This disease occurs very rarely in the lower animals, and most probably when it does happen it is connected with other diseases which have been described, i.e. pericarditis and endocarditis, in both of which the inflammation may extend to the muscular structure.
The alterations which are occasioned in the muscular structure will depend upon the activity of the inflammatory condition. Among them may be mentioned softening, and different degrees of degeneration, which weaken and impair the functions of the organ.
In its chronic form the disease tends to the development of hypertrophy or enlargement, hardening of the muscular structure, and different forms of fatty and fibroid degeneration, and in some instances small abscesses are formed in the muscular walls.
Symptoms of myocarditis are not of a sufficiently definite character to lead to a correct diagnosis. In the majority of cases the pulse is weak, sometimes hardly detectable, generally increased in frequency, and the respiration is rapid and carried on with difficulty. Sometimes, when caused to turn, the animal grunts, and deep pressure over the region of the heart causes pain. There is also weakness, incapacity for work, a fastidious appetite, and occasional attacks of vertigo, especially in the advanced forms of the disease. There is an absence of the morbid sounds which are observed in cases of pericarditis and endocarditis, and in valvular disease.