If your new cat is showing signs of illness, please do not introduce it to your existing cat until you consult with your veterinarian.
Make sure the new cat has a place to himself for at least a few days, allowing him time to adjust. Spend time with your new cat out of sight of your existing cat. Be sure to lavish attention on your existing cat as well.
Allow your original cat to follow his usual routine. He will be aware that something is different and will seek out the location of the new cat. Allow them to sniff under the door.
After a few days or so, swap the cats so they can investigate each other’s areas. Getting used to each other’s scents is an integral part of adjustment. If all seems to be going well, allow the cats to meet one another through a screen, baby gate, etc. It is normal for cats to hiss and growl at each other, but an all-out physical attack is very inappropriate and rare. However, be prepared with a water spray and a towel or blanket to separate the cats, as allowing them to fight will elongate the introduction period or may even damage the process altogether.
After a comfortable period of introductions through a barrier, or monitored encounter, increase the area and time the new kitty can explore outside his safe room. Allow only short periods of contact, increasing as time goes on.
Once the new cat has been allowed access to the rest of the house, leave his safe room intact so he may take refuge there. Improvement in attitudes toward each other should begin in a week or so. Do not be discouraged if it takes longer. The length of adjustment will depend on the personalities of the cats and the length of time your existing cat was a solitary pet. It may, though, take as long as a month or more for the new cat to be accepted.
Be sure to lavish attention on all your cats to make everyone feel part of the family. The social interaction level may vary; you may have cats that groom, play and sleep with each other, or just tolerate the existence of the other—and that is okay, too!
First, prepare a safe room and have a clear escape route set for your new cat, as a precaution to the encounter. Make sure the dog does not have access to the cat’s food and water and especially the litter box! Use a baby gate or cat door to block the dog’s access.
When introducing your new cat to a dog, put your dog on a leash. Allow them to see each other (try to keep the leash loose) and then distract the dog with a treat or praise.
With a flat collar on the dog, you can let him drag the leash around the house so you can grab it if necessary. It is normal for the cat to hiss and growl at first, especially if she has not lived with a dog before. Cats take time to adjust to changes; the more the animals are together in the same room, with you to supervise, the quicker they will adjust. You can also throw a towel over the cat if she attacks the dog. Make sure the cat has an escape route or can leap to a high place. Do not let the dog chase and/or corner the cat or vice versa.
Separate them when you are not there to supervise until you are comfortable with the situation. Some dogs are too predatory to adjust safely to a cat. Proceed with caution and, if you find this is the case, never leave them together unsupervised.
If your cat hides, don’t worry. Keep the well-behaved dog around as much as possible so the cat can get comfortable with his presence. Make sure the cat has a private place not accessible to the dog where she can eat and drink in peace and feel safe. Ensure that the cat has unthreatened access to her litter box at all times.
Allow the pets to adjust in their own time—be patient and act calm and relaxed. Often they become friends; sometimes they just tolerate each other. Either way you should respect the arrangement.