The following criteria have been proposed for establishing a diagnosis of Lyme disease in dogs: a history of tick exposure in an endemic area, clinical signs, positive serology, and prompt response to antibiotics. With the exception of cases where glomerular disease has developed, results of complete blood counts, serum chemistries and urinalysis on these patients are generally normal. The presence of hematologic abnormalities, such as an elevated white blood count or thrombocytopenia should prompt a search for concurrent disease. Dual infections with rickettsial organisms may occur in some endemic areas. Joint taps reveal elevations in cell counts of up to 50,000 cells/mm³, with neutrophils as the predominant cell type. Differential diagnoses include immune-mediated arthropathies, degenerative joint disease, and other infectious causes of arthritis, such as Rocky Mountain spotted fever and ehrlichiosis.

Serology remains the most practical tool in veterinary medicine for the diagnosis of Lyme disease. Antech offers an ELISA test for IgG, and an IFA test for IgM. Clinical signs lag behind infection for 2–5 months, so that clear serologic evidence of infection is almost always present. IgM titers may remain elevated for prolonged periods after infection, and are not definitive of recent infection. IgG antibodies are detectable 4 weeks after infection and levels peak at 3 months. Antibodies in untreated dogs may persist for 12–18 months. Titers in treated animals may drop translently, but because the organism is not completely cleared from the dog by treatment, titers may persist. Treatment will reduce the number of spirochetes and alleviate clinical signs, but small numbers of the organism remain and elicit a persistent antibody response. Dogs who have been vaccinated for Lyme disease can have positive IgM and IgG titers as early as 7–10 days after vaccination (earlier than in natural infection).

The Western Blot test separates antibodies for Borrelia spp. based on the antigen they target. Flagellar antibodies from natural exposure can be distinguished from the other surface protein antibodies induced by vaccination. With this technique, when sufficient antibody is present, a band forms on the nitrocellulose test strip. The intensity of this banding will vary with the amount of antibody, although the test is only subjectively quantitative. This test is indicated when evaluating vaccinated dogs to determine whether natural exposure has occurred. Results may indicate the presence of vaccine-induced antibodies, antibodies from natural exposure, or a combination of both.

Isolation of the organism is difficult and expensive, and so it is rarely performed. The paucity of organisms in blood and body fluids render both culture and PCR techniques unrewarding. Use of collagen-rich tissue samples, such as skin, fascia, peritoneum and synovium may be more productive, but the poor sensitivity and high cost involved remain as deterrents.

Tetracycline and ß-lactam antibiotics are both effective in the treatment of Lyme disease. Due to the slow growth of the organism, 21–28 day courses of therapy are recommended. Doxycycline at 10 mg/kg SID, or amoxicillin at 22 mg/kg BID are both appropriate choices. The clinical response is usually prompt, with improvement evident in 1–2 days. Dogs that experience relapses usually respond equally well to therapy. In the event of a poor clinical response, the diagnosis should be reevaluated.

A common clinical dilemma is posed by the question of using antibiotics in asymptomatic seropositive dogs. Only a small percentage of seropositive dogs ever develop clinical signs, and many dogs remain seropositive despite successful treatment of their symptoms. In the absence of symptoms, therefore, it can be argued that the use of antibiotics is unjustified, except in those dogs receiving ongoing immunosuppressive therapy. Also, as dogs in endemic areas are at risk for repeated exposure, continual re-treatment is impractical. It is not known whether treatment of such dogs would prevent or lessen the rare, more serious complications of Lyme disease in dogs (e.g., chronic arthritis or glomerulonephritis).

The use of tick repellents and the prompt removal of ticks are important practices to decrease exposure to B. burgdorferi in endemic areas. Two commercial vaccines are currently available to veterinarians; a bacterin, and a single protein recombinant vaccine containing outer surface protein A. While a decreased incidence of clinical illness has been demonstrated in vaccinated populations of dogs, the low rate of disease despite the high rate of infection in endemic areas warrants careful evaluation of the risk:benefit ratio of vaccination for each patient. It is unclear at this time whether vaccinating seropositive dogs is beneficial, and some experts believe it presents the increased risk of adverse reactions (immune-complex arthritis).