Could a dog suffer from flea allergy even though he has no signs of fleas, having been treated with a spot on? Asking as presumably if the dog  picks up a flea (juvenile hedgehog living in garden) it with not die until it bites the dog? Thank you.

Yes, but if you have used a Spot on, which is very concentrated and applied to a small area, it will take time for it to spread around the body and of course could cause a local reaction itself.

I have had dogs react to Frontline spot on, who never reacted at all to the spray.

I used to sue the Spray every three months because my oldest dog at the time was flea allergic, but the vet and I never found any sign of fleas on her or the others.

It certainly seemed to combat the sumemr itchies and hot spots she sometimes got.

I don't use flea treatments routinely any more though, she died in late 2005.

Like you don't routinely treat for fleas as there has been no need, stupidly when one had a couple (around the time juvenile hedgehog moved into garden) used Frontline combo that I had. He was three months into feeding raw & his coat looked fantastic & no red areas. Within a couple of weeks he was the worst he'd ever been hot, red & damp areas in groin, along either side of penal shaft, under front legs & dewlap . Came home yesterday after 3 days away, large bald areas and hair falling out by the handful took him to vet who mentioned flea allergy & did he suffer from stress when I was away! 

Poor love. 

My flea allergic bitch I treated with Frontline SPRAY every three months and never had any more bother. 

Interestingly we have never had a flea issue in the 20 years + I have had the dogs, though we do have hedgepigs, and did have an issue with ticks  in the last couple of years, but only with puppies not the adults.

Otto looking like an old moth eaten coat & continues to loose a lot of hair. Blood test found nothing, bio chemistry results showed no problems with organs, but urine contained white blood cells & keytones. After a month of antibiotics, yesterdays urine sample was back to normal. I wasn't here to take the call but vet mentioned that he had been concerned about TT levels, could someone tell me what they are, thank you.

Perhaps I should have started a new post?