The first thing you should understand about cats is that they are very difficult to understand. Cats are mysterious creatures and the things they do cannot always be explained. Disabuse yourself of any ideas that cats only purr when they are contented. Cats purr for other reasons.
Cats purr for any number of reasons. It used to be thought that cats only purr when they are contented or happy. Dr. Lorraine Kassarjian, a South Florida veterinarian, says this is untrue because she, her staff and colleagues, have observed cats purring who are in in great pain, anxious or fearful. The conclusion Kassarjian draws from this is that cats sometimes purr for the same reasons people take deep breaths, to calm themselves. So, if your cat is purring, don't assume it's because she is enjoying whatever activity you are sharing with her at the moment. She could be extremely frightened or angry and is purring to help calm herself down. In this instance, biting is the next natural order of events since a frightened or angry cat will bite to defend herself.
Purring and Biting
A commonly reported scenario is as follows: a cat owner will be absentmindedly petting a purring cat who is lying contentedly on said owner's lap. The owner is not really paying much attention and therefore doesn't pick up on the extremely subtle signs that things are about to take a turn for the worse. The cat will suddenly, and seemingly without warning, bite the hand of the person petting him. It seems like such an insult in human terms. One minute, you're getting along so well and then next your sweet little pussycat has become the cat from hell. What's it all about? There are several schools of thought on this scenario. If he was purring in the first place because he's not feeling well, it's possible you touched on a sore spot and set off the unfortunate chain of events. But this is not the most probable explanation.
If Kitty is purring because she is content and happy to be engaged in a mini-lovefest with you while you pet her and she is rubbing against your hand in an obvious show of "yeah that, do more of that," she may just become overstimulated. Experts call this "petting aggression" and it simply means that the cat has decided the petting has gone on long enough and is seeking to end it. There are some very subtle clues the savvy owner can pick up on to avoid the bite. The cat's tail may begin swishing from side to side ever so slowly, or flicking upward. Her ears may flatten. She may stare into space, and you may notice pupil dilation. She may look directly at your hand just before biting. She could get very still, or walk away. She may lick her paw in an effort to redirect her aggression. If you can pick up on these signs, you can avoid the bite. If you are just not that observant, well, keep the hydrogen peroxide handy.
Some cats are quite territorial and do not like it when new people or animals come into the environment. They may purr in order to calm themselves down, but in the end their true colors win out and they may bite. Some cats will be upset at a stranger but then bite their owners. The only way this can be explained is that it's like when a person has a bad day at work and comes home and kicks the dog. The dog didn't do anything, but the person knows he can get away with it with the dog, not so much with the boss. The cat will snap at his owner because the stranger, and what he is capable of, is unknown to him. Other aggressive tendencies arise when the cat sees you as an equal. He may be territorial over a bed, a person or the couch. If you try to get on the bed, sidle up to the person or sit on the couch, a purring cat, whether he's purring out of contentedness or anger, may suddenly bite.
Michelle A. Rivera is the author of many books and articles. She attended the University of Missouri Animal Cruelty School and is certified with the Florida Animal Control Association. She is the executive director of her own nonprofit, Animals 101, Inc. Rivera is an animal-assisted therapist, humane educator, former shelter manager, rescue volunteer coordinator, dog trainer and veterinary technician.