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Hello, I'm Dr Gary. I've been practicing veterinary medicine since 2007. I look forward to helping with your questions/ concerns.
This location is very suggestive of fleas. Even though you don't see any, they could still be getting exposed. The topical preventatives are great, but fleas can still get on and bite them. They just die soon after. The issue is that the flea bites can lead to flea allergy dermatitis and we can see these crusty lesions on the neck and the base of the tail. You may want to try alternating products and using Revolution or Advantage as some fleas will get immune to certain products when used long term.
I would probably do a round of antibiotics like Cephalexin or Amoxicillin on them. This is in case of a staph infection. Sometimes we'll see secondary staph infections that lead to these lesions. Alternatively, you could make a diluted Chlorhexidine rinse and clean the areas once daily for 7-10 days to see if that helps. Make sure you rinse it off well after applying.
I don't think this is a mite or any other topical parasite.
I hope this helps, let me know if you have any other questions.