What Makes a Dog or Cat Lose Weight? How to Know if Your Pet is Underweight--and What to Do About It

What Makes a Dog or Cat Lose Weight?
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There are several ways to tell if your pet is underweight, and the problem should be addressed right away. See what can cause a dog or cat to be under their ideal weight.

Is your pet underweight? There’s a lot of attention given to the problem of obesity, but it’s flipside, overly skinny pets, is of equal concern.

When cats and dogs are underweight it can indicate a more serious health problem, or can reveal that your pet is feeling anxious and stressed out. Weight loss can lead to a decreased quality of life for cats and dogs--they feel cold faster, can become lethargic, and may experience damage to their internal organs. Shelter pets are frequently underweight. Regardless of the cause, weight loss should be taken seriously: it’s tough on a pet’s body, and even if the cause is just a matter of your pet being finicky about food, the underlying issue should still be tackled and resolved.


Many illnesses--including diabetes, cancer, and heart diseases--can have weight loss or a loss of appetite as a symptom. Parasites can inhibit a pet’s ability to absorb nutrients, and dental problems can make eating painful, resulting in a reluctance to eat. Finally, don’t discount the power of your pet’s feelings; changes large and small can affect your pet’s appetite, and depression may be a factor.


To some extent, the symptoms of underweight pets are precisely what you’d expect: a loss in weight compared to your cat or dog’s standard. You may also notice your pet acting lethargic, lacking the usual nicely groomed coat, and experiencing stomach issues from vomit to diarrhea. As well as seeing the difference in your pet’s weight and appearance, your sense of touch can also help to reveal weight loss. Feel along your dog or cat’s side--while you should be able to feel the rib cage, you should not be able to see any bones.


If your pet’s weight loss is the result of an illness, parasites, or another condition, tackling the underlying cause may help restore your pet’s appetite and ability to maintain a healthy weight. Some basic tactics to get your pet’s eating habits back on track are to provide several meals each day and to tempt animals with homemade or warmed up food. If necessary, you can feed pets through a syringe, and provide higher calorie food to help make every bite that your cat or dog eats count toward helping restore a normal weight.

When you notice that your pet has lost weight, or no longer seems interested in eating, a visit to the vet is likely in order. If simple at-home techniques do not restore your pet’s appetite and ability to keep on weight, the veterinarian will perform a physical, ask some basic questions, and potentially do some tests to determine what’s causing your cat or dog’s eating problems. The vet may take blood samples to check for certain diseases, check your pet’s stools for signs of parasites, or perform X-rays. To catch any problems early, watch for any changes in eating patterns, and take them seriously.

Why is My Dog or Cat Losing Weight? 

What Causes Cats and Dogs to be Underweight

Dogs and cats can be underweight for various reasons. Here are the most common causes.

Pets can lose weight for a wide range of reasons, including disease, parasites, and anxiety, to name just a few. Regardless of the cause, take any weight loss seriously. Being underweight can impact a pet’s quality of life and result in health problems. If your pet has been losing weight, or if a newly adopted cat or dog seems a bit too svelte, read on for some possible causes.


There are several illnesses that are known to have weight loss as a symptom:

  • Diabetes: While strongly associated with obesity as a cause, a common early sign of diabetes is weight loss.

  • Diseases including certain cancers, such as lymphoma and feline leukemia, liver disease, kidney disease, and heart disease.  

  • Inflammatory bowel disease: A loss of appetite can point to IBD, an illness that interferes with a pet’s digestion.

As well as these diseases, dental problems can also result in cats and dogs losing their appetites, since chewing and swallowing can become painful.


Your pet doesn’t have an easy way to let you know about stress, depression, and anxiety, but a loss of appetite can be a big clue. Changes–from another new pet in the household to travel to a new home–can easily throw a pet off and lead to a decreased appetite.

Picky Eaters

Some pets are very particular about their food. For finicky pets, be careful to slowly transition between brands and varieties of food, and to make sure that food is available at consistent times.


Hookworms, whipworms, giardia, and other parasites can all interfere with either a pet’s appetite or ability to absorb and utilize the nutritional value of food. If this is the cause of your cat or dog’s decreased appetite or weight loss, getting rid of the worms is the first step to restoring weight and appetite, and is an essential step in preventing further harm to your pet and possibly to yourself. Some of these parasites can spread to people through contaminated water or through oral routes, so see a veterinarian right away.


Pet food that lacks nutritional value can result in a loss of weight, as pets eat their regular amount, but do not receive all the nutrients and fats they require. And of course, if pets are not fed a sufficient amount of food, weight loss is a natural result.

Vaccines and Medications

One common side effect of vaccinations is a temporary a loss of appetite. This often only occurs for a few hours or a day, and amounts to a missed meal or so. In some cases, the diminished appetite lasts for several days, which may be cause for concern. If your cat or dog doesn’t seem interested in food for several days following a vaccine, and particularly if your pet has other symptoms like a fever or irritation at the site of the injection, it’s a good idea to consult your veterinarian.

Some medications also tend to have a loss of appetite as a side effect. Talk to your vet about whether to expect this when your pet starts a new medication, and how to prepare for it.

More on Keeping Your Pet a Healthy Weight

The Visual Check Chart
How to Tell if Your Dog is a Healthy Weight

This information is for informational purposes only and is not meant as a substitute for the professional advice of, or diagnosis or treatment by, your veterinarian with respect to your pet. It has, however, been verified by a licensed veterinarian for accuracy.

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