Final answer:
Ivermectin should not be given to ivermectin-sensitive collies as it can cause neurological toxicity. The correct answer is 2.
Step-by-step explanation:
Ivermectin should not be given to ivermectin-sensitive collies. Collies and some other breeds have a genetic mutation that makes them particularly sensitive to ivermectin, an anti-parasitic medication commonly used in dogs. This mutation affects the blood-brain barrier, allowing ivermectin to enter the brain, leading to potential neurological toxicity.
Instead, alternative medications or treatment methods should be used in ivermectin-sensitive collies to avoid any adverse reactions. It's crucial to consult with a veterinarian who can provide appropriate guidance and prescribe safe alternatives for treating parasites in these dogs. Therefore the correct option is 2.