Hello, thanks for your question today! I am Dr. Drew, and I am a licensed veterinarian. I'll be happy to help you in any way I can.
There is no reason why Advocate can't be used in Collies.
UNfortunately, the information you received regarding this issue, is inaccurate, and based on the sensitivity some collies and other breeds have to related drugs such as Ivermectin.
During safety trials, ivermectin-sensitive Collies were tested at more than 25 times the recommended dose of Advocate, with no ill effects.
it's a good product, and there's no reason to avoid its usage.
Why then have I been told that my dog's erratic behaviour is likely to have been caused since using this substance
I can't explain why you were told that - but it would not be an expected result of using this product.
now, an allergic reaction "could" occur in any breed,
but it would not be the "MDR" mutation that makes collies sensitive to certain drugs.
How can you be so certain that this drug has NOT caused sensitivity?
Your question was "why should Collies not be given Avocate treatment for fleas and ticks?". I explained that based on safety trials, there's NO reason why. Now, this doesn't mean an individual patient may not have an issue with ANY medication. Anything is possible, each dog is an individual just like each person.
I didn't say it's impossible that the drug was the cause, but there is NO reason why the entire breed should not receive Advocate.