When should your pet’s weight loss concern you? The standard is when it exceeds 10% of normal body weight (and when it is not due to fluid loss). There are many things that can cause weight loss, including chronic disease. It is important to understand this because the animal's entire body will probably be affected by the weight loss, and it ultimately depends on the cause and severity of the underlying medical condition.

Causes 

• Insufficient calorie intake
• Poor quality of food
• Taste (palatability) of food
• Spoiled food/deterioration from prolonged storage
• Reduced appetite (anorexia)
• Inflammatory bowel disease
• Chronic protein-losing intestinal disorder
• Intestinal worms (parasites)
• Chronic infections of the bowel
• Tumors of the intestine
• Blockages in stomach/gut (gastrointestinal obstructions)
• Surgical removal (resection) of segments of bowel
• Disease of the pancreas
• Liver or gall bladder disease
• Organ failure (heart, liver, kidney)
• Addison’s disease
• Diabetes
• Hyperthyroidism
• Chronic blood loss (hemorrhaging)
• Skin lesions that ooze and cause loss of protein
• Disorders of the central nervous system that interfere with eating or appetite
• Paralysis of the esophagus
• Neurologic disorders that make it difficult to pick up or swallow food
• Increased physical activity
• Prolonged exposure to cold
• Pregnancy or nursing
• Fever or inflammation
• Cancer
• Bacterial infections
• Viral infections
• Fungal infections

Diagnosis 

Your veterinarian will begin with a variety of diagnostic tests to find the underlying cause for the weight loss. After an initial health assessment, the following are some tests that might be recommended for your pet:

• Fecal studies to look for chronic intestinal parasites
• Complete blood count (CBC) to look for infection, inflammation, leukemia, anemia, and other blood disorders
• A biochemical profile that will evaluate kidney, liver, and pancreas function, and the status of blood proteins, blood sugar, and electrolytes
• Urinalysis to determine kidney function, to look for infections/protein loss from the kidneys, and to determine hydration status
• Chest and abdominal x-rays to observe heart, lungs, and abdominal organs
• Tests to evaluate the condition of the pancreas
• Ultrasound of the abdomen
• Bile acids test to evaluate liver function
• Hormone assays to look for endocrine disorders
• Using a scope to view the intestines (endoscopy) and biopsy
• Exploratory surgery (laparotomy)

Treatment 

At times your veterinarian may recommend treating your pet's symptoms, especially if they are severe. This is not a substitute, however, for treating the underlying cause of the weight loss.

Once the appropriate treatment has been assigned, make sure a high-quality diet for your pet is provided. It may be necessary to force-feed, with nutrients given intravenously as necessary. The diet must be supplemented with vitamins and minerals. Appetite stimulants are also used occasionally to get the animal to start eating again.

Living and Management 

A proper medical follow-up is vital, especially if the animal does not show improvement quickly. Monitoring during this period is also critical. The underlying cause of the weight loss will determine the appropriate course for home care. This includes frequent weigh-ins for the animal. Follow your veterinarian’s recommendations for treatment. And if your pet does not respond to the treatment, contact your vet right away.