It’s hard not to notice when your normally energetic dog drags on his morning walk or suddenly sleeps all day, indifferent to the box of toys he normally raids. Veterinarians have two terms for this type of behavior, displayed by both canines and felines: exercise intolerance and lethargy, respectively.

Lethargy and exercise intolerance can often overlap, but veterinarians regard them as two separate, albeit related, symptoms. Regardless of whether you suspect your pet is suffering from lethargy or exercise intolerance, whether it comes on suddenly or has been happening for some time, it’s important to report these symptoms to your vet. Lethargy and exercise intolerance can signal potentially devastating — although usually treatable or manageable — conditions, including infections, diabetes, heart failure and gastroenteritis (among many others).

Causes of Lethargy and Exercise Intolerance

A variety of ailments can lead to lethargy and exercise intolerance, such as these common causes:

Lethargy in Young Pets

Infectious diseases — panleukopenia, parvovirus, FeLV, and FIV, to name a few — are serious causes of lethargy in young cats and dogs. Other causes include fever, congenital heart disease, pneumonia, anemia and malnutrition. Even parasites, like hookworms, can cause lethargy in young dogs and cats. And because fleas and ticks drink blood, a severe infestation can lead to anemia, which can cause an affected pet to be lethargic.

Lethargy in Older Pets

The list of possible causes of lethargy in older animals includes excess weight, cancer, osteoarthritis, pain (orthopedic, dental, and cancer-related pain are the most common), diabetes, heart disease, canine hypothyroidism and infections, such as tick-borne diseases in dogs and feline FIV.

Exercise Intolerance in Young and Older Pets

Several factors can come into play when it comes to canine and feline exercise intolerance:

Anemia. Infectious diseases, chemotherapy, parasites and chronic kidney disease are just a few things that can lead to anemia, a condition in which normal red blood cell levels decrease, hampering the body’s ability to deliver oxygen to tissues and organs. A patient with anemia can experience exercise intolerance.

Heart diseases. Any disease in which the oxygen supply to the body is affected by poor circulation can cause exercise intolerance, including heart disease.

Lung diseases and upper respiratory diseases. Low oxygen levels can also result from the body’s inability to process oxygen inhaled from the air. This can occur for two reasons: The lungs themselves don’t work properly because of pneumonia, cancer, asthma or emphysema, for example. Or the upper respiratory tract is somehow occluded due to a collapsing trachea, laryngeal paralysis, cancer or infection.

Neuromuscular diseases. Exercise intolerance is the hallmark of several diseases that fall into this category, including exercise-induced collapse and myasthenia gravis.

Endocrine diseases. Hypothyroidism is a classic example of a hormonal condition that causes sluggishness and unwillingness to exercise; diabetes and Cushing’s disease can also manifest in this way.

Obesity and excessive weight. Extra poundage is the No. 1 reason for exercise intolerance and lethargy in pets. And it’s highly treatable!

What Your Veterinarian Will Do

Here’s what you can expect from your vet if your pet exhibits signs of lethargy or exercise intolerance:

1. History. Most veterinarians will begin by asking a few basic questions to help them understand the history of the problem: When did you first notice the lethargy or exercise intolerance? Has it changed? How has your pet been acting otherwise?

2. Physical examination. Examining the entire body is a necessary part of the process. 

3. Laboratory testing. In most cases, a vet will order blood, fecal and urine testing. Aside from a basic urinalysis and a CBC and chemistry panel, your vet may order additional tests to help identify specific infectious, metabolic, or endocrine diseases.

4. X-rays and imaging studies. If your vet suspects orthopedic pain, cancers or certain other processes, she will order X-rays. Some pets may require additional imaging, such as ultrasounds of the chest and abdomen, CT scans or MRI studies.

5. ECG. Vets who believe that a patient may have heart disease can perform an electrocardiogram.

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The correct course of treatment depends entirely on the underlying cause of the lethargy or exercise intolerance.

What You Can Do at Home

Any pet who appears lethargic or exercise intolerant — especially if the symptoms surface suddenly — should visit a veterinarian. If you can’t schedule an immediate appointment, there are some measures you can take to keep your pet comfortable in the meantime.

Confine him. Keep your pet enclosed in a smaller space, such as a cozy crate, for easier observation.

Let him rest. Long walks and exercise, in general, should be ruled out until you see the veterinarian.

Take his temperature. Many pets with fevers will be very sluggish. A temperature over 102.9 degrees requires prompt veterinary attention.

Monitor his symptoms. If your pet exhibits other unusual symptoms, such as poor appetite and pain, head for your vet or an animal emergency hospital. These can signs that your pet needs emergency care.

This article was written by a Veterinarian.