MDR1 :

What is MDR1:

In the normal healthy dog, the brain tissue and central nervous system are protected against high concentrations of drugs circulating in the vascular system. The "Multidrug Resistance gene 1" (the MDR1 gene) plays an important role in the barrier between blood capillaries and brain tissue. It encodes a large transmembrane protein, P-glycoprotein, which is an integral part of the blood-brain barrier. P-glycoprotein functions in a protective capacity to transport a variety of drug substrates, including Ivermectin, from brain tissue back into capillaries.
The phenomenon of "Ivermectin toxicity in Collies" was first described in 1983. Ivermectin (an antiparasitic agent) causes neurologic toxicity in some, but not all Collies, at doses that are 1/200th of the dose required to cause toxicity in other dogs. Neurologic manifestations of Ivermectin in susceptible dogs include hypersalivation, ataxia, blindness, coma, respiratory compromise, and death.
Dogs suffering from oversensitivity for Ivermectine have been documented to be susceptible to neurotoxicity induced by many other drugs as well.

Breed policy:

Of the utmost importance is the control of hereditary disorders in breeding programs. Thus, in order to reduce the frequency or eliminate altogether a genetic defect, the further spread of the mutant gene has to be prevented in a family or entire breed. When we find an inherited disorder in our population (in our breed), especially when the frequency of affected animals is quite high, most of us would want to exclude all animals that carry the defective allele on the shortest possible term.
That's not always the best thing to do. Eliminating all affected animals and all carriers from our breeding programs in just one generation may result in a tremendous reduction of the breed's gene pool, thus causing an enormous increase of the level of inbreeding. As a consequence problems with the vitality of the population will increase and we probably will see outbreaks of other genetic disorders.
Each dog with the defective allele has many good and important genes that may be worthwhile to preserve in the population. Selection programs should be carried out slowly and carefully, taking a sufficient number of generations to prevent overselection. That is where the breeders' organizations should take their responsibility towards their breeds. With the availability of a DNA test for disorders like the MDR1 defect, selection programs can be extended to the number of generations needed to make sure that no harm is done to the breed's gene pool.
Carriers can still be bred to clear dogs, thus avoiding the birth of highly susceptible affecteds. Pups that will be used for breeding the next generation can themselves be DNA tested to determine whether they are clear or carrier.
Of course, where the MDR1 defect is involved, extra attention should be paid to the heterozygous animals (the carriers, MDR1/mdr1-1?). There have been reports of a (limited) susceptibility for certain drugs. So whenever medication is required, checking the latest updates of the listings of "dangerous drugs" is advisable.