Many veterinarians and other's believe that all herding breeds are affected by ivermectin drug sensitivity.  This problem is usually associated with other collie like breeds such as the "Lassie" Collie, Shetland Sheepdogs (shelties), Australian Shepherds, Border Collies as well as a few other breeds which aren't even in the herding group.  The of the location of the mutant gene that causes this drug sensitivity has been discovered, the gene has been labeled MDR1.  Now we can safley identify the breeds which are and are not affected by MDR1.  There had not previously been any cases of Pembroke Welsh Corgi's having an otherwise normal reaction to ivermectin drugs but now with the test we are positive that they are not an affected breed.  Many of the Wind Dial Farm dogs have even had blanket testing done and have been proven to be free from the gene.  Should this topic come up between you and your vet concerning a deworming program, this information is posted to help you broaden your treatment options.  Ivermectin's can also quite effectivley treat various types of external parasites such as mange, ear mites, and most other microscopic extenal parasites.