Why does my dog have diarrhea with blood?

Diarrhea is characterized by frequent loose or liquid bowel movements. It can be caused by something as simple as a change in diet or a more serious illness or infection. Diarrhea may be sudden in onset and short in duration. It can also last for weeks to months or occur off and on. A single bout of diarrhea is generally not a cause for concern in dogs -- but if it persists for more than a day, it can lead to dehydration, or it may indicate an underlying health issue and should be checked out by a veterinarian