Why is my cat suddenly attacking my other cat?

What it often means

• Redirected aggression: a strange cat at the window, a noise, a scare. He cannot reach the cause, so he attacks the cat beside him, and the tension between them can outlast the incident.
• A foreign smell: back from the vet or a boarding facility, one cat comes home smelling strange and the other no longer recognizes him. This is called non-recognition aggression.
• Pain or illness: a cat who hurts gets irritable, and a sick cat behaves differently. In older cats, hyperthyroidism can make him edgy.
• Social maturity, around 2 to 4 years: a young adult may start controlling hallways, perches or the litter box. If he is not neutered, hormones join in.
• Competing resources: too few litter boxes, bowls, beds or perches, or all in one spot, so one cat guards while the other waits.

Signals to read at the same time

• Play or fight: play is nearly silent, roles switch, with pauses; a fight comes with hissing, growls, yowls, fur standing up, and one cat ends up fleeing.
• What comes before: a hard stare, stiff body, lashing tail, ears swiveled back, a direct approach. In the cat being targeted: crouched body, flattened ears, dilated pupils.
• Place and time: near a window, after a cat passes outside, after a vet visit, at mealtime, in a hallway.
• The friendly gestures: a 2004 review showed that in groups, cats have preferred partners they rub against and groom. If these two no longer do, their relationship has changed.
• The targeted cat’s state: he hides, eats less, soils outside the box, overgrooms. Silent tension sometimes does more damage than the fights.

What you can do

• During a fight, slide a cushion, a piece of cardboard or a blanket between them, or make a sharp noise from a distance. Never your hands: an aroused cat bites whatever touches him.
• Then separate them in two rooms, each with a litter box, water, food and a hiding spot, and do not pick either one up while he is still tense.
• Reintroduce them as if they had never met: swap scents with cloths, feed them on either side of a closed door, then let them see each other through a gate, in short, calm sessions.
• Remove the trigger: frosted film on the lower part of windows if a cat prowls outside; after a vet visit, keep the returning cat apart and rub him with a cloth that smells of home.
• Multiply and spread out resources, one of the five pillars of the 2013 feline environmental needs guidelines: one box per cat plus one, several food and water stations, perches, in different rooms.

When to see a vet

See your vet if the aggression is new in a cat who was calm until now, if either cat hides, eats less, soils outside the box or limps, or if there were bites: they often get infected and form an abscess a few days later (swelling, fever, lethargy). Pain, hyperthyroidism or another illness in either cat can trigger the conflict. If attacks keep coming back despite your efforts, a veterinary behaviorist or certified behavior consultant can help plan the reintroduction.