Can't figure out where I posted it, but someone else asked a similar question, I think last week. I found an aluminum medal, somehow charged, that works with a critters own heart/ chemistry. It WORKS. It's about $80, but works for 2yrs. It must stay on, so would need a waterproof collar, but it works so well (no troubles with fleas, ticks, or even mosquitos, on my little Charlie, since I put it on her, that I'm seriously considering getting one to wear, myself, because the ticks here are TERRIBLE.
Here's the link:
https://drjudymorgan.com/products/fleasgone-tag-non-toxic-flea-and-tick-prevention