Reduced ability to exercise—exercise intolerance—weakness, fatigue and lethargy are occasional complaints for animals with heart disease. Often, reduced exercise ability is attributed to old age or arthritis, when the heart is really an important cause. Since many animals do not regularly engage in strenuous activity, noticing a decreased ability to exercise can be difficult. Dogs with heart disease are often intolerant of hot and humid days, so they may get short of breath early, but this can be mistaken for normal panting. Exercise intolerance is often only present in dogs with more advanced stages of heart disease, as mild heart disease rarely causes major physical limitations to routine activity. However, if your pet stops participating in familiar or common activities, it may be a sign of heart disease.