Two cats were presented for bradyarrhythmia. The cats were evaluated further with hematology, serum biochemistry with electrolytes and thyroxine concentrations, systolic blood pressure measurement, echocardiography, electrocardiography and infectious disease testing. 

Infectious disease testing included serology for Toxoplasma gondii, Ehrlichia canis, Anaplasma phagocytophilum and Borrelia burgdorferi, and PCR for B burgdorferi antigen in both cats. One cat was also assessed by PCR for Bartonella henselae antigen and the other was assessed for Dirofilaria immitis by serology. 

All infectious disease tests, other than for B burgdorferi, were negative. One of the cats was diagnosed with Lyme carditis based on marked bradydysrhythmia, positive B burgdorferi serology, a structurally normal heart and clinical resolution with appropriate treatment with a 4-year follow-up. 

The second cat was diagnosed with Lyme carditis based on marked bradydysrhythmia and positive B burgdorferi PCR; however, this cat had structural heart disease that did not resolve with treatment

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First report of Lyme borreliosis leading to cardiac bradydysrhythmia in two cats.