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Since it only seems to happen about once a month or so, it may be an allergic reaction to something that he is only coming into contact with once a month or so. It doesn't have to be his food. It could be a human food treat, a doggie treat, a lawn treatment, almost anything including a pollen or cleaning supply. The possible allergen could be anything.
The allergy would likely start with a rash as a reaction which would cause the dog to itch a little, damage the skin and allow staph which is usually present on a dog to develop into a staph 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. 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
Given his age, allergy testing might not be cost effective, but it would be helpful in allowing you to avoid the allergen. You might also see if desensitization shots might stop the issue.
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