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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
Usually results from a septicaemia or viraemia E.g Parvovirus infection in puppies. Also includes:
- Foot and Mouth Disease.
- Leptospirosis.
- Pasteurellosis.
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.
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.
Chronic
Myofibrils replaced by fibrous tissue during healing. May be a sequale of previous acute myocarditis.
Parasites
Parasites also may cause myocarditis. parasitic larvae may encyst in the myocardium, including:
- Cysticercosis.
- Sarcocystis.
- Toxoplasmosis.