Because American Doctors aren't trained about parasites they only train Doctors about what pill to give for other stuff so if you mention parasites they think you are mental & tell you to go to a psychiatrist I went through this when I bought my purebred dog which was supposed to be Vet checked & he was 8 weeks old had Diarrhea & actually seen these worms freaked me out the Vet knew they were hook worms I had never heard of them & giggled them to find out they transfer to humans they gave my dog pills to kill them, but when I went to family doctor he said " We don't have hook worms in the US" he refused to help me so lucky for me I found a Doctor that had studied disease in Mexico & knew about them he gave me a script same as my dog larger dose just in case because he said they spread fast & suck your blood I went to every pharmacy in my town & they all said that drug was discontinued years ago so told the doctor he was so upset he said your dog can get it .