the active disease or primary exposition to the pathogen. Studies on antibody titers in the sera of dogs either naturally or experimentally infected, and on their associations with clinical symptoms, indicate the same limitations of serological tests as in the diagnostic of human Lyme disease; therefore, PCR is the most appropriate method of diagnosis.