Thursday, February 16, 2012

Veterinary Mythbusting

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.


Sue said...

Ah, you answered my question.. is there a test to see if a dog is sensitive? It's good to know. Thanks.

Karissa said...

Well this was a great post to read! I always figured that was a hard & fast rule, so it's good to know that I don't have to worry so much. It's not nearly the concern now that it was back in my horse days, though -- it would be pretty common for one of the horses to spit a gob of dewormer back on the floor and it was always so fun trying to keep the dogs & cats away from it until you could get it cleaned up. lol

I wonder how it became such a viral myth, though, that "all collie breeds" will die if they get ivermectin? Curious.

Chris and Ricky said...

Thank you so much for this post! Ricky has always taken Heartgard - my vet told me it was safe for him - but I had heard the myth and had a hard time not believing it. You should shout this information from the rooftops! :)

Nicki said...

An anon commenter left a comment earlier about testing dogs that weren't in the database (ACD) and about Ace sensitivity in boxers. I don't know where that comment went but anon-if you are still interested here is my answer.

The MDR1 mutation can in theory appear in any breed, it's just that most dogs are not tested, especially if they are not on the suspicious list so there may not be enough data to list them on the WSU website. It won't hurt to check your ACD but unless he needs to be on one of the problem drugs it is likely unnecessary. I had not heard of an issue with boxers and ace so I did a search-there are some anecdotal reports of adverse events when using ace in boxers but it seems to be primarily with dogs that are from other countries. Since it's hard to say which dogs have ancestors from foreign lines it won't hurt to avoid ace in these dogs, although many veterinarians use this drug commonly without incident. It does not appear to be an MDR1 related issue. Thanks for the question-I learned something!