Multidrug Sensitivity in Dogs 


"What is Multidrug Sensitivity in dogs?"

Some dog breeds are more sensitive to certain drugs than other breeds. Collies and related breeds, for instance, can have adverse reactions to drugs such as ivermectin and loperamide (Imodium). 

Drug sensitivities result from 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 cannot 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.

"How do I know if my dog is affected?"

Approximately three of every four 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, English Shepherds, 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.

"What drugs should I look out for?"

Many different drugs and drug classes have been reported to cause problems in Collies and other herding breed dogs that carry the MDR1 mutation.  We suggest that you link to the Washington State University web site which has a list of drugs that have been researched because information on these drugs changes often.

The most common drugs known to cause neurological signs are listed below.  We suggest that these drugs be avoided completely unless you are absolutely sure the amount to be administered is in the "safe" dosage zone.

Invermectin (antiparasitic agent).

Acepromazine (tranquilizer and pre-anesthetic agent).

Butorphanol (analgesic and pre-anesthetic agent).



Selamectin, milbemycin, and moxidectin (antaparasitic agents).

Vincristine, Vinblastine, Doxorubicin (chemotherapy agents).


Important: Please ensure that your vet is aware of the possible drug sensitivity of your dog.


For more details on these drugs, and others, and the amounts that are recommended to be administered please follow this link to:


Click here for the WASHINGTON STATE UNIVERSITY website


Note: The above infomation has been sourced from the Washington State University website Feb 2010, but due to ongoing research in this field the above information is subject to change at any time, we suggest therefore that you thoroughly research any drugs yourself, and are aware of their side effects before you administer them to your dog.


This is the most common of the Collie eye diseases, but it should be noted that Collies share this disease with several other breeds. Researchers first noted this problem nearly fifty years ago. It was later found to exist in most Collies and in most Collie families. This so-called syndrome, meaning a group of conditions which appear in conjunction with each other, is present prior to birth. Collie Eye Anomaly can be easily checked when the puppies are 5-6 weeks old, by a qualified Ophthalmologist. It is something that your average Veterinarian is not qualified to diagnose. The eyes must be dilated prior to the examination, so the interior of the eye can be examined with an Ophthalmoloscope.
A dog is either given a “Normal” or "Affected" rating. If given "affected" the type of abnormality is noted. It used to be popular to use a Grading System and still is being used by some Ophthalmologists. Currently there is no universal, standardized grading system.
Normal: A “Normal” eye rating is of course the best grading there is. (There are also "Go Normals", which are so mildly affected at a young age, that later, the pale areas disappear, leading to what is termed a "Go Normal". Keep in mind that these are still in fact affected with CEA). There are variations even in "Normal" eyes. These correspond somewhat to a dog's coat color. Thus it is often difficult to judge the pigment in a Blue Merle's eyes as it is diluted along with his coat color.
Choroidal Hypoplasia, Chorioretinal Change: These refer to abnormalities in the coloring or pigmentation of the choroid or central layer of the eye's lining. This is the most common abnormality found in Collie eyes. Often referred to as mild CRC, Grade I or Grade II, it is the least harmful and least severe form of CEA. Most dogs with this eye grade function normally with no ill-effects or loss of vision.

Staphyloma, Coloboma, Ectasia: While not completely synonymous, these terms all refer to a cupping or bulging in the eyeball usually in the area of the optic disc.

Vascular Disease, Tortuous Blood Vessels: Defects in the vessels of the eye which are responsible for its blood supply or "nourishment." These may be malformed, undersized, or even lacking.

Retinal Detachment: Loosening or separation of the inmost, or retina, layer from the wall of the eye. This may involve a tiny area or the entire retina. It can be either one or both eyes. The complete detachment of the retina results in blindness in that eye.
There is a new Genetic Test for Collie Eye Anomaly / Choroidal Hypoplasia.

Can the Collie's eyes become worse? Might he later go blind?
The basic answer is "No" as CEA is present prior to birth. However, a dog born with a severe Staphyloma or with Vascular Disease may later suffer loss of sight if a detachment or severe hemorrhage occurs. The majority of dogs that are slightly affected, will have perfectly adequate eye vision throughout their life. (Even a dog with one blind eye will adapt perfectly well in his surroundings.)
What have breeders done to improve Collie eyes?
When the eye problem was discovered more than 50 years ago, it was estimated that 90% of the Collie population was afflicted with some form of eye disease. Because CEA has involved such a large percentage of the breed, eradication has been slow.  ALL reputable breeders eye check not only their breeding stock, but all puppies that are offered for sale.
How is CEA inherited?
Most of the specialists agree that Choroidal Hypoplasia is carried as a simple recessive. For a dog to show symptoms, both parents, even if they show no signs themselves, must carry a gene for the condition. Evidence exists that some other parts of the syndrome are inherited differently. Staphyloma, for instance, rarely occurs except in the presence of Choroidal Hypoplasia. The ideal, of course, is to eliminate all but the clear, non-carriers, from the breed. Please note: Even among the dogs that examine "normal", most are carriers of the gene. They have a "hidden" or recessive gene for the condition and will transmit the gene to half its offspring.
Two other conditions not part of the CEA, which can occur, should be noted:

Hypoplasia of the Optic Nerve: An undersized nerve which is noted where it enters the eyeball. In extreme cases, this can cause blindness.

Corneal Dystrophy:This condition comes on when the dog is mature, often during stress. Opaque spots appear centrally on the surface of the cornea. (It is often confused by the layman with cataracts which occur in the lense.)

The other Collie eye problem that can occur in rare instances is Progressive Retinal Atrophy (PRA). Since the name is just what it implies, it can be a progressive disease, that may not appear until later in life. This is a completely different and unrelated disease to CEA. As the name indicates, PRA is a progressive disease which refers to retinal degeneration. It can result in complete blindness in one or both eyes. However, Collies seem to be blessed with the fact that PRA seems to have an early onset. Fortunately, this is an eye disease that has largely been eradicated thanks to breeders efforts of test breeding potential carriers.  Currently there is no genetic test for PRA, but rumor is that researchers are very close to finding a DNA marker.
PRA has proven to be a simple recessive in all the breeds studied. Again, this means that even though the condition is not present at birth, both parents must be carriers. If one parent has PRA, half the puppies may develop PRA, but all will be carriers for the disease. Early signs of the problem may be noticed by the owner as "night blindness." The dog has trouble seeing in dim light and will bump things.  An expert may detect early signs in the eye at six months or younger.
CEA and PRA information is sourced from Collie Health Foundation at

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