The thread about the Rottweiler Puppies reminded me...
My lasty dog was a Rottweiler/Shepherd. I used the Seresto flea and tick collar on him. He developed a tumor right under the collar that killed him. I checked online about those collars - thousands of dogs were made sick by them, and also people, including kids. And, my dog still got Lyme disease.
Looking back, I used those collars on previous dogs and they also died from tumors. I can't prove their tumors were caused by the collars, but the tumor on my last dog developed right under the collar.
https://www.greenmatters.com/p/are-seresto-collars-safe