Today I did some research on this. The most helpful site I found was this one. Apparently blood in the stool is called hematochezia. It seems that in older cats, it can be a sign of cancer or tumors. However, in younger cats it is a less serious problem, like parasites.
I called the vet and made an appointment for Saturday. Since I'm not sure which cat passed the stool, I will have to bring them both in, though I suspect that Lewis is the troublemaker. I am not worried that it's something serious, but rather hope we can figure it out and get him all better!
Below are some of the causes of hematochezia (copied from the site I linked to above):
- Viral infections (panleukopenia), bacterial infections (Salmonella), protozoal agents (coccidiosis), or intestinal parasites (hookworms and roundworms)
- Dietary intolerance from eating spoiled food, overeating, ingesting foreign material (especially bones), a sudden change in diet, or eating people food
- Dietary allergy to certain food substances, such as to particular proteins, lactose, high fat content, etc.
- Masses of the colon, rectum or anus such as benign (polyps) and malignant (cancer) tumors
- Inflammation of the colon (colitis)
- Trauma of any sort (bite wounds to the anal area, fractures of the pelvis that disrupt the colon or rectum, the passage of sharp ingested objects, or the insertion of instruments or materials into the rectum)
- Bleeding disorders (coagulopathies) of the body may result in bleeding from the lining of the lower bowel. Examples include ingestion of rat poison that contains anticoagulants, inherited clotting disorders (rare in the cat), decreased numbers of platelets (uncommon in the cat), disseminated intravascular coagulation (DIC) from massive infections or organ failure, and severe liver disorders.
- Intussusception (the telescoping of one part of the bowel into another) secondary to foreign bodies, tumors, or parasites
- Stricture (narrowing) of the anus or colon, secondary to previous trauma, inflammation, cancer or a foreign body may result in bleeding, especially as stools are passed
- Chronic or intermittent constipation and attempted passages of dry, hard stools
- Anal sacculitis (inflammation of the anal sacs) or anal sac abscessation can change the consistency of the fluid in the anal sacs to a bloody liquid that may coat the stools as they are defecated (this is uncommon in the cat)
- Proctitis is inflammation of the rectum and is often associated with colitis