Ear mites

I took one of my gyps to the vet for a blood test for tick diseases, and I knew she had been shaking her head a lot and scratching at her ears so they checked and she did have ear mites, so he told me to put 1 cc ivomec in each ear and massage the ear to make sure it gets down in the canal before she starts shaking her head. He also said they spread from dog to dog easily so I'll probably have to treat all my other dogs. He also said that if I had been worming monthly with ivomec like I was supposed to the dog would have enough in their system to be immune to the ear mites.