Answer

Hello,

This is Dr.Christian. Welcome to JustAnswer. What you are describing is consistent with colitis which is an inflamed colon or large intestine. Colitis can cause blood in the stool. It is usually caused by stress, diet changes or sometimes the cause is not determined. The wall of the colon becomes inflamed and seeps mucous and blood. Many times these dogs will try to have a bowel movement and only produce the blood. They feel as if they have to go due to the irritation but in reality there is nothing is no stool in the colon. Here's a link:
http://www.petplace.com/dogs/colitis-in-dogs/page1.aspx

Colitis can sometimes progress to hemorrhagic gastroenteritis (HGE) which is a life threatening condition that puts the dog in to shock. Treatment involves fluids, medications and sometimes vomiting medications. Here's a link:
http://www.petplace.com/dogs/hemorrhagic-gastroenteritis-hge/page1.aspx

If he starts to act lethargic, stops eating or seems sicker rather than improving then he should be checked for HGE immediately. None of these conditions are contagious to your other dogs. If your vet did not check a fecal sample for intestinal parasites then that would be a good idea. Intestinal parasites could be contagious to your other dogs. Examples of intestinal parasites are giardia, coccidia and hookworms. These parasites cause irritation to the lining of the colon and in the case of the hookworm actually latch on to the inside of the intestine and suck blood. Giardia in particular can be contagious to humans. Here's a link:

http://www.petplace.com/dogs/gastrointestinal-parasites-in-dogs/page1.aspx
If you have more questions please let me know.