Collars are nice but they do not solve the problem. If the diet has never been changed then whatever you are feeding is suspect. Most dogs that do this are sensitive to the feed grains in the dog food they are eating.
Some dog food companies use moldy feed grains. [not supposed to] Dog foods that are inexpensive that do not have feed grains like 4 Health, Blue Buffalo, Taste of the Wild, Nutro Natural Choice Grain Free, can help eliminate this problem.
It will take 6 weeks or longer to notice a change. The toxins in some feed grains have to come out of the body somewhere and the skin is where when their body is overwhelmed. Veterinarians are not nutritionists as a rule and they are like Drs they ease the symptoms without going after the underlying cause.
I am not a raw feeder or over the top health nut tree hugger. I just know what works from over 20 years experience, with several dogs and several breeds. Prevention is cheaper than a vet bill.
Most of my dogs eat Purina Dog Chow. The ones that cannot eat it I feed the grain free.
Another cause of this condition is yearly vaccinations other than rabies. They are totally unnecessary [IMO and others] and could compromise the immune system. Here are some articles on that subject.
http://www.drbeckersbites.com/pet-vaccinations
I know you did not ask but I cannot help myself. I hope your doggie gets better.