Merck Manual

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Professional Version

Myocarditis in Dogs and Cats


Mark D. Kittleson

, DVM, PhD, DACVIM-Cardiology, Department of Medicine and Epidemiology, University of California, Davis

Reviewed/Revised Jan 2023 | Modified Jun 2023

Myocarditis is a focal or diffuse inflammation of the myocardium with myocyte degeneration and/or necrosis. It is rare in companion animals, although there are numerous causes, including several viruses and bacteria. Canine parvovirus Canine Parvovirus Canine parvovirus is a highly contagious virus that commonly causes GI disease in young, unvaccinated dogs. Presenting signs include anorexia, lethargy, vomiting, and diarrhea, which is often... read more , encephalomyocarditis virus Encephalomyocarditis Virus Infection , and equine infectious anemia Equine Infectious Anemia virus can cause myocarditis.

Treatment should be aimed at improving cardiac contractility, relieving congestion, and decreasing vasoconstriction. Pimobendan and digoxin are used most commonly to improve contractility. Furosemide is indicated to control clinical signs of pulmonary edema. Corticosteroids are often used when cardiac isoenzymes are increased and a viral infection is deemed unlikely.

Miscellaneous Toxic and Degenerative Myocardial Diseases

Mineral deficiencies (eg, deficiencies in iron, selenium, or copper) can also result in myocardial degeneration (not myocarditis). Deficiencies of vitamin E or selenium may cause myocardial necrosis. Cardiac toxins include ionophore antimicrobials such as monensin and salinomycin, cantharidin (blister beetle toxicosis Cantharidin Toxicosis ), Cryptostegia grandiflora (rubber vine), and Eupatorium rugosum (white snakeroot). The diseases caused by these agents have clinical signs typical of CHF.

A heart murmur of mitral or tricuspid regurgitation, as well as an irregular rhythm, is usually audible. Atrial fibrillation is common, and ventricular or atrial premature complexes may also be evident. Echocardiography reveals chamber dilation and poor contraction with essentially normal valves. Neutrophilic leukocytosis and hyperfibrinogenemia are common. The activities of cardiac isoenzymes (CK, troponin, and lactate dehydrogenase) are often increased.

Chagas Myocarditis in Animals

Trypanosoma cruzi, a flagellate protozoon transmitted by kissing bugs, causes Chagas disease Chagas Disease Tsetse-transmitted trypanosomiasis refers to a group of diseases caused by protozoa of the genus Trypanosoma and affects all domesticated animals. The major veterinary species are T... read more Chagas Disease . Acutely, ECG abnormalities (eg, AV block, right bundle-branch block, sinus tachycardia, depressed R wave amplitude) may be noted, but animals are rarely presented during the acute stage. Usually there are no echocardiographic abnormalities during the acute phase; however, sudden death is a concern. Though treatment protocols are lacking, anti-trypanosomal and anti-arrhythmic medication would be indicated at this stage. An asymptomatic latent phase then develops for 27–120 days in dogs, followed by a chronic stage demonstrating systolic dysfunction indistinguishable from dilated cardiomyopathy Dilated Cardiomyopathy in Dogs and Cats Dilated cardiomyopathy (DCM) is characterized by the progressive loss of myocyte number and/or function, along with a decrease in cardiac contractility. DCM is most prevalent in dogs and is... read more . Treatment for the chronic phase is the same as for DCM; however, it is typically ineffective at controlling clinical signs of progressive myocardial failure. Chagas myocarditis is most commonly identified in southern states (eg, Texas) and Latin America.

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