Thanks for the additional information. It is helpful.
It sounds like she might be having an allergy reaction especially if the scratching started after the change in food. Often manufacturers change a formula as well and a dog is suddenly itchy without a change in diet. It can even be a new dye or preservative in the food. I asked about moving because sometimes if you move to a different area, the dog is exposed to new plants which can set off a reaction.
In addition to allergies causing a reaction, there may be a skin condition such as staph dermatitis or yeast dermatitis. These skin conditions are generally seen with allergies. Staph usually occurs on the lower regions of your pet and tends to have small pimple type bumps. Shampoo containing Chlorhexiderm and/or Oatmeal can help with this condition. Yeast typically shows as a greasy area that has a sweet musty odor. Sometimes the skin can become inflamed, darker and thickened due to itching which sounds like what your dog is experiencing on her breast area. Yeast likes areas such as between toes, armpits and ears. Selsun Blue Shampoo can help with Yeast dermatitis. When shampooing, lather and leave on 15 minutes before rinsing. These shampoos are not meant to be a cure, just a relief until your pet can see a Vet. Benadryl can be given to your dog, the dose is up to 2mg per pound every 8 hours for itching due to allergies.
You can read about allergies here: http://www.vcahospitals.com/main/pet-health-information/article/animal-health/allergy-general-in-dogs/428
Here is a site that goes over itching in dogs.
If the change in food happened before the itching, then you need to switch back and seek treatment for the skin dermatitis again. The allergen needs to be eliminated and then the dermatitis cleared to stop the cycle. If the food was changed after the symptoms, you are going to need to try and isolate what might be causing the allergic reaction. Your vet may have treated a dermatitis but if the underlying cause was not eliminated, the dermatitis might come right back.
Now the swelling of the breast area could indicate a localized reaction to an allergen, but it might also indicate an infection of the breast or a mammary tumor. You can read about both of these conditions here:
I'd probably give the benadryl a try to see if you notice a reduction in symptoms and have her breast area looked at by your vet if there is no improvement. A breast infection can cause serious problems if not addressed quickly.
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