Your cat and skin problems
Your cat may be excessively scratching due to a skin problem, rather than a parasite. This could be a fungal infection, such as ringworm, which enters the skin via a lesion, bite or direct contact, and then begins to irritate the outermost layer of the skin. Another cause of scratching can be acne, which causes itchiness and shows as inflamed, red lesions commonly found on their chin.
If your cat's diet isn't fulfilling all their nutritional requirements, it may result in a poor-quality coat and skin which in turn can cause itching. For example, if their diet doesn't include the right combination of omega 3 fatty acids and other vitamins to help their skin retain moisture and strengthen their role as a barrier to irritating material, it may dry out, flake and cause your cat to scratch.
Those cats that regularly go outside may end up scratching vigorously to remove environmental debris which has stuck to their fur or skin, such as splinters or seeds. If these get lodged into your cat's coat, they will groom themselves frequently as an attempt to get them out.
Allergies and your cat
Aggressive scratching and grooming can also be caused by an allergy, whose symptoms manifest themselves in your cat's skin. There are three main causes of allergic reactions in cats: fleas, food and environmental allergens. Some cats can develop an allergy to fleas, which results in their immune system overreacting to a flea bite and causing irritation. Certain environmental allergens, such as grass seeds or dust mites, may also aggravate your cat's system. Cats can also be allergic to specific components of their diet, which develop through overexposure; rather than treating the element as harmless, the immune response cells in their system are wrongly triggered, leading to itchiness.
A cat's grooming process is an important way for them to keep themselves clean and healthy, but it should never become excessive. Visit your vet if you see your cat repeatedly scratching, itching or licking itself, and they'll be able to advise you on the best course of action.