Male cats are indeed in a relatively dangerous period on the first night after spaying/neutering surgery. This is because post-operative cats may experience several challenges:
Anesthesia recovery: Cats are anesthetized during surgery. After the effects of the anesthetic wear off, cats may experience discomfort and pain.
Stress response: The surgery itself and postoperative environmental changes may cause stress in cats, which may lead to heart and respiratory problems.
Decreased immunity: After surgery, a cat's immunity will be temporarily lowered, which may cause diseases that were originally dormant in the body to suddenly break out, such as feline panleukopenia and parvovirus.
Wound care: If the cat licks or bites the wound, it may cause infection and affect recovery.
