Bacterial Endocarditis

Bacterial endocarditis, also called infectious endocarditis (IE), is inflammation of the endocardium, caused by bacteria that entered the bloodstream. The strains of bacteria vary with predisposing factors, such as congenital heart defects or heart valve diseases. It is a disease of primarily middle-aged to older, large-breed dogs that is associated with high mortality. Bacterial endocarditis presents many challenges with respect to diagnosis and effective treatment. Infection of endocardium typically involves one of the heart valves, although endocarditis of the cavity's wall may also develop. In dogs, horses, and cats, the aortic and mitral valves are most commonly affected.

The tricuspid valve is rarely affected, and pulmonic valve infective endocarditis is exceedingly rare.3 Organisms causing infectious endocarditis in dogs and cats most often have been Staphylococcus species, Streptococcus species, and Escherichia coli. Other organisms isolated from infected valves have been Corynebacterium species, Pasteurella species, Pseudomonas aeruginosa, and Bartonella species 5. Streptococci are the most common cause of IE and are more likely to infect the mitral valve and be associated with polyarthritis. Dogs with IE caused by Bartonella spp infection rarely have fever, are more likely to develop congestive heart failure, and have shorter survival times. Vascular encephalopathy is a commonly encountered clinical complication of infectious endocarditis in humans, but it has been infrequently reported in dogs.

Petri dish

There are several factors contributing to the development of infective endocarditis. One factor is an injury to the endothelium leading to the formation of a coagulated mass of platelets, fibrinogen, fibronectin, and fibrin. The fibronectin receptor on platelets and extracellular matrix proteins avidly bind bacteria that contain microbial surface components recognizing adhesive matrix molecules (MSCRAMMS). The microorganism becomes embedded and incorporated into the lesion and multiplies. The lesion may extend to adjacent structures and cause rupture of chordae tendinae (heart strings, cord-like tendons that connect the papillary muscles to the tricuspid valve). The end result is severe mitral regurgitation and congestive heart failure4. Other factors are disturbed blood flow, immune responses, and bacterial virulence. Highly virulent organisms or a heavy bacterial load can infect normal valves while bacterial clumping promoted by agglutinating antibodies may facilitate attachment of the bacteria to the valves 5.

Bacteria released from the infected valves enter the circulation and can infect other organs. Therefore, infective endocarditis can produce a wide variety of signs: fever that comes and goes, lameness, blood and pus in the urine, difficulty breathing, lethargy, and depression. Various blood tests may be needed for diagnosis. X-rays may show enlargement of the heart chamber. Treatment is directed at controlling signs of congestive heart failure. The outlook is poor in most dogs.


  1. Evaluation of the relationship between causative organisms and clinical characteristics of infective endocarditis in dogs: 71 cases (1992-2005). Sykes JE, Kittleson MD, Pesavento PA, Byrne BA, MacDonald KA, Chomel BB. Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California-Davis, USA.
  2. Endocarditis due to Staphylococcus aureus after minor dog bite.Bradshaw SE. Cambridge University School of Medicine, Cambridge, England, United Kingdom.
  3. Merck Vet Manual. Infective Endocarditis
  4. Infective Endocarditis in Dogs: Diagnosis and Therapy Kristin MacDonald, DVM, PhD. Veterinary Clinics of North America: Small Animal Practice Volume 40, Issue 4, July 2010, Pages 665-684
  5. Wendy A. Ware. Cardiovascular Disease in Small Animal Medicine

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