Old Dog Diarrhea and Blood: Causes and What To Do

1. Gastrointestinal Infections

Bacterial infections (like Salmonella or Clostridium), viral illnesses (like parvovirus, although rare in older vaccinated dogs), and parasitic infestations (like whipworms or giardia) can lead to bloody diarrhea in old dogs.

These infections irritate the lining of the gut, leading to inflammation, tissue damage, and sometimes bleeding.

Old dogs are more susceptible to these infections due to a weaker immune response.

Signs to watch:

• Frequent, foul-smelling diarrhea

• Blood and mucus in the stool

• Vomiting

• Fever

• Lethargy

2. Colitis or Inflammatory Bowel Disease (IBD)

Colitis, or inflammation of the colon, is a common reason for old dog diarrhea and blood.

It can be acute or chronic and is often caused by infections, stress, food sensitivities, or autoimmune diseases. Inflammatory Bowel Disease (IBD) is a chronic condition that may be more common in aging dogs.

Signs include:

• Straining to defecate

• Small amounts of bloody diarrhea

• Increased urgency to go

• Weight loss over time

3. Dietary Indiscretion or Sudden Food Change

Older dogs may be more sensitive to abrupt dietary changes or consuming inappropriate items (garbage, spoiled food, non-food substances).

This behavior can trigger bloody diarrhea in senior dogs by irritating the digestive lining or introducing harmful bacteria.

Other signs:

• Vomiting

• Bloating

• Lethargy

• Gas or abdominal discomfort

Even treats, table scraps, or fatty foods can upset an aging dog’s digestive system.

Related: Old dog diarrhea and not eating (Here’s why)

4. Cancer or Tumors in the GI Tract

One of the more serious causes of old dog diarrhea and blood is cancer.

Tumors in the stomach, intestines, or rectum may bleed, resulting in visible blood in the stool.

Unfortunately, cancer is more common in senior dogs and can sometimes go unnoticed until symptoms like bloody diarrhea appear.

Symptoms can include:

• Unexplained weight loss

• Loss of appetite

• Vomiting

• Lethargy

• Visible abdominal mass

5. Hemorrhagic Gastroenteritis (HGE)

HGE is a condition characterized by sudden onset bloody diarrhea in dogs, often with vomiting.

Though the exact cause is unknown, it may involve bacterial toxins or hypersensitivity reactions.

While it can affect dogs of any age, older dogs are at higher risk for severe complications.

HGE signs:

• Profuse, watery, and bloody diarrhea

• Vomiting

• Dehydration

• Sudden lethargy

Prompt veterinary intervention is crucial.

Related: Old dog diarrhea and losing weight (Causes, symptoms and what to do)

6. Liver or Kidney Disease

In older dogs, organs like the liver and kidneys often begin to fail, which can lead to a cascade of digestive symptoms, including diarrhea with blood.

Toxin build-up in the body and changes in blood clotting function may contribute to gastrointestinal bleeding.

Look for:

• Increased thirst and urination

• Vomiting

• Pale gums

• Jaundice

• Weakness

Routine bloodwork in older dogs can help detect these issues early.

7. Side Effects of Medication

Many senior dogs on long-term medication—such as NSAIDs for arthritis or steroids—can develop gastrointestinal side effects.

These medications may erode the stomach lining or disrupt the gut flora, leading to ulceration, inflammation, and bloody diarrhea.

Watch out for:

• Black, tarry stools (melena)

• Vomiting blood

• Loss of appetite

• Restlessness or signs of pain

Always follow your vet’s dosage instructions and monitor for adverse effects.

8. Anal Sac Disorders or Rectal Injuries

Sometimes, senior dog diarrhea and blood, isn’t from the gastrointestinal tract but due to injuries or inflammation near the rectum.

Impacted anal glands, rectal tumors, or even trauma from hard stools can lead to blood in the stool.

You might notice:

• Scooting or licking the rear

• Pain when defecating

• Blood only on the surface of the stool

What to Do If Your Old Dog Has Diarrhea With Blood

When you notice old dog diarrhea and blood, prompt action is essential to prevent dehydration or worsening of an underlying disease.

Here’s what you should do:

Immediate Steps to Take at Home

• Withhold food for 12–24 hours, giving the gut time to rest (only if your vet approves this based on your dog’s age and health).

• Offer small amounts of water frequently to avoid dehydration.

• After the fasting period, introduce a bland diet such as boiled chicken and rice in small portions.

• Monitor your dog’s behavior, hydration status, and whether the diarrhea improves or worsens.

When to Seek Veterinary Help Immediately

You should contact your vet right away if your senior dog:

• Has bloody diarrhea lasting more than 24 hours

• Shows dark, tarry stools (internal bleeding)

• Is vomiting repeatedly

• Appears weak, lethargic, or in pain

• Has pale gums or other signs of anemia

• Is not eating or drinking

• Has a pre-existing condition like kidney, liver, or heart disease

• You suspect poisoning or medication reaction

Your vet may conduct blood tests, stool analysis, ultrasound, X-rays, or endoscopy to determine the exact cause.

How Vets Treat Old Dog Diarrhea and Blood

Treatment depends on the cause but may include:

• Fluids and electrolytes to correct dehydration

• Antibiotics or antiparasitic meds if an infection is present

• Dietary changes or hypoallergenic diets for IBD

• Discontinuation or adjustment of medications causing side effects

• Surgery or chemotherapy if cancer is found

Managing diarrhea and blood in senior dogs also involves ongoing monitoring, especially if your dog has chronic health conditions.

Key Takeaway

Old dog diarrhea and blood is a potentially serious condition that should never be overlooked.

While some causes are minor and treatable at home, others require urgent veterinary care—especially in aging dogs with weaker immune systems or chronic conditions.

Prompt diagnosis and treatment not only improve outcomes but also reduce the risk of dehydration, anemia, or further complications.

If your senior dog experiences diarrhea with blood, act quickly—your attentiveness could save their life.

By understanding the possible causes and knowing when to seek help, you can provide your older dog with the best chance at recovery and comfort in their golden years.