The Interceptor Shortage – Today…

Nancy Kelso, D.V.M
Interceptor seems to have disappeared from the market completely.

Sentinel (Interceptor and Program, a flea growth regulator) is now available.

The main ingredient in Interceptor (milbemycin) is also available in Comboguard and Trifexis, with a flea prevention product, too, but no one is currently manufacturing milbemycin alone.

Interestingly, milbemycin (the active Interceptor ingredient) is also a p-glycoproteins substrate that can affect mdr1 mutant dogs adversely, but like the vast majority of heartworm preventatives used AT THE HEARTWORM PREVENTION dose, is safe.

With that being said, it is hard to leave your comfort zone when out of Interceptor.  A reaction with any medication is possible and should not be ignored, but at heartworm preventative doses, even ivermectin, is safe. Many collie kennels and collies have made a transition to ivermectin, and are happy (especially at the reduced cost of this product).  If you and your veterinarian choose to use ivermectin, it is suggested that you weigh your dogs and be precise about the dose. A reaction to heartworm prevention, including interceptor, is very rare, and fortunately mild and transient.  In the mid-atlantic area hundreds of doses of Revolution (selamectin) have been used on collies, initially for the treatment of scabies, and now as primary heartworm prevention. Revolution is heartworm prevention, as well as a flea, tick and scabies mite treatment (common in the mid-atlantic region). Discuss with your veterinarian and use the heartworm prevention you are comfortable with, any heartworm prevention is far better than the alternative of getting heartworms. Interestingly, ivermectin is also part of the treatment of heartworms.

All heartworm prevention products can be good alternatives but you need to partner with your veterinarian to decide what is best for your collie. Proper dosing is critical.

MDR1 testing, it’s not just about heartworm prevention:
Unfortunately, some vets are still unaware, and need educating.  After recently speaking at a regional meeting on mdr1, and it was amazing how many vets were still unaware, or who now understood why a dog in the past reacted or acted oddly after prescribing a drug listed on the Washington State University list (see For now it will continue to be the owner’s job to inform their veterinarians.

One good thing that CHF hopes will come from the interceptor dilemma is that more people do MDR1 testing.  Rather than assuming and accepting that all collies are mutant/mutant, do the test! Statistics say 1/3 of collies are normal/normal, 1/3 mutant/normal, and 1/3 collies mutant/mutant. The significance of testing goes past heartworm prevention to include how to dose many other drugs.

Nancy Kelso, D.V.M.
Chairman Collie Health Foundation Grants Committee
Medical Director VCA Columbia Animal Hospital

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