Vaccination against Lyme disease is crucial.

  • Up to 73%92 of dogs tested positive for Lyme disease when left unvaccinated
  • Prevalence has increased by approximately 250% in north central states93 and more than 320% in northeastern states93

Disease Overview

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

Most Lyme disease in the United States is spread by deer ticks and 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.


  • 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
  • The number of counties in the US considered to be high risk for Lyme disease has increased by 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 by Ixodes spp. ticks, commonly called deer ticks. The ticks carry the bacterial cause of Lyme disease, Borrelia burgdorferi. After attaching to a dog, a tick takes a blood meal which stimulates the B. burgdorferi in the midgut of the tick to change their outer surface protein from Osp A (outer surface protein A) to Osp C. This outer surface protein change enables the bacterium to move into the tick’s salivary glands where it can be injected into the dog’s bloodstream. Once in the bloodstream the organism is carried to multiple locations including the 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, the warming weather and migration of the white-tail deer have caused the spread of the tick populations. Because of this, Lyme disease is now prevalent in the majority of the U.S., increasing the number of dogs at risk.
  • With the increase in dog travel to Lyme endemic areas, more dogs are at risk. 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 the tick attaches to a host when
    B. burgdoeri is passed through the salivary glands of an infected tick into the animal.84
  • 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.84


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 OspC.1,5,80 Shown to be effective against Borrellia 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