Canine Lyme Disease

There’s an increase in ticks, and tick-related diseases, with the number of reported tickborne diseases more than doubling in the last 13 years.81

Disease Overview

Lyme disease is a bacterial infection transmitted by ticks to dogs, as well as to other animals and people.

Most Lyme disease in the United States is spread by the deer tick, which is found primarily in the Northeast, Southeast, and upper Midwest. The western black-legged tick is the primary cause of Lyme disease transmission in the western states (map pictured below).


  • Ticks that can transmit Lyme disease have expanded their geographic range and are now being found in places they weren’t seen 20 years ago82
  • Counties with high risk for Lyme increased 320% since the 1990s82
  • CAPC data shows Nationwide, dogs are being exposed to Lyme disease outside of traditionally endemic areas54
  • Approximately 75% of unvaccinated dogs in endemic areas will eventually test positive for Lyme disease55
  • The best way to protect dogs is year-round flea and tick control and vaccinating those dogs that visit or live in endemic areas56

U.S. Canine Lyme Disease Prevalence83


Lyme disease is transmitted to through the bite of a tick that is carrying the bacterial spirochete, Borrelia burgdorferi. When the tick bites the dog and takes a blood meal, the blood causes the Borrelia to change its outer surface protein (Osp) from OpsA to OspC which allows it to move from the tick’s midgut into the dog’s blood stream. Once in the bloodstream, the Lyme disease organism is carried to many parts of the body and is likely to localize in joints.


  • Arthritis
  • Lameness
  • Fever
  • Anorexia
  • Fatigue


  • Lyme disease was traditionally thought to be limited primarily to the Northeastern region of the U.S. where deer populations were most dense. However, with the warming weather and the spread of the tick populations, Lyme disease is now prevalent in the majority of the U.S., increasing dogs at risk.
  • Geography is no longer a rule out for Lyme risk with many people traveling with their dogs. Positive cases of canine Lyme disease have been reported across the U.S., with 39 states now reporting high and moderately high prevalence of canine Lyme disease.83
  • Dogs at increased risk for Lyme disease include:
    • Active dogs that walk or hike in the woods
    • Dogs that play in fields
    • Dogs that spend time in the yard


  • Most commonly, dogs may present with fever, shifting leg lameness, swollen joints, enlarged lymph nodes, lethargy, depression and anorexia.
  • Less commonly, dogs will develop a protein losing nephropathy that can result in hypoalbuminemia, edema and eventually renal failure, which is known as Lyme nephritis.


  • Infection occurs 24-48 hours after attaching to a host. B. burgdorferi is passed through the salivary glands of an infected tick into the animal.
  • Once the animal is infected with spirochetes from the tick, the spirochetes spread through connective tissue and may result in infections in the joints, heart, and neural tissue.78


Diagnosis is based on history, clinical signs, elimination of other diagnoses, laboratory data, epidemiologic considerations, and response to antibiotic therapy. Mixed infections should be considered when clinical signs are apparent.

Long incubation periods, persistence of antibodies for months to years, and the disassociation of the antibody response from the clinical stage of disease make diagnosis by blood testing alone impossible.

Merck Animal Health Vaccines


Two inactivated isolates of Borrelia burgdorferi initiate the production of borreliacidal antibodies against OspA and OspC1, 5, 80. Shown to be effective against Borrelia burgdorferi and subclinical arthritis caused by Borrelia burgdorferi.

Professional Resources and Educational Materials

Keep your clinic and staff informed and aware of diseases and outbreaks.

Lyme Disease Pet Parent Guide


Share this printable brochure to educate pet parents about their dog’s risk for Canine Lyme Disease.


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For technical assistance or to report a suspected adverse drug reaction, contact Merck Animal Health at: 1-800-224-5318