1st, I am so glad you caught the problem on Birch and that all is well with that, phew. Unless they are in the bath, or blowing coat, stuff can hide in the forest of their fur!!!
2nd- what is with these people when their dogs go after your dog they DO nothing?

That makes me livid. Having had encounters where I am pulling off other dogs from my dog while the owners are oblivious makes me so livid.

It is amazing to me how when needed you can hoist up a hefty dog and get rid of it to help your own dog. Then somehow that offends the dog owner, ugh. There is NO way under heaven I am allowing ANY dog to bite my dog, pin my dog, or hurt my dog in any way if I can intervene. Thumbs up for you grabbing that offending mutt.

There is a HUGE difference between dog play and dog attacking. Unfortunately, shelties can trigger the prey instinct in bigger dogs.