Gungahlin Vet Hospital

Aggression in Cats after separation (especially hospitilisation)

Unfortunately, it is quite common for cats which may have lived harmoniously together for months or years to be very aggressive when reintroduced after a period of separation. The separation may be as short as a few hours, such as a visit to the vet. This is not a status (dominance) issue, but rather a form of Territorial Aggression and is associated with lack of recognition of the smells on the returning cat’s fur. Aggression can be profound, and very upsetting to observe.


  • Staring at the other cat
  • Attack, chase and paw blows
  • May see signs of fear in victim (defensive aggression). This appears to trigger an attack


Letting the cats fight it out does not work, and may result in injuries. Best is to conduct very gradual exposure (systematic desensitisation)

  • Keep the returning cat in a cat carrier outside. Rub a towel over the home cat’s fur, then use the same towel to rub the returning cat, then repeat several times, to spread the smells and encourage recognition in a non-threatening way.
  • Bring the cat in, but leave him in the cat carrier, which should initially be placed up high (e.g. on top of a cupboard).
  • Confine the returning cat until it is calm, and until all resident cats are showing curiosity about the returned cat.
  • If there is no sign of aggression, put the carrier on the floor and let the cats sniff through the cage door. If all is well, let the cat out, but observe for problems.

If there are problems

  • Keep in separate rooms with no visual contact; feed near separating door or in cages. The amount of visual contact may be gradually increased
  • The cats can be put in separate cages for an hour each day and fed. The cages are gradually brought closer together each day. The cats must be kept completely separated at other times. The cats can be put in alternate cages every day, so as to become familiar with the other's odours.
  • The cats can be exposed to each other on leashes. This is the only time they get attention. The time when they are exposed to each other must be the best time of the day: Play with the cats, use food treats, feed them while they are exposed to each other.
  • Once the cats are loose together again, interact with them only when both are present. Toss a treat to the aggressive cat when it approaches the victim in a friendly way, and reward the victim for staying relaxed.
  • Pheromones (Feliway) can act as a social facilitator for cats by reducing anxiety and increasing confidence. The Feliway diffuser should be introduced and be running for some days before the cats are reintroduced.
  • In some cases, drugs may be used on one (or both) cats (in the aggressor to reduce aggression, in the victim to reduce fear)
  • Consider bringing both (all?) cats to the Hospital next time, so they are all sharing the same experience.

Territorial aggression can be directed to people as well, or an upset cat may display Redirected Aggression against a human or other animal if it cannot get to the cat it wants to attack. Be careful!

Georgia Knudsen and Michael Hayward

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