Editor’s Note: This excerpt is from Understanding Equine First Aid by Michael Ball, DVM. 

Blood coming from the rectum can represent an extremely serious veterinary medical emergency. If a relatively healthy horse is observed to be bleeding from the rectum, the most likely cause is some type of trauma. I have seen this result from malicious insertion of an object into the horses rectum and from a stallion raping another horse.

These situations need to be evaluated as soon as possible due to the potential for tearing of the rectum deep enough to allow fecal contamination of the abdominal cavity. Should this occur, the prognosis is grave. If there is a tear and the abdomen has not yet been contaminated, immediate attention is required to evaluate the injury and perform reparative measures.

If the bleeding is a result of thermometer insertion or breaking inside the rectum, it is not usually deep enough in the rectum to cause abdominal contamination, but still should be carefully monitored or evaluated due to the potential for perirectal (around the rectum) abscess formation. A thermometer should be well-lubricated and never forced into the rectum