Dogs on drugs: FDA warns of dangerous drug interaction
By Christie Keith
June 26, 2008
Dogs who are taking the flea preventive drug Comfortis (spinosad) at the same time they are being given high dosages of the drug ivermectin, such as those used in the treatment of demodetic mange, are at risk of ivermectin toxicity.
The maker of the drug, Eli Lilly’s companion animal health division, does not believe that there is any risk to using the regular heartworm-prevention dose of ivermectin with Comfortis; the daily dosages used to treat some stubborn cases of demodetic mange are as much as 100 times the monthly dosage used to prevent heartworm infection. In an informational release, the company cited a supporting field study that involved hundreds of dogs (PDF file):
The administration of Comfortis and approved canine formulations of ivermectin at doses labeled for heartworm prevention has been tested and shown to be safe, including in a North American field trial involving over 450 dogs that were required to be on monthly heartworm prevention throughout the three-month study. Laboratory work has found that, even at doses of 5 times the monthly dose of spinosad combined with 10 times the monthly dose of milbemycin oxime in ivermectin-sensitive collies, there were no signs of neurotoxicity (Sherman et al., publication pending).
The FDA Center for Veterinary Medicine is advising that the two drugs not be prescribed at the same time.
Ivermectin toxicity can be fatal and requires immediate veterinary care. Early signs include vomiting, weakness, drooling, tremors and coma. Dogs also often become blind, although this is usally reversible.
With aggressive veterinary care including hospitalization with round-the-clock nursing, most dogs will recover.
High-dose use of ivermectin is considered “extra-label,” which means a use of an approved drug to treat an illness for which it has not been approved. Such use is legal under the Animal Medicinal Drug Use Clarification Act of 1996; without extra-label prescribing, half or more of the drugs used in veterinary medicine every day would be unavailable, including most antibiotics.
Is this a problem only for dogs who have the MDR-1 mutation, or for all dogs?
Comment by H. Houlahan — June 26, 2008 @ 6:59 pm
All dogs. But I can’t help but think dogs with the MDR-1 mutation would be at greater risk. I believe there is a study underway to look at this issue more carefully now that this interaction has been identified.
Comment by Christie Keith — June 26, 2008 @ 8:47 pm
My first thought when I read this was that MANY people using “extra-label” Ivermectin for HW prevention are mistakenly using the mange dose which is a much higher dose than needed for HW prevention. I see this all the time on various boards. Granted, they are overdosing monthly - not daily - but OTOH they are unaware they are overdosing.
Comment by slt — June 27, 2008 @ 4:00 am
Agreed about the off-label ivomec. I can’t tell you the number of times that, when confronted with the math (here’s the dose in mcg per kg of Heartguard; here’s how you dilute the ivomec to get to that dose, or close to it, look I’ve shown my work), the guy who is giving his Sheltie a full ml of cattle ivomec every month will object that MY VET TOLD ME THIS IS THE DOSE.
FWIW, I do the off-label ivomec, and don’t go through the bother of diluting with glycerol of propylene glycol. Every dog gets an overdose — one small drop of ivomec on a biscuit. But every dog I own has either been tested, or is known by parentage, to be MDR1 normal/normal. Including the German shepherd! I keep Heartguard on hand for the rescue fosters that come in with unknown pedigree for MDR1, or are known to be at risk for the mutation.
If Heartguard wasn’t overpriced on such a robber-baron scale, the bottle of cattle wormer in the fridge wouldn’t be such an obvious solution for so many rural dog owners. I just wish they’d do the math instead of substituting the letters “DVM” for research, and I wish they’d get at least dogs from at risk breeds or suspect mixes tested for their mutation status.
(I’m also a bad, bad, disobedient consumer, and buy the largest vials of Frontline from Australia, then carefully measure with an insulin syringe and split the doses between 2-3 smaller dogs. Cuts my tick control costs by well over half.)
Comment by H. Houlahan — June 27, 2008 @ 7:20 am
I do similar to what you do H and for the same reasons. And yes, have personally corresponded with (or seen on boards) several Vets who give the mange Ivomec dosage for HW prevention. Now some people are concerned that HWs are developing a resistance to Ivermectin and I wonder if that goes beyond the usual “parasites eventually develop resistance to drugs in widespread usage” and crosses into so many people overdosing their dogs as a contributing factor.
Comment by slt — June 27, 2008 @ 7:35 am
When I was visiting my rancher friend in Texas, we went to the feed store. (Yeah, party down in ranch country!)
For the first time I saw Ivermectin in bottles face to face. (Our citified local feed store doesn’t carry it.) And I thought, as I always do, that it would be better for veterinarians to help their clients skip the brand name and protect their pet. ESPECIALLY in poor rural areas with bad heartworm problems, like where I was in Texas.
Comment by Gina Spadafori — June 27, 2008 @ 8:16 am
My dog Benny is taking Ivermectin for congenital parasites, he also received a dose of comfortis last night (both prescribed by vet). This morning Benny was panting, drooling, and ataxic, after an emergency visit to the vet I was told these meds should not be given together.
Comment by Pam Goins — June 29, 2008 @ 12:23 pm
I hope Benny recovers and is okay!
You should report this as an “adverse event”. Here is a page on it:
http://www.avma.org/animal_hea.....events.asp
Comment by The OTHER Pat — June 29, 2008 @ 1:48 pm