A certain sub-population of collies is more susceptible to certain wormers and other drugs than non-affected collies. This article attempts to explain why this is, which drugs to avoid or use with caution and how to get your dog checked for the drug sensitivity.

Ivermectin

In the 1980s a new wormer for cattle was launched containing ivermectin. The wormer was very effective in cattle and had the bonus of also treating lice and other ectoparasites. Reports of deaths and adverse reactions in collies following administration of ivermectin soon appeared. The ivermectin was causing neurotoxicity in a small population of collies. These dogs were 200 times more sensitive to the ivermectin that other breeds. Levels of ivermectin in the brains of affected dogs were very much higher than in non-sensitive collies.

P-glycoprotein

A leakage through blood –brain barrier was suspected. It was not until 1994 that the absence of a particular glycoprotein (P-glycoprotein) that affects permeability was identified as the reason for the increased sensitivity. The coding for this glycoprotein is controlled by a gene called the multidrug resistance 1a (MDR1a) gene. The defective gene has been traced back to a mutation in a single dog in UK before the different collie type breeds developed in the 1800s.

MDR1a Gene

All genes are carried on chromosomes that are paired. The defective MDR1a gene can be carried on one or both chromosomes. Dogs that have the gene on both chromosomes will be more affected than those with only one defective gene. Dogs with one or two affected genes are capable of passing on this hereditary disease to their offspring. A genetic screening test is available. This should aid breeders in eliminating this hereditary disease.


 

Breeds Affected

 

Table 1. Showing Results from a 2011 study showing prevalence of the defective gene in 5 common sheepdog breeds. As can be seen from the table, the prevalence of the defective gene is high in all these breeds but very high in Border Collies and Old English Sheepdogs.

 

As can be seen from the table, the prevalence of the defective gene is high in all these breeds but very high in Border Collies and Old English Sheepdogs.

Drugs Involved

Antiparasitic agents sold for Use in Dogs A lot of the ivermectin group of products is only licensed for use in cattle, sheep pigs and horses. Some however, are also licensed for use in dogs.

Drug JPEG

Summary

If you have a dog in the high-risk group, consider genetic testing to find out if your dog has the MDR1 gene mutation. Select worm and flea treatments very carefully. If you dog is going to the vet for sedation or a general anaesthetic remind the vet of your dog’s susceptibility to ACP and Torbugesic.