One of the most compelling arguments for spaying and neutering pets is that it decreases aggression. So what is going on when your newly spayed cat shows more aggression than ever and what can you do about it?
Aggression in newly spayed cats is usually caused by the anesthesia that remains in your cat's system after her surgery. These anesthesia after-effects can last from several hours to several days depending on the amount and type of drugs used and your cat's sensitivity. Think of your cat as being on a bad trip - she may be sensing and defending herself against threats that exist only in her drugged-out feline imagination. This will wear off. In the meantime, keep her in a calm, safe space far away from other pets.
If your cat was in heat, pregnant, or lactating when you had her spayed her body will still have mama cat hormones for days or weeks after the surgery. If she was in heat, her smell will still trigger courting behavior from males and territorial behavior from other females, both of which can lead to fights. If she was pregnant, she'll still have the nesting instinct for a while and will defend her territory from your other pets. If she is lactating she will continue to feed and defend her babies until they are more grown up and spaying will not alter her aggression one bit when she is protecting them.
Pain and Defensive Behavior
Surgical pain is another reason that newly spayed cats may show aggression. Your cat just had major abdominal surgery and a complete hysterectomy. She is going to experience a lot of discomfort for a couple of weeks. This pain can make her feel threatened by your other pets and cause her to lash out at them. It is a good idea to use the pain control drugs prescribed by your veterinarian but note that these can also make your pet behave in unusual ways and trigger more aggression. If this is an issue, continue to keep your newly spayed cat separated from other pets until her healing is more complete.
Aggression between your newly spayed cat and your other pets can be triggered by the mere fact of removing and reintroducing your cat to the household. Pets have pecking orders and social relationships, and when these are disturbed dominant animals can get upset and lash out, and subordinate animals can decide the time has come to challenge the "boss." For all of these reasons, most veterinarians will recommend that you keep your newly spayed cat away from other pets for at least her first 48 hours home, and be on the lookout after that so you can separate if there is any aggression.
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