Lyme disease (borreliosis) is an infectious, tick-borne disease first recognized in dogs in 1985. It is caused by Borrelia burgdorferi, a type of bacteria called a 'spirochete.' The common deer tick Ixodes scapularis (formerly called Ixodes dammini) is the primary carrier of B. burgdorferi in the Northeast and upper Midwest. The tick Ixodes pacificus is the primary carrier in the western United States.

Although Lyme disease was first diagnosed in Connecticut in 1975, there is evidence that it has existed in wildlife for many years. Tissue samples taken from a white-footed mouse in Massachusetts in 1894 have now undergone DNA testing that show the mouse was infected with Lyme disease. Certain environmental factors have caused Lyme disease to be more prevalent in people in the twentieth century. Before 1900, the habitat in the East and upper Midwest was heavily deforested by early settlers. Deer and their associated ticks were greatly reduced. The reforestation of these areas and the resulting booming white-tailed deer population in these areas are probably a big factor in the increase in the incidence of Lyme disease. This coupled with increased awareness and testing capabilities has led to the greatly increased reporting of the disease.

Blood tests are available to assist in the diagnosis of Lyme disease. The standard blood test detects antibodies made by the dog in response to infection with B. burgdorferi. Many dogs show positive test results, but are not actually infected with the disease. These animals have been exposed to the organism, but fought off the infection on their own. These animals will have antibodies to B. burgdorferi but not have the disease. Thus a single positive result means only that the dog was exposed. As mentioned earlier, only around 10% of the exposed dogs actually contract the infection.