Borreliosis is the most frequently diagnosed tick-borne disease caused by spirochete bacteria belonging to the genus Borreliae - Borrelia burgdorferi sensu stricto (s.s.), Borrelia afzelii and Borrelia garinii. Clinical manifestations in dogs include fever, lameness, polyarthritis and glomerulonephritis. Diagnosis is mainly serological and is based on an immunoenzymatic test followed by a Western blot confirmatory test. Early treatment with antibiotics such as doxycycline or amoxicillin, for four weeks, usually reduces the risk of chronic disease. Tick control, including tick repellents, is highly reliable in preventing transmission. Vaccines are available to reduce transmission and the clinical manifestations of infection in dogs. Bernese Mountain Dogs are a breed that often test positive for antibodies against B. burgdorferi without showing any clinical symptoms of the disease. Quantitative determination of the immunoglobulin level for spirochetes has indicated that Bernese Mountain Dogs may have an increased susceptibility to Borrelia spp. infections of a hereditary nature.