When Aggression Has a Physical Basis

Its emergence in older cats calls for special concern because they typically mellow with age.

Your cat is shy but generally sweet natured and affectionate. Imagine your shock, then, when you reach to pick him up and he tries to bite you — for no apparent reason. Did your cat’s inner tiger emerge?


When the onset of feline aggression is sudden, there’s a good chance that the source could be a health problem, and you need to make an appointment with your veterinarian.

“A medical cause for aggression in cats is relatively rare, but it must always be ruled out,” says Katherine A. Houpt, VMD, Ph.D., diplomate of the American College of Veterinary Behaviorists and emeritus James Law Professor of Animal Behavior at Cornell University’s College of Veterinary Medicine. “The primary cause of medically based aggression is pain, no matter what the source of that pain.”

The emergence of aggression in older animals is a particular cause for concern, Dr. Houpt says, because cats tend to mellow with age. Moreover, pain is easy to miss in geriatric felines, who will spend a good deal of time sleeping. Owners might think the cat is simply slowing down when, in fact, he’s hurting.

Two painful diseases. Primary sources of undiagnosed pain in cats include periodontal disease — in the worst case, a receding or abscess of the gums that makes eating difficult — and arthritis. Dental work and medications such as corticosteroids, respectively, are usually effective in mitigating the problems.
Several diseases can also cause dramatic behavioral changes. The following are the most common:

Hyperthyroidism. Caused by an excess of thyroid hormone, hyperthyroidism occurs quite often in middle age and older cats. Signs include weight loss and increased appetite. The disease is easily diagnosed with a blood test and successfully treated with medication, radioactive iodine or surgery.

Rabies. Outdoor cats who haven’t been vaccinated for rabies may develop this usually fatal disease as a result of a bite from carriers such as bats, skunks, foxes or raccoons. Any unvaccinated cat who comes home with deep scratches should be taken to the veterinarian immediately for possible quarantine.

Feline ischemic encephalopathy. In summer, outdoor cats in the Northeast United States and Southeast Canada may pick up a parasite called a cuterebra, which lays its eggs on the host’s skin. If the larvae migrate to the cat’s brain, this can cause aggression. The disease is difficult to treat unless anti-parasite medication is administered during the first week, but signs such as epilepsy can be controlled.

Meningiomas. Magnetic resonance imaging can diagnose these benign tumors of the meninges — thin membranes that cover the brain and spinal cord. They’re the most common type of brain tumors in cats, but because they occur on the surface of the brain, they can often be removed surgically with complete success.

Aggression isn’t generally a problem in cats, Dr. Houpt says. “People tend to be more respectful of a cat’s need for space, but if we have an animal that presents for aggression with no known cause, we always perform at least a good physical exam, a chemistry screen and a complete blood count to rule out medical causes.”

In most cases, veterinarians can treat medical causes of aggression and you’ll have your happy, friendly cat again.

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