Cat at Vet - photo © cattoo
Cat skin rashes can be caused by Atopic Dermatitis; although there are numerous possible causes. Atopy is an allergic hypersensitivity to an allergen (a non-parasitic antigen causing hypersensitivity by over-production of the antibody immunoglobulin E - IgE). Humans have the same problems and a classic is being allergic to cat dander.
For cats (who are similar in many ways physiologically to humans - hence animal testing on cats) and humans the allergic reaction to substances in the atmosphere could be due to an immune system that is malfunctioning either due to an inherited condition and/or environmental factors.
I believe (or at least it is possible to believe) that some cats that have been deliberately breed within a tight gene pool may have compromised immune systems (I am thinking of Modern Siamese cats, for example). As Atopic Dermatitis is due to an immune system that is working against the welfare of the cat (over reacting due to being compromised) there may be a link between skin rashes and purebred cats. I have not seen any statistics to support this and this is pure speculation and chit chat. But it may be that skin conditions occur more frequently in purebred cats than mixed-breed cats. Anyone have any figures?
The actual mechanism for causing Atopic Dermatitis is unclear but in animals with this condition there is an imbalance between two different types of white blood cells. White blood cells provide the protection to invasion from foreign bodies.
There may be raised antibody immunoglobulin E (IgE) presence as well. I'm rambling a bit here but there is an instance of a cat being allergic to propane fuel (for a gas oven) for example. The skin became red and the hair fell out. In time and on removing the source of the problem (and with steroid treatment) a scab forms which eventually falls of leaving good skin. The hair grows back.
What I am saying is that skin rashes are not always caused by the usual things like fleas and over grooming. It could be something in the environment. A little judicious testing by the person keeping the cat could isolate the offending allergen and a resolution found. Thinking wider, if breeding purebred cats can jeopardize the cat's welfare some thought needs to be given to the wider issues of cat breeding generally.
A possible example of purebred breeding that may be having a negative impact on cats is the high incidence of heart disease in Bengal cats (specifically HCM) and Maine Coons. I am speculating that cat breeding without full knowledge of the medical consequencees can result in long term health issues associated with the entire cat breed and wider (as outcrossing takes place).