The rough-coated collie, the smooth-coated collie and the Shetland sheepdog all originated in the UK, where heartworm is rare. But heartworm is present in most of the rest of the world, so any dogs of these breeds are susceptible to infection. According to the "Encyclopedia of Dog Breeds," however, these three breeds can be allergic to the preventive once-a-month medication ivermectin.
Before giving any preventive medications, test the collie to see if it is already infected with heartworms. If already infected, the collie will need other medications to kill the heartworms before it can be put on a preventive medication. Ivermectin sensitivity is caused by a recessive gene mutation known as "mdr1-1 delta." According to Cindi Bosshart, DVM, and Director of the Collie Health Foundation, only 35 to 45 percent of collies carry this gene. There is a test available to determine whether or not they have this gene. If they don't, then it should be safe to use ivermectin, which is considered to be the best heartworm preventive available, with fewer side effects in collies than other heartworm preventives.
Collies can receive a twice-a-year injection called ProHeart 6 that uses moxydectin as the active ingredient. However, the American Working Collie Club reports that some collies show an allergy to ProHeart 6. Stay in the vet's office for a half-hour after receiving a ProHeart 6 injection to be sure the collie isn't allergic. Symptoms include problems breathing, coordination problems, vomiting, sudden itchiness and seizures. Be sure the collie has been off any other kind of heartworm preventive medication for 30 days before getting a ProHeart 6 injection.
If ivermectin or moxydectin are not viable options, then daily pills can be given. The active ingredient in these is called diethlycarbamazine citrate, better known as DEC. Brand names include Filaribits, Carbam, Carcide, Difil, Dirocide, Hetrazan and PET-DEC. These come in chewable pills and oral tablets. The chewable type is best, since the collie may spit out non-chewable pills. DEC also prevents against roundworms as well as heartworms. DEC should be given with food in order to prevent nausea. It is important the pills are given every day, because even one missed dose can leave the collie open to infection.