Chronic diarrhea in a pet can be frustrating to care for and to resolve. When chronic diarrhea is associated with vomiting, lack of water intake, fever, depression, or other symptoms, it often indicates that your pet has a potentially serious underlying disease. Such signs should prompt an immediate visit to your veterinarian.

Causes

There are numerous diseases and disorders that can lead to chronic diarrhea., and they include:

Infectious diseases can cause chronic diarrhea in any age and breed of dog, and include a variety of agents, such as the following:

Intestinal parasites, such as whipworms, roundworms, hookworms, and tapeworms

Bacteria, including Salmonella, Clostridium, Campylobacter, Yersinia

Fungal agents, such as histoplasmosis, aspergillosis, phycomycosis, candidiasis

Protozoal parasites, such as giardiasis and coccidiosis

Infection with blue-green algae, namely protothecosis

Inflammatory bowel disease is a microscopic infiltration of the intestinal wall with inflammatory cells. The cause is unknown, although it is suspected to have an immune basis. It may affect the small intestines, large intestines, or both. Vomiting and weight loss are common but do not always accompany the diarrhea.
        

Dietary intolerance or allergy is most often attributed to a particular protein in the diet, but can be associated with gluten, lactose, high fat content, and certain food additives. It can develop slowly, over weeks or months and cause chronic diarrhea.

Drugs and toxins are more often associated with acute diarrhea; however chronic diarrhea may occur following the administration of certain antibiotics, nonsteroidal anti-inflammatory drugs, certain supplements, etc.

Gastrointestinal cancer can cause chronic diarrhea by either destroying part of the intestinal walls or by causing a partial blockage of the intestine. The most common tumors seen are lymphosarcoma and adenocarcinoma.

Partial obstruction or blockage may develop with cancer, ingestion of foreign bodies, intussusception (telescoping of the bowel into itself), cecal inversion, or stricture of the intestines.

Metabolic disorders including kidney and liver disease, diabetes mellitus, and hypoadrenocorticism, are often associated with systemic signs of illness (e.g. vomiting, weight loss, chronic diarrhea, etc.).

Pancreatic disease, especially exocrine pancreatic insufficiency (EPI) gives rise to chronic diarrhea because there is inadequate production of digestive enzymes.

Small intestinal bacterial overgrowth (SIBO) is characterized by an overgrowth of normal intestinal flora (bacteria) usually secondary to other gastrointestinal diseases, but occasionally from chronic administration of antibiotics and other medications.

Many diseases can cause malabsorption of food stuffs or poor digestion of food stuffs in the intestines, and subsequently chronic diarrhea. Malabsorption/maldigestion problems include lymphangiectasia (abnormality of lymph vessels of the intestines), inactivation or lack of digestive enzymes, inadequate supply of bile salts, severe inflammation of the lining of the intestines, inability to absorb sugars or proteins across the intestinal wall, etc.

Short bowel syndrome is created after a large portion of the intestinal tract is removed surgically. The remaining bowel is too short to provide an adequate surface for digesting and absorbing nutrients.

Irritable bowel syndrome (spastic colon) is a chronic, intermittent dysfunction of the lower bowel for reasons not entirely understood. This condition may be aggravated by stress.