Ivermectin

Ivermectin should never be used in collies, Shetland Sheepdogs, Old English Sheepdogs, Australian Shepherds and sometimes other herding dogs. Vets have a saying regarding Ivermectin: White feet? Don’t treat!

Ivermectin is a anti-parasite medication and is effective against most common intestinal worms (except tapeworms), most mites, and some lice. While normally used to treat animals, it is also prescribed to humans to treat infections of Strongyloides stercoralis and onchocerciasis (river blindness). It is sold under brand names Stromectol® in the United States and Mectizan® in Canada.

The white feet saying relates to the gene {si~si} (sucrase-isomaltase) which is the "Irish spotting white" allele. It can be found in many breeds, and in many species, but in dogs and cats, when mixed with other white factor genes, it becomes what is called a "double lethal". That term does not mean it kills the animal, it is just a term for an animal that has inherited two copies of the same dominant gene, and sometimes causes health problems and sensitivities to medications and vaccines.

Old English Sheepdogs, and Ragdoll cats are most widely known for these anomalies because of breeders attempting to control the si~si gene to create specific coat patterns. Collies with Ivermectin sensitivity have been found to have a mutant gene for what is called the "P-glycoprotein." The P-glycoprotein has been studied largely because overexpression of this protein (i.e., having more of it than normal) results poor function of chemotherapy drugs in the treatment of cancer. The P-glycoprotein appears to be involved in keeping drugs out of certain body tissues. Having excess P-glycoprotein keeps chemotherapy drugs from reaching the tumor; having a mutant/non-functional protein fails to keep medications like Ivermectin out of the central nervous system. In other words, Collies (and their cousins: Old English Sheepdogs, Shetland Sheepdogs, Australian Shepherds, etc.) have less P-glycoprotein than normal. Having less P-glycoprotein means certain drugs gain access to protected body tissues more readily. This phenomenon is responsible not only for Ivermectin toxicity in sensitive breeds but also Loperamide toxicity. Loperamide is the drug in Imodium.

Approximately 35% of Collies appear affected by this condition. It is probably best to avoid Loperamide in Collies and their relatives. Imodium contains a small amount of Ivermectin, and therefore, could cause your dog to go into toxic shock and 99% of them do not survive.

FROM The Washington State University Veterinary School website: It is well known that Collies and related breeds can have adverse reactions to drugs such as Ivermectin, Loperamide (Imodium®), and others. It was previously unknown why some individual dogs were sensitive and others were not. Advances in molecular biology at the Veterinary Clinical Pharmacology Laboratory at Washington State University's College of Veterinary Medicine have led to the discovery of the cause of multi-drug sensitivity in affected dogs. The problem is due to a mutation in the multi-drug resistance gene (MDR1). This gene encodes a protein, P-glycoprotein, that is responsible for pumping many drugs and other toxins out of the brain. Dogs with the mutant gene can not pump some drugs out of the brain as a normal dog would, which may result in abnormal neurologic signs. The result may be an illness requiring an extended hospital stay--or even death.

A test has recently been developed at Washington State University to screen for the presence of the mutant gene*. Instead of avoiding drugs such as Ivermectin in known susceptible breeds, veterinarians can now determine if a dog is normal, in which case the drug can be administered or abnormal, in which case an alternative treatment can be given.  Owners and breeders can submit samples for testing. All that is needed for the test is a cheek brush sample that can be obtained by the owner and sent by mail for analysis.

Affected Breeds
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.

Ivermectin is not approved to be used in this manner.  So your veterinarian should let you know this prior to getting your permission to utilize Ivermectin therapy in the treatment of scabies in dogs and cats.  You can retain some confidence to know that it is in common usage, and has been for a number of years, for the treatment of scabies in pets.  It simply has not been tested by the manufacturer and approved by the FDA to be used in this manner.  You are on your own, you and your veterinarian, in the decision-making arena regarding whether or not to use it to treat sarcoptic mites.   Your alternative is to use insecticide dips and sprays.  Additionally, if the wrong dosage is given, the pet can have a very serious and even fatal reaction to Ivermectin.   The correct dose MUST be given and great care taken not to give too much.

All pets in contact with an affected animal should be treated since there can be asymptomatic carriers (they have the disorder but aren’t showing any signs of disease) of the sarcoptic mites. Pfizer Animal Health has released a new product called Revolution that is approved for use on dogs for the elimination of sarcoptic mites.  Be sure to ask your veterinarian about this.