Myocarditis of probable viral origin in pups of weaning age.

Myocarditis in 4- to 8-week-old pups from 10 litters was characterized by sudden death. Histopathologic findings included mononuclear cellular infiltration and interstitial fibrosis in the myocardium of the left ventricle. Basophilic intranuclear inclusion bodies were seen in myocardial cells in 4 of 18 pups necropsied, suggesting a viral origin of the disease. Other pathologic changes were variable, but all were attributable to cardiac failure. Of 8 surviving pups examined, 7 had evidence of cardiac failure, including pulmonary edema, cardiomegaly, and cardiac arrhythmias.