Simple SummaryEndocarditis in dogs is a rare but severe disease with a bad prognosis. Early diagnosis and improved treatment plans could be beneficial to the outcome of the treatment. However, diagnosis is difficult because of the nondiscriminatory symptoms of the disease. In this report, we describe a case of endocarditis in an 8-year-old male dog caused byStreptococcus canis, a bacterium that is part of the normal microbiota of a dog’s skin and mucous membranes. The treatment that he received was unfortunately unsuccessful. We decided to investigate the infected heart valve and the bacterium to understand his disease better. We discovered a biofilm on the heart valve, which explains the difficulty of treating the infection, because bacteria in biofilms are more resistant to antibiotics. We propose more research towards better treatment of bacteria in biofilms, especially in the case of endocarditis, which is such a lethal disease without a prompt and effective treatment.AbstractAn 8-year-old male Rhodesian Ridgeback was presented with fever and severe thrombocytopenia. Clinical and laboratory examination, echocardiography, blood culture, and pathohistology revealed evidence of infective endocarditis, ischemic renal infarcts, and septic encephalitis. Treatment was started immediately but the dog’s condition worsened, and the dog had to be euthanized. The causativeStreptococcus canisstrain was detected by blood culture and MALDI-TOF MS and analyzed using whole-genome sequencing and multilocus sequence typing. Antibiotic susceptibility testing did not detect any resistance. The affected heart valve was analyzed using FISH imaging, which showed a streptococcal biofilm on the heart valve. Bacteria in biofilms are recalcitrant to antibiotic treatment. Early diagnosis could be beneficial to treatment outcome. Treatment of endocarditis could be improved by researching the optimal dosage of antibiotics in conjunction with the use of biofilm-active drugs.