What can cause a cat that has no flea collar nor new treatment of any kind to lose its top coat in patch skin without the presence rash or fleas?

Several reasons. Most common are:

1. Allergies: Cats can develop allergies to various things, such as food, pollen, and dust. When a cat has an allergy, its skin may become inflamed and itchy, leading to hair loss.

2. Infections: Bacterial or fungal infections can also cause your cat to lose its fur. These infections can be caused by a variety of things, such as cuts, scratches, or underlying health conditions.

3. Stress: Stress can also lead to hair loss in cats. When a cat is stressed, its body may produce hormones that damage hair follicles.

4. Nutritional Deficiencies: A lack of certain nutrients, such as biotin or zinc, can also cause hair loss in cats.

5. Underlying Medical Conditions: Some medical conditions, such as thyroid disease or Cushing's syndrome, can cause hair loss.

6. Age: As cats get older, they may start to lose their fur. This is usually a gradual process, and it is not usually a cause for concern.

If your cat is suddenly losing its fur in patches, it is important to take it to the vet to rule out any underlying medical conditions.