Welcome! My name isXXXXX am a UC Davis graduate, and currently, a Medical Director of a veterinary hospital.
I am sorry to hear about this concern.
Regardless of any testing done, there very well may still be a food allergy, and absolutely, other environmental allergies.
First, let's talk about the food.
Summary: The primary protein in most dog foods is lamb, chicken, or beef. The primary carbohydrate is rice or corn. So, you need to change both. The most popular alternative diets are Venison and Potato, Duck and Potato, or Fish and Potato. It is available in prescription formulas (i.e. Science Diet D/D) or you can find some similar foods at the big pet stores like PetSmart or PetCo. I always recommend picking a “limited-ingredient diet” from your vet to ensure you are doing the trial properly. Just make sure that when they say Venison is the "main ingredient" they don't sneak in other stuff like lamb meal or fish meal. A non-prescription option would include Dick Van Patten's brand called Natural Balance. Keep in mind, holistic or all-natural foods are not necessarily any better when trying to remove a food allergy. Chicken is chicken, and rice is rice. When you perform a diet trial, you must stick with the food for 2-3 months to see if there is any benefit. NO OTHER treats or human food can be given during this time period. Remember, a grain-free diet is not necessarily any better. Over 75% of the time, it is the protein in the diet that is the problem. Lastly, even a limited ingredient diet, as explained above, is not good enough. Sometimes, you need to use a food that is processed in such a way, that the body cannot react to it. A popular choice is Science Diet Z/D low allergen or Ultra allergen. They utilize hydrolyzed protein. The body cannot react to the protein in the food. Food Allergies
So, remember, if you are feeding a special limited ingrediet diet but also giving any other proteins or treats, then the food may not be fully helping.
Now, what about environmental allergies. I don't know what blood or skin test was done by the vet, but usually it would really require a specialist consult to have found out that dust mites are the problem. Regular vets can do special blood tests to check for environmental allergies, so I am assuming that's what your vet did.
Now, what about the scratching!
First, dust mite spray. Not enough.
You got to start vacuuming any carpet at least twice a week for next few weeks.
Second, dust mites eat human dander. So, guess where there is the most! In your bed. So, you need to reduce buddy's exposure to whereever the most dust mites will be. That means, if he sleeps in your bed, you got to stop allowing him there. Also, make sure to thoroughly wash any of his bedding.
In the meantime, you can try giving an antihistamine to try and help allergies, but ultimately, the fastest way to relief is getting some steroids from your vet. Steroids don't have to be done long-term but will get him some relief.
Benadryl can be given at a dose of 1mg per pound of body weight. Keep in mind, Benadryl tablets and liquids come in different sizes. So, an approximately 25 pound dog can get a full 25mg tablet or a half of a 50mg tablet. I usually avoid the liquid Benadryl in larger dogs (you would have to give too much of it). You can give Benadryl every 8-12 hours.
Another favorite for dermatologists is Zyrtec (cetirizine) over the counter. Available in 5mg or 10mg tablets. A small dog 10-20 pounds can get a 5mg tablet once a day in the morning, but can be given every 12 hours. Any dog over 30 pounds, I typically recommend the 10mg tablet every 24 hours (or twice a day).
Keep up those medicated baths. Most people think that dog allergies are "inhaled" like us. That is not correct. They are absorbed by the skin to cause problems. So, bathing, can help quite a bit.
Very sorry and definitely understand about costs. Fortunately, steroids are relatively inexpensive.
I hope that information has been helpful.
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