7 Medical Causes Behind Weight Gain

Your pet is overweight, and being the conscientious pet owner, you have made the necessary changes to your pet’s diet and activity levels, but your pet is still overweight. In fact, not only is he still overweight, he seems to be gaining more weight. If diet and exercise are not solving the problem, what else is there?

There are other valid reasons for weight gain besides eating habits and lack of activity. Here are seven of the most likely offenders.


This is the most obvious cause of weight gain and potbellied appearances. Although it may seem obvious, some pet owners are completely unaware that their cat or dog is pregnant until there is a litter of little ones staring them in the face. If a female dog or cat is not spayed, she can become pregnant, and it does not take long for it to happen. A few unattended minutes in the backyard can lead to an unintended pregnancy.

So don't go putting your dog on a strict diet or exercise regimen just because she's gaining weight for no obvious reason. She may just be “expecting.”

Fluid Retention

A common side-effect of heart disease is a condition called ascites, the medical term used for excess fluid in the abdomen. The outward symptom is of an enlarged belly that is not coincident with overeating or lack of exercise. Other conditions can also cause the body to react in this way, including tumors or diseases of the internal organs. In very young animals, abnormal amounts of fluid in the abdomen may be the result of abnormal blood flow in the heart due to a congenital defect. Another cause of ascites may be linked to a portosystemic shunt, also referred to as a liver shunt, where the circulatory system bypasses (shunts) the liver.

In cats, feline infectious peritonitis (FIP) is one of the main causes of abdominal fluid retention.

Prescription Drugs

There are some prescription medications that can also lead to weight gain, especially if they are taken over a long period. If your pet is on any kind of medication and is also having a weight problem that you cannot control through simple food management and moderate exercise, you will need to consult with your veterinarian to see if the medication is related to the weight, and if a different medication or lower dose can prevent further weight gain.


Internal parasites, especially the type that lodge in the abdominal walls and intestines (though not limited to those types), will often cause fluid to build up around the area of infestation, causing a potbellied appearance. This is often seen in young animals whose immune systems are not yet strong enough to resist the effects of parasitic infestation, and is more severe when there is a heavy load of internal parasites.

In the course of a standard examination, your veterinarian will take blood, fluid, and stool samples, one or more of which will show the presence of parasites in the body. Once the specific type of parasite is determined, your veterinarian will be able to prescribe the appropriate parasiticide.

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