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Smooth Collies are one of the healthiest of dog breeds but it has long been recognised that they in common with a number of herding and hound breeds are particularly sensitive to a range of drugs.  Such dogs lead perfectly long, happy and healthy lives as long as they are not given certain drugs which are known to cause a problem by crossing the blood-brain barrier and attacking the nervous system.  The condition is known as Multi Drug Resistance-1 (MDR1).  Some vets remain unaware of the problems that some drugs can cause and we together with the Rough Collie Breed Council and its constituent clubs have developed a range of information to make owners, breeders and vets aware of the condition.
 
The problem first came to light in 1983 when several Collies died from Ivermectin poisoning and, since then, the veterinary profession has accepted this drug should never be given to Collies. It used to be prescribed to treat mange and heartworm.  More recently a Rough Collie died from eating horse faeces (Ivermectin is commonly used for worming horses and cattle and any excess drug passes out with their faeces), and another Collie may have died from an excess of sedative.

Researchers have since found that approximately 60% of Rough and Smooth Collies appear to be susceptible not only to Ivermectin, but to a wide range of other drug substances. The MDR1 (multi-drug resistant) gene is responsible for enabling the body's blood-brain barrier to function normally. In healthy dogs the brain and central nervous system are protected by the 'blood-brain barrier', which prevents high drug concentration from circulating in the blood stream.

However, in MDR1-affected dogs the function of the blood-brain barrier is compromised allowing certain drug compounds to leak into major organs like the liver, or into the central nervous system, causing toxic reactions and even death. Symptoms include excessive salivation, ataxia, blindness, coma, and respiratory problems.

An MDR1 Normal dog (+/+) receives a healthy MDR1 gene from each of its parents and can therefore only pass on healthy genes to its offspring. The healthy + genes are dominant and such animals do not exhibit drug toxicity.

A 'Carrier' (+/-) is a dog that has received a normal [dominant] MDR1 gene from one of its parents, and a defective gene [recessive] from the other parent which is 'carried' by the dominant + gene. Please bear in mind that a carrier can pass either a normal or a defective genes onto its offspring resulting in approximately 50% of the puppies inheriting a defective MDR1 gene. Theoretically the 'carrier' animal should not be susceptible to drug toxicity but unfortunately the dominance of the MDR1 + gene has been found to be incomplete as some 'carrier' animals also appear to be susceptible to high doses of those drugs that cause problems in MDR1-affected dogs, that is, those with two defective MDR1 genes (-/-).

An Affected dog (-/-) receives a defective or mutant MDR1 gene from both its parents, so such dogs are double recessive and will display toxic reactions to a wide range of drug compounds (see list below). In 2007 a genetic test was made available for MDR1 and so it is extremely important that breeders try and use Normal (+/+) dogs in their breeding programmes so as to eliminate the defective MDR1 genes as soon as possible.
 
At a Rough Collie Breed Council seminar in February 2011, speakers Simon Tappin and Jeff Sampson told delegates that breeders must strive to eradicate the condition wherever possible but that this would in all probability take 30 years.  They recognised that the condition can be controlled by proper management and breeders need not exclude affected dogs from their breeding programmes.  This common sense approach was welcomed by Smooth Collie delegates - our small gene pool means that if we were to exclude affected dogs from breeding plans, breed type would very quickly be lost.

If you have an MDR1-affected Collie (-/-) or carrier (+/-) you could be in a position to save its life by providing your veterinary surgeon with a copy of its MDR1 Certificate and the list of drug compounds that your Collie should never be given. Of those vets who have already been asked to put MDR1 test results onto a Collie's records, all have been aware of the Ivermectin problem but have had no idea about the broad spectrum of drug compounds that can severely threaten an MDR1-affected dog. 

Please note there are usually alternative, safe drugs your Collie could be given instead.

The table here shows three classes of drug compounds: Class A includes substances that have been proven to pass through the blood-brain barrier in MDR1-affected dogs and cause problems; Class B lists substances which have shown interactions in animal tests, whereas Class C substances can be given without problems, even to affected dogs:
 

Latest update:  27 August 2010