Thank you for your patience. What you are experiencing with Diesel is actually very common and really comes down to her age. As cats get older, they are more prone to arthritis in their spine and joints, and as such, the practice of grooming (which most cats spend one third of their awake time doing) is too painful. That is more than likely the case here with Diesel, and as a result of not being able to groom herself, she is having the issue with matted fur, which in turn leads to dried out skin and excessive dandruff. So what can you do about it? You are already on the right track here, as the key is regular grooming for her! If she has really firm tight mats, then you may even need to get her along to your local veterinary surgery for the nursing team to clip these mats out, otherwise the skin issue will worsen even further.
If you can groom her every day, that will be the best bet to trying to help her coat condition, and hence longer term the condition of her skin. There are a couple of very good brushes I use on my cat that both work very well. One is called a Furminator and helps to thin the hair out a little. You can purchase this online from here for example: http://www.furminator.com/pet-deshedding-solutions-tools/cat-grooming-deshedding-tools.aspx. In combination with this, I would definitely recommend a 'zoom groom' and you can purchase one of these here: http://www.cattoys.com/catzoomgroom.html . These are great and cats love being brushed with them as they have soft plastic large bristles!
Carrying out brushing every day would be ideal and should stop these matts from occurring. Also make sure Diesel is on a very good cat food and preferably a senior cat food from your local vet or pet store. If she hasn't had a full geriatric blood test recently, then I would also recommend that in the next couple of weeks as well to ensure she doesn't have an underlying issue that could be causing this skin issue as well. The fish oil supplements are great too, but I would recommend a feline specific one to ensure it isn't too high in mercury and other trace elements. I hope all of the above makes sense? If you have more questions or if I can help in any other way, please do not hesitate to ask! If you would like to accept my answer, please press RATE OUR CONVERSATION (I am not compensated in any other way). Bonuses are always welcome. Thanks! I hope to work with you again soon!
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