Multi-drug Sensitivities in Dogs

A number of dog breeds are more prone to serious side effects from certain drugs than other breeds. For example, Collies can have severe adverse reactions to the antiparasitic drug, ivermectin.
The cause of this sensitivity is a DNA sequence change (mutation) in the dog’s multi-drug resistance gene, known as the MDR1 gene. The function of this gene is to prevent dangerous drugs from entering the CSF. Dogs with the mutated MDR1 gene have an alteration in the blood brain barrier affecting transport of drugs such as ivermectin. Exposure to these drugs may result in serious neurological signs, such as hypersalivation, ataxia, blindness, tremor, respiratory distress and even death.
As well as protecting the brain, the MDR1 gene plays a vital role in drug elimination. Dogs that have a
mutated MDR1 gene can have reduced drug elimination compared to others, resulting in elevated plasma drug levels and an increased tendency for toxicity.
A number of drugs have been shown to cause problems in dogs with a mutated MDR1 gene.
These include:
Acepromazine (tranquilliser)
Butorphanol (analgesic)
Cyclosporin (immunosuppressant)
Digoxin (cardiac inotrope)
Doxorubicin (antineoplastic)
Ivermectin (antiparasitic)
Loperamide (antidiarrhoeal)
Vinblastine (antineoplastic)
Vincristine (antineoplastic)
(Please note that this is not a complete list of drugs that may affect dogs with the MDR1 gene mutation)

Approximately 75% of Collies in Australia have the mutated MDR1 gene. The mutation has also been found in Shetland Sheepdogs (Shelties), Old English Sheepdogs, German Shepherd Dogs, Long-haired Whippets and a variety of mixed breed dogs.

You can test your dog for multidrug sensitivity and prevent serious adverse drug reactions.

Washington State University
Gribbles Molecular Science and Gribbles Veterinary Pathology


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