Endocarditis

Usually due to bacterial infection. More common in cattle, pigs and sheep. Often due to a chronic bacteraemia or pyaemia, spread from adjacent myocardium is rare.

Bacterial endocarditis. Courtesy of A. Jefferies

Organisms commonly isolated include:

• Cattle: Actinomyces pyogenes
• Pigs: Erysipelothrix spp.
• Sheep: Streptococci.

Pathogenesis:

Contributing factors include:

• Trauma: haemodynamic turbulence.
• Valve tissue ageing.
• Avascularity of valves; poor healing capacity.
• Tissue exposure; constant contact with blood-borne pathogens.

Vegetative endocarditis

Predisposed by valvular damage as thrombi occur on the surface of the valves exposed to blood flow. Needs sustained or recurrent bacteraemia. Some bacteria have particular adherence properties and may only need a single episode of bacteraemia to set up an endocarditis.

Vegetative endocarditis (dog). Courtesy of T. Scase

Pathophysiology:

Loose thrombi of platelets and fibrin form which build up to larger vegetations. Bacterial colonies are found within the mass. Attempts to organise the vegetation occur but complete healing very rarely occurs. The mass may become ulcerated. Progression to rupure of the chordae tendinae is possible, along with spread of the infection to the adjacent mural endocardium. Valves may become stenotic, incompetant or both.

Death usually results from either embolisation of the vegetation or congestive heart failure due to significant valvular damage.

Species differences:

• Cattle: predominantly affects the tricuspid valve, perhaps due to bacteria arising in the GI tract and liver. Congestive right sided failure is manifested as ascites (including bottle jaw) and embolisation to the lungs. Anaemia is often present as the red blood cells are damaged as they pass through the vegetation.
• Pig and dog: Lesions occur particularly on the mitral valve, perhaps due to the higher pressure blood flow on the left side of the heart leading to more valvular damage. Left sided heart failure and pulmonary oedema are seen clinically, as are emboli in various organs, particularly the kidney.

Ulcerative endocarditis

Commonly seen along with renal failure in dogs. Uraemia irritates and damages the endocarium, particularly in the left atrium. Oedema is seen in the sunendocardial tissue with deposition of glycosaminoglycans.

Healing may occur by fibrosis or the lesion may progress to a necrotising endcarditis and, in extreme cases, left atrial rupture. If renal sufficieny is re-established then healing of the endocardial lesion is possible.