You might try changing diet to a unique protein and filler like venison and potato and soaking the paws and using the creams I suggested to help clear up the skin dermatitis that might be present. You might also want to use an e-collar on him to prevent his chewing. You can get e-collars (cone) at most pet stores. That will give the paws and legs time to heal.
Provide him with a kong toy with treat compartment. Fill with yogurt or peanut butter without Xylitol then freeze it. Give this to him to help keep his mouth occupied so he won't want to lick and chew on his legs.
If you can't figure what the allergen is then you might need to do allergy testing. If the vet did not test for mites, you might want to do that as well. Allergy testing can be expensive.
Benadryl just helps relieve the symptoms of the allergies. If it helped the itch he was experiencing then an allergy might be the cause. He is pretty young for a thyroid issue. Mange mites might cause intense itchiness but non contagious mites usually tend to strike the head first while the sarcoptic mange mites tend to start lower like the legs. A skin scraping would diagnose these. Read about this type here and treatment suggestions.
Yellowish green areas would indicate infection. I think you really need to have him seen. Try these resources to make the visit less expensive. Remember you get to determine what tests your vet performs, so ask what the tests they want to run are and the cost so you can make an informed decision on whether that particular test is necessary. I would start with a skin scraping. It is likely the vet will give an antibiotic, recommend a shampoo and prednisone to help with inflammation and itchiness.
Veterinary Bill Assistance Programs:
1. The Big Hearts Fund: http://bigheartsfund.org/
2. Cats in Crisis: http://www.catsincrisis.org/
3. Diabetic Cats In Need: http://fdmb-cin.blogspot.com/
4. The Dog And Cat Cancer Fund: http://www.dccfund.org/
5. Frankie's Friends: http://www.frankiesfriends.com/
6. Handicapped Pets Foundation: http://hpets.org/
9. The Mosby Foundation: http://themosbyfoundation.org/
10. The Onyx and Breezy Foundation: http://www.onyxandbreezy.org/
11. Paws 4 A Cure: http://www.paws4acure.org/
12. The Pet Fund: http://thepetfund.com/
13. Pets-DC: http://www.petsdc.org/
14. Shakespeare Animal Fund: http://www.shakespeareanimalfund.org/
This site may have additional resources as well.
These vets may offer a free initial visit though you may have to pay for extra procedures and medications. See the next one for the coupons.
If he says allergies, you might ask him for information concerning Atopica which is helpful in cases where the allergen can not be avoided such as grasses and pollens.
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