Here are 20 reasons why your cat is lethargic or apathetic. I’d suggest that all but two (chronic constipation and URI) require a proper veterinary diagnosis. Lethargy is a natural consequence of feeling ill and therefore is present in most illnesses. You’ll also see loss of appetite accompanying lethargy. Eating poorly will exacerbate the lethargy. This list is probably not entirely comprehensive.
This is a dangerously low body temperature. You’ll know if this reason is relevant because it will depend on the climatic conditions at the time. If your cat has been lost outside for some time or trapped and when found it lethargic, hypothermia might be the reason. There will also be shivering. At the extreme it results in collapse and coma.
When symptoms are shown (often they are not) the most common are lethargy, loss of appetite, rapid breathing and coughing.
Bacterial infection – Bordetella
This is an upper respiratory infection (URI). It occurs more severely in young cats and in shelters. Symptoms include: fever, lethargy, sneezing, coughing and discharges from nose and eyes.
Immune related skin problems
There is an inappropriate attack by the immune system on one of the layers of the skin. Symptoms include: pustules on the skin developing into crusts, itching, fever, lethargy and loss of appetite in severe cases.
This is rare in cats. It is a deficiency in thyroid hormone. The coat becomes thin. Other symptoms are: lethargy, weight gain, constipation and mental dullness.
This is an irritation of the stomach lining. Signs are severe vomiting, stopping eating after vomiting, lethargy, head hangs over water bowl. Most common cause: ingesting an irritant or poison.
Constipation causes a cat to look bloated and she picks at food. Lethargy is a symptom.
Impaired liver function
Cats appear weak and lethargic. Loss of appetite and weight.
Symptoms include: lethargy, loss of appetite, weakness, vomiting, acetone breath, dehydration, laboured breathing.
The initial signs of pancreatitis in half of cats suffering this condition include: lethargy, not eating, increased breathing rate. This condition is painful in people. I’d expect it to be the same for cats.
Lethargy, depression and weight loss but often no direct respiratory signs.
Signs of an infectation include: cough especially when exercising, lethargy, weight loss and poor coat condition.
An infection of the uterus. This is life-threatening. Symptoms include: fever, lethargy and weight loss, urinates a lot and drinks lots of water.
Birthing process infection
This is called acute postpartum metritis. An infection of the uterus lining spreading through the birth canal. Symptoms include: lethargy, refusal to eat, hangs head, high rectal temperature.
This is insufficient blood flow to meet the needs of the cat. Signs include apathy (lethargy).
Bacterial infection Tularmia
A rare infection for cats causing the usual signs such as loss of appetite, lethargy, fever and depression.
Viral respiratory infection
It is to be expected that this illness can cause lethargy and loss of appetite with fever.
This is feline panleukopenia. AKA feline distemper. A viral infection. Highly contagious. It causes severe lethargy, fever and loss of appetite.
Acute infectious enteritis
An infection of the gastrointestinal tract. Signs: vomiting, diarrhoea, fever, lethargy and depression.
A very common cause of death in elderly cats. My cat died of this. One day she stopped eating and looked at me. I knew it was the end. She had been lethargic for a long time sleeping all the time.
P.S. All these diseases are covered in detail on this site. Please use custom search to find them.
Image source: Fundo vetore desenhado por Photoroyalty – Freepik.com
Source: this site and Cat Owner’s Home Veterinary Handbook.