6 Signs of Illness in Your Pet (cont.)

Lack of Appetite or Decreased Activity

These two pet symptoms are vague, but if they persist, a veterinarian should check for causes. Dogs and cats stop eating for a host of reasons, including fever, pain, or stress. "A reduced or absent appetite, especially if it lasts for more than 24 hours, is a reason to bring your animal to the veterinarian," says John Randolph, DVM, diplomate of the American College of Veterinary Internal Medicine and professor of medicine at Cornell University's College of Veterinary Medicine.

Typically, cats suffer more dire consequences when they stop eating, according to Sawchuk. "A dog may be a picky little snit and doesn't eat his dinner because he's waiting for human food," she says. "With cats, you've got to be real careful because if they just even go a couple of days without eating, especially an overweight cat, they can develop fatty liver." In this potentially fatal disease, excessive accumulation of fat in the liver can cause liver failure. A cat that stops eating should see a vet promptly because fatty liver can be treated.

What about this pet symptom: the cat or dog whose energy level drops? "It's just one of those very vague things that tell us that something is not right," Sawchuk says. Many things can cause lethargy, including major problems, such as heart disease.

A pet whose lethargy can't be pinned on an obvious reason, such as from an extra-long run at the dog park, may need to visit the vet, especially if other symptoms arise, such as change in exercise tolerance, weakness, collapse or loss of consciousness.