TL;DR: Sudden aggression between cats usually stems from external triggers or medical stress and requires immediate separation and a veterinary evaluation to rule out underlying pain. Taking prompt action helps prevent physical injury and preserves the cats' long-term social bond.
What causes my cats to suddenly start fighting?
Sudden aggression between cats who previously got along—often called redirected aggression or non-recognition aggression—is a distressing behavior change. It typically occurs when a cat is startled or hyper-aroused by an external trigger (like a stray cat outside) and directs that frustration toward the nearest housemate. It can also happen after one cat returns from a veterinary visit smelling like a clinic.
Is sudden aggression between my cats a veterinary emergency?
- The urgency level for sudden aggression is Medium; while rarely life-threatening, it requires prompt attention to prevent injury and preserve the cats' relationship.
- Seek a veterinary exam immediately if either cat is showing signs of physical injury.
- Immediate medical evaluation is needed if the aggressor is also acting lethargic or losing their appetite to rule out underlying pain or illness.
Why should I provide a video or photo of my cats' behavior to the vet?
- Veterinary professionals analyze subtle cues like ear position, pupil dilation, and tail posture to differentiate between play, defensive fear, and offensive aggression.
- Seeing the physical environment where the conflict occurs allows the team to suggest specific environmental modifications to restore peace.
- Visual evidence provides a clearer picture of the cats' body language than a verbal description alone.
Clinical Context (Merck Veterinary Manual)
To manage sudden aggression between household cats, the immediate priority is to ensure the safety of all cats by physically, visually, and ideally, olfactorily separating them to prevent further injuries; early intervention is crucial. Cats should remain separated until calm, which may take days to months. Reintroduction involves desensitization and counterconditioning using favored rewards like toys and treats, starting with odor exchange via grooming tools and feeding separately in a common area divided by a partition, gradually decreasing the distance between them only when calm. Training one or both cats to wear a leash and harness aids in controlled reintroductions, while a bell on the aggressor alerts the victim. Providing more vertical space with climbs and perches, and ensuring ample resources spaced sufficiently apart, can further reduce conflict. Reward training using simple verbal cues can assist in managing cats during introductions or de-escalating potentially aggressive encounters; punishment should be avoided as it increases fear and anxiety.
Chapter: Behavioral
Source: The Merck Veterinary Manual, 11th Edition (Page 1574)