Why did my friendly cat turn aggressive after we got new kittens?

Oh my, that's a lot of cats, especially for a household that started with a cat who wanted to be an only cat. It sounds like cat E also would prefer to be an only cat. I would immediately separate E from both A and the B C D group. You have yourself 3 different fractions here and each needs their own space. I wish I could give you an answer that is more pleasing to hear. It's theoretically possible that cat E has some medical problem instead, but from your description, it very much sounds like a personality/behavior issue. The majority of cats would be stressed by this arrangement, unless you have happen to live in a very large mansion so they can spread out. It's just hard to integrate new cats in to households. Wishing you the very best of luck.

Cats are very sensitive to other cats in their territory. Unlike most dogs that will accept another dog into their house relatively easily, for cats it can be a different story. Your cat is upset by the presence of the strange cat in your house. Animals behave that way sometimes for reasons that are unclear, taking their anger out on you. She may smell the foster cat on you. I would definitely keep them separated from each other and give her some time and space to possibly accept the fosters presence. Don't get hurt! I wouldn't force your attention on her for now. Maybe in a few days she'll be easier to approach.

I hate to hear that. This situation can very very stressful and it's hard to know why this is happening. It's possible Kyo is sick and his odor has changed. It may be worthwhile to have him examined by the veterinarian to make sure everything is ok. It's also possible the cats are seeing or smelling another cats outside and this is causing stress and bullying behavior. Wipe down doorways and windows to eliminate odors. Here are a couple articles to help with cat aggression and stress: https://www.petcoach.co/article/5-tips-to-handle-an-aggressive-cat/ https://www.petcoach.co/article/causes-of-stress-in-cats/ You can help relieve stress by setting aside 30 minutes a day for vigorous play for each aggressive cat, provide climbing opportunities with cat shelves and cats treats, use a pheromone diffuser and provide separate litterbox and feeding stations. Lastly, your veterinarian can provide medications or a special diet called Calm which may help the cats. Best of luck and please feel free to post any additional questions.

That is very unusual and sounds scary! There could be a few reasons for this behavior. Sometimes there could be pain or a medical condition causing a change in behavior. Since Oscar can't talk and tell you what is happening, I recommend to have him examined by the veterinarian to be safe. Sometimes this behavior is related to the stress of seeing another cat outside. For some cats, this causes frustration and they can then exhibit bullying type behavior to other pets or people within the house. If the veterinarian thinks this is behavioral, there are a few things you can do. Try enriching the environment. Here is a great article about cat's instinctual needs: https://www.petcoach.co/article/why-every-cat-needs-a-place-to-climb/ Use a Feliway diffuser, set aside 30 minutes a day for vigorous play (really get him moving) and/or switch the food to Royal Canin Calm. If he is the only cat in the household, you may try feeding him out of a food puzzle. Hide food for him to find. Do not use these last two mentioned options if you have other cats. Provide a stand or resting place by a window so he can look outside. You may also try clicker training him for positive behavior. Here is a link about training cats: https://www.petcoach.co/article/can-i-train-my-cat/ Wash all windows and doorway exteriors where a cat may be spraying outside. Try to drive out the cat if possible. You may also need to block the window where you think Oscar may be viewing the cat most frequently. Most cats response best to combination therapy. The veterinarian can also prescribe medications which can help with aggression/stress. I hope this helps and please feel free to post any additional questions.

Thanks for using PetCaoch! It’s not uncommon to hear about behavior like Alyssa. It can take a lot of time and patience for them to get along but here are some tips for decreasing stress for Alyssa in the short term. -Feliway- calming cat pheromone- comes in many forms from pet stores or online. 

 - Adding a litter box. Studies show that the ideal number of litter boxes for a home is #of cats +1 . This is regardless of how clean you keep his current litter box. 

- making sure she has plenty of “high space”. The safest (and therefore least anxiety-inducing) place for a cat is up. If you don’t already, get a cat tree or put up a shelf or two that she can go perch on. 
 Feeding them in separate rooms with two or three water bowls throughout the home is also a good idea as perceived competition for resources is a common problem in cats. Best of luck to you and your kitties!

Changes in the social structure in a household like this can be very stressful for cats. They are not a very social species by nature, and it's often very hard for them to accept "newcomers" - even if these are her sister's kittens. I would recommend making sure that the aggressive cat has her own area where she can get away completely from the kittens and your other cat whenever she wants to - this should be a comfortable room with a litter box, food and water bowls, a cat tree or other resting spot, etc. This way, she can be on her own if she prefers to, but can still come out an interact with the other cat and her kittens if she chooses. Feliway diffusers in the areas where she spends most of her time may also help to reduce her stress. It may take her weeks, months, or even longer to adjust, so be patient - some cats never really adjust, unfortunately, and may need to be kept in separate areas of the house to keep everyone happy.

The first thing I would do for now is put Attila in a room by herself with a litter box, food and water bowls, comfortable resting places, etc. and leave her alone for the next few days apart from coming in briefly to refill her food and water and change her litter. When cats become upset and aggressive like this, for whatever reason, it can take up to a week for them to fully calm down and return to normal - until then, it's best to leave her completely alone and give her plenty of space. Some cats can have serious aggression issues that are triggered by a strange or scary noise, an unusual smell, or anything else that startles them. It's possible that Attila may have had a scary dream that frightened her, or she could be painful due to a pinched nerve, arthritis, or some other issue that may have been aggravated by petting. If she calms down over the next few days and things return to normal with no further problems, you may not need to do anything else. If she is still acting aggressive a week from now, or if this becomes a recurring problem for her in the future, I would strongly recommend making an appointment with a veterinary behaviorist - he/she can evaluate Attila in person and help you determine what may be triggering these episodes, and will also give you some specific suggestions for managing her when this happens. Prescription medication is helpful for some cats with aggression issues, so this is something that could be discussed also. You can search for a board-certified veterinary behaviorist in your area here: www.dacvb.org