Collies (and other herding breeds) and Ivermectin
A lot people have heard that collies, aussies, shelties, border collies (any dog with white feet) cannot have Ivemectin containing products including the popular heartworm preventative, Heartgard. This is false. There are no Heartgard sensitive breeds or dogs. Yes, there are Ivermectin sensitive dogs but the dose in Heartgard is well below the level at which any problems are seen, even in affected dogs. After all, there is a Border Collie pictured on the appropriate sized Heartgard Box. Also veterinarians would not willy-nilly distribute a product that was unsafe for significant portion of dogs (herding breeds and herding breed crosses). Well, I wouldn't, maybe others would?
Ok. So myth busted. But why are there Ivermectin sensitive breeds? Some dogs-primarily collies, but others as well (more on that later) are affected with a mutation called the Multi-drug Resistance Gene Mutation or MDR1. This mutation specifically causes a defective pump (the p-glycoprotein pump). This pump is responsible for moving certain substances into and out of cells around the body. When the pump does not work effectively higher concentrations of the drug ends up in places it shouldn't (in the case of Ivermectin that would be in the brain). Not all drugs are moved by this pump so not all drugs need to be adjusted or avoided in dogs with the mutation. However, Ivermectin is also not the only one. Others include Butorphanol, Loperamide, Digoxin,Vinblastine, Mexiletine, Doxorubicin, and Acepromazine.
It should also be noted that a dog can be homozygous normal for the mutation (not affected), heterozygous for the mutation (partially affected) or homozygous abnormal (fully affected). It's important to know what the status of your dog is before starting on these medications because it will affect the dose used in some cases.
The most commonly affected breed is the Collie with around 70% carrying the mutation. Other breeds affected include Aussies at 50%, Shelties at 15% and Border collies at less than 5%. These numbers (as well as stats on other breeds) are from the Washington State University Veterinary Diagnostic Lab. Note that those percentages include the heterozygous dogs as well as the homozygous mutants. Mixed breeds can also carry the gene, especially if it is a herding breed mix.
The WSU lab can check your dog's status with a simple test. They also have additional details about which drug are problems and how to avoid complications.