
Milbemax is being used in dogs of unknown parentage all over the country without a single report of a problem. You have to bear in mind that there will be a lot of cross breeds which don't look like they have any herding ancestors but they may carry the mutant MDR1 gene. Looking at the silken windhound you wouldn't think there was a chance that they might carry it, but some do. The fear arises from the safety data sheet which states that tolerance in young collie breeds has not been tested and that the correct dosage procedure must be followed.
My vet prescribe it as the wormer of choice now to any breed but with every breed the correct dosage must be followed, which is fairly easy as it's one tablet for any dog up to 25kg.
If I had a litter of collie pups and I wanted to be very safe I wouldn't leave a box of Milbemax lying around where the pups could get it, same goes for slug pellets, antifreeze etc :-)
Do SWD carry the defect MDR1 gene Diane? It's worth DNA testing a few to see if you don't know, as there are other (more dangerous) ivermectin related drugs to be aware of if they do, for example imodium or eating horse dung from an animal that has just been wormed.