Thank you for your patience. As above - no prescriptions can be given out for dogs without a full veterinary examination, so you will definitely need to follow up with your local vet again. This will give you some ideas on what to follow up on however. Your boy's skin sounds very red, very itchy and it must be pretty frustrating for the both of you. As you may have already discussed with your local vet, there are a number of possible causes for this and we definitely have to consider the possibility of an allergy of some description here.
Firstly - you need to make sure that your boy is definitely up to date with flea control. Make sure you are using a flea product appropriate for his weight. Even if you cannot see any fleas on him doesn't mean there are one or two lurking and oftentimes this is all it takes for a dog with flea allergy dermatitis. In your boy's case it does sounds like he may have an underlying allergy of some sort with secondary infection, however this could also be caused by parasites or fungi (ringworm or yeast).
Basically dogs are allergic to four main things - food, fleas, grasses/plants (contact allergy) or environmental allergens (canine atopy - the dog version of ezcema). Flea allergies are avoided by strict flea control - every 4 weeks with Advantage or Advantage Multi or every 6 weeks with Frontline. You must also ensure any other cats or dogs in the household are treated at the same time. It is great you have had a skin scraping done as that will rule out mites, however I would still recommend ongoing monthly treatment with an appropriate flea/mite treatment as a preventative, particularly so in case your boy is allergic to flea saliva (flea bite allergy dermatitis).
Contact allergies are best treated by avoidance of the plants that are to blame, but identifying the exact cause of a contact allergy can be very hard. A referral to a canine skin specialist may be the way to go in your dog's case for either a skin patch test or a blood allergen test. It sounds like your vet has already recommended this, and while it is expensive, given your boy's age and degree of symptoms, this really is a possibility here.
As above, the skin scrape your vet has done has thankfully helped to rule out both Demodex (less itchy) or Sarcoptes mites (VERY itchy). Do continue ongoing with a flea/mite treatment as discussed above.
Ongoing, you should also use a medicated shampoo that will deal with both yeast and bacteria, and the product Malaseb is the most ideally suited for this. Given the smell you are getting from your boy, a superficial yeast and or bacterial infection is more than likely. The Malaseb requires you to make a lather and leave this lather on for 10 minutes before washing off! For many clients this is easier said that done, but it is SO important. You should be able to get this shampoo from your local vet without an issue and it will help with the smell. If there is a bad infection present, then your boy likely will need some antibiotics, and to get these you will need to have a consultation with your local vet in order to see if there is bacteria present (via cytology) and if so - which antibiotic is the most suitable. Finally - you could try some benadryl for your boy. The typical dose is 1mg per pound of body weight up to twice daily. This is effective in around 40 - 50% of dogs with this type of symptom. You can read more about the use of benadryl in dogs online here: http://www.petplace.com/drug-library/diphenhydramine-benadryl/page1.aspx
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