This
page from the American Working Collie Association has lots of excellent information regarding the gene mutation (mdr1-1delta) that makes collies sensitive to ivermectin and other related drugs. It's the source of information I've relied on for years for sensible information on the topic.
All herding breeds that have descended from (or are otherwise related to) collies have the potential to carry the mutation. However, it should be noted that the last I heard on another list (border collie related) no border collies that have been tested have been found to carry the mutation. That doesn't mean every individual in the breed is free of the mutuation, just that none of the dogs tested so far has been found to have it. CJS has already posted the link to the WSU CVM, but here is a relevant quote from there:
Approximately 3 of every 4 Collies in the United States have the mutant MDR1 gene. The frequency is about the same in France and Australia, so it is likely that most Collies worldwide have the mutation. The MDR1 mutation has also been found in Shetland Sheepdogs (Shelties). Australian Shepherds, Old English Sheepdogs, German Shepherds, Long-haired Whippets, Silken Windhounds, and a variety of mixed breed dogs. The only way to know if an individual dog has the mutant MDR1 gene is to have the dog tested. As more dogs are tested, more breeds will probably be added to the list of affected breeds.
If you read closely the AWCA link I posted, you will see that milbemycin oxime is in the same class of drug as ivermectin: the avermectins. Any of the drugs in this class could cause an adverse reaction in dogs with the mutation, and a list of some of the more common ones is listed on that page.
I have not tested my border collies for the mutation and I do give them ivermectin (at a low dose suitable for HW prevention but not for control of intestinal parasites; incidentally, the doses that work for HW prevention are way below the dose threshold known to cause problems in dogs with the mdr1-1delta mutation, which is one of the reasons I don't test my dogs) monthly. Like Becca, I start my pups on Interceptor and then switch them over at about 8 months to a year. One of the reasons I don't like Interceptor is that I don't necessarily think it's a good idea to continually dose dogs for intestinal parasites. As a sheep farmer, I see how resistance of worms to dewormers develops and I can't help but think that using medications like that (basically continuous dosing) is just setting us up to create superparasites. I think using Interceptor on a low-dose regimen is a wise idea.
I'm sure some of you already know all of the information posted here, but I think it's helpful for people new to the topic to have as much information as possible. I think the Working Collie Association site is invaluable, especially considering the fact that it points out the other drugs of the same class that could cause problems in dogs with the mutuation.
J.