Why is my cat lethargic and limping after a vaccine?

The other night a cat came to my office at 2 am. Her owner was very worried. The cat was lethargic and she was limping on her right rear leg. The patient’s intake paperwork indicated that she had been previously healthy, she took no medications, she had no allergies, and her vaccines were current. A technician had measured the cat’s vital signs, which were normal except for a mild fever.

I entered the exam room to evaluate the cat in question. She was a one-year-old, very sweet Burmese. (Has there ever been a Burmese who wasn’t sweet?) She was lethargic, but not catastrophically so. When I entered the exam room she slowly emerged from under the owner’s chair (the preferred location for cats in veterinary exam rooms) to greet me. She was limping on the right rear leg, but it was relatively mild. She was still putting weight on it.

I watched the cat walk. She was slow, and she was tender on her right rear leg. She sniffed my hand and then pressed her head into it. She reacted with a beatific smile and she moved her whiskers forward when I stroked her chin. She wasn’t feeling well, but it was clear that she wasn’t dying.

When you practice veterinary medicine for a long enough, a list of possible diagnoses courses through your mind the moment you meet a patient. Broken leg? No, not enough pain. Soft tissue injury such as a sprained ankle? No, she wouldn’t be lethargic. Laceration to the foot pad or nail injury? No, there was no bleeding and again, she wouldn’t be lethargic. Bee sting? Maybe, although she’d probably be more painful. Fever of unknown origin? Maybe, but the soreness that occurs with the condition usually does not affect one leg in particular. A rare condition such as immune-mediated arthritis or septic arthritis? Maybe, but probably not, because rare conditions are, as it turns out, rare. Abscess? Probably. In fact, almost almost certainly.

There are two types of abscesses in cats.

Dental abscesses generally occur in older cats whose teeth become severely infected. They cause facial swelling and oral pain. Obviously, this was not the type of abscess I suspected.

The type of abscess I suspected was a cat fight abscess. When a cat is bitten by another cat, an infection is likely to develop. In the early stages the affected cat may suffer from lethargy and pain in the affected area. If the cat were bitten on the right rear leg, the symptoms would exactly match those of my patient. Later, the area might fill with pus and then rupture in a dramatic and disgusting fashion.

The owner and I began to talk. She confirmed that the cat had no prior medical problems. She was not aware of any trauma. The cat received regular flea and heartworm preventatives. There was no known cause for the symptoms. She assured me that the cat did not go outdoors, and had not escaped from the house, ever. There were no other cats in the house.

Those last few items blew my theory out of the water. There was no way the cat had been in contact with another cat. A cat fight abscess was out of the question.

Of course, it didn’t make much difference. Although I often reach a tentative diagnosis before examining my patient, experience has taught me that one must never get tunnel vision. A thorough physical exam and and an open mind are always necessary.

Evaluation of the cat revealed that she was not dehydrated. There was no evidence of trauma. Her teeth and gums looked good. Her eyes, ears, nose and throat appeared normal. Her heart and lungs sounded good. There was nothing wrong with her abdomen. A thorough evaluation of all of the legs, including the right rear, revealed no focus of pain or swelling or irregularities. The cat was sore on the right rear leg, and she was a bit lethargic. That was all.

This called for more questioning, and probably a bunch of tests. Was there anything, I asked, that was outside of the ordinary about the cat’s day before the symptoms occurred? The owner thought for a while, and then said, “Well, we went to the vaccine clinic for some shots earlier.”

Scratch the tests.

I asked which vaccines the cat had received. It turned out that she had received an FVRCP (the most basic vaccine, which protects against the most important illnesses) and a rabies vaccine. Rabies vaccines generally are administered in the right rear leg.

Vaccines work by stimulating the immune system. That is their purpose. A stimulated immune system may lead to lethargy. There may be some soreness at the site of the stimulus, which in the case of vaccines means the place where the shot went in. Rabies vaccines are supposed to be administered in the right rear leg. Hence the limping. The vaccines were the source of the symptoms. The prognosis was excellent.

The last time I had a tetanus shot, my arm became sore and I felt lousy for a day. I spent the day lounging on the sofa and watching TV. I recommended the same treatment for my patient.