Thank you for the further information regarding Ginger.
She is so lucky to have had you to safe her from what sounds to be a horrific situaiton.
If Ginger is only vomiting hairballs, then this makes disease processes that would cause nausea and then vomiting less likely in her case (which is good). Rather in Ginger’s situation, her GI signs are being triggered by her overgrooming which is likely induced by anxiety/stress (very understandable given her history). This means that the hairballs and vomiting are just a side effect or secondary issue here. This also means that hairball diets and hairball treatments can only hope to manage and lower the occurrence of GI signs but will not be able to solve her hairball issue. Instead, the anxiety overgrooming needs to be tackled.
Now I understand that Ginger’s current situation is much better then where she was previously. Still if she underwent a lot of stress then, a lot of change when she came to you, and is a gentle sensitive soul (which the Himalayan breed usually are) then she may still be struggling. In a way, the behavior is an outlet for their frustration of being powerless to change their situation. As well, even with so much behind her, it is quite possible that part of her overgrooming is also now motivated by habit (like we might chew our nails when we are stressed but then start doing it without stressful stimuli). Therefore, even with a peaceful life ahead of her, she cannot stop this behavior (similar to the post traumatic stress soldiers experience after coming home from terrible battles). And this is a very common for kitties and leads to continuous baldness in those 'easy to reach' places (ie flank--example belly--example, tail, and legs) and hairballs.
Just to make a small note here, I would like to note here to direct you to an interesting wee article from an owner who's cat started overgrooming due to stress ((example/article)). It is quite a nice wee read and has some good pictures of how significant hair loss can be due to this.
So in this situation, we do want to take steps to reduce her anxiety and help her let go of her coping habit. To do so, the ideal first step is to consider using some de-stressing tools to help provide her with a general peaceful environment and reduce her anxiety. In these situations, we often we will use Feliway, also known as Comfort Zone in the US pet stores, which is a synthetic cat pheromone that helps to relieve stress. This can be used as a spray or a plug-in diffuser. There is also a diet on the market called Calm by Royal Canin. This contains a number of supplements that have been found to provide stress relief to cats. As well, there are nutritional supplements like Zylkene (example ), which use a casein protein to soothe anxious cats. Some people have even found treats like Composure (LINK) or Bach Flower Remedy (LINK) to be helpful for settling kitty tension. And as these are not 'drugs', you can use any of these together to help settle her anxiety.
Overall, while they are the sign you appreciate the most, her hairballs and GI signs are actually a knock on effect of her stress induced overgrooming. Therefore, the key to decreasing the hairballs is to decrease her drive to overgroom. And the key to this is to reduce her stress and help her cope with her new life. So, I would suggest the above supports for Ginger. If you find she is still struggling and her overgrooming doesn’t settle then that would be your cue to get her vet involved and consider drug therapy to address this for her. And in that case, there are drugs that your vet can dispense to decrease feline arousal and dampen her drive for anxious overgrooming. These can be short term and long term treatments depending on the cat and often will help you settle the behavior for her.
I hope this information is helpful.
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