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Most often due to an infective agent. In most cases myocarditis is a result of a generalised infection, myocarditis is rarely an isolated primary condition. Classified as:

Acute Suppurative

Follows from the dissemination of septic emboli form suppurative foci therefore always follows pyaemia. Original septic foci may be:

  • Joint ill.
  • Umbilical abcess.
  • Metritis.
  • Mastitis.
  • Valvular endocarditis.

May also be a result of extension from the endocarium or pericardium.

Acute non-suppurative

Parvovirus (dog). Courtesy of T. Scase

Usually results from a septicaemia or viraemia E.g Parvovirus infection in puppies. Also includes:

  • Foot and Mouth Disease.
  • Leptospirosis.
  • Pasteurellosis.

And can be classified according to the type of exudate:

  • Necrotizing
  • Lymphocytic
  • Hemorrhagic
  • Esinophilic

Parvovirus Infection of puppies:

Parvovirus affects rapidly dividing cells which are found in the GI tract, and in the growing pup, the myocardium. Myocardial cells divide up to approximately 2 weeks of age and so this is when the infection has an effect although clinical signs may not become apparent until later in life.

Parvoviral Myocarditis. Courtesy of A. Jefferies

Two cardiac syndromes exist:

  • Sudden death, 3-8 weeks: Cardiac failure with pulmonary oedema etc. Histologically see multifocal myocardial necrosis due to viral replication within the myocardium. May see large basophilic intranuclear inclusion bodies.
  • Puppies >8 weeks: See dyspoea, weakness, collapse and death within 24 hours. In this case cardiac failure occurs due to myocardial fibrosis.


Myofibrils replaced by fibrous tissue during healing. May be a sequale of previous acute myocarditis.


Parasites also may cause myocarditis. parasitic larvae may encyst in the myocardium, including:

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Myocardial Pathology Flashcards