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Ivermectin Use in Dogs
Ivermectin is a widely-used anti-parasite drug that has been around for decades. At low doses it is effective against heartworm larvae, and in fact ivermectin is the active ingredient in Heartguard, the well-known heartworm medication. At higher doses, ivermectin is active against such things as demodectic mange, and even though ivermectin has not been approved by the FDA for this use, it is widely considered to be a safe and effective treatment for demodectic mange. In most patients, ivermectin is well-tolerated, and there are no significant side-effects.
However, in certain families of dogs, a particular gene mutation makes them react in an unusual way to ivermectin. In these individuals, we see things like dialated pupils, depression, tremors, staggering, siezures, and even death. At the low dose levels found in Heartguard, ivermectin is safe for use even in these susceptible dogs. However, at the higher doses used in the treatment of demodectic mange, signs of toxocity can occur.
Most dogs with the particular gene mutation are at least part Collie. In general, we do not use ivermectin at high doses at all in dogs known to be Collie, Border Collie, or Australian Shepherd. However, it is possible for a mixed-breed dog to have some Collie breeding and not look particularly like a collie. For this reason, in mixed breed dogs, we usually start ivermectin therapy slowly in a test-dose fashion.
In dogs of unknown genealogy, we start with a very low dose of ivermectin, one that it considered safe even in Collies. We give the first dose, and watch carefully for sign of problems, such as dialated pupils, depression, tremors, staggering, or siezures. If everything goes well, we give a little more the next day, and watch closely for any adverse symptoms. We keep increasing the dose a little each day until the maximum dose is reached, and then we continue at that dose until treatment is complete. If signs of toxicity develop, we immediately stop giving the ivermectin. Dogs medicated in this way generally do not develop severe symptoms if they do react at all, and they generally experience a full recovery in a short time.
It often takes not days or weeks, but months of therapy to cure severe demodectic mange. Ivermectin is given until we feel that recovery has been complete, however long that takes. Then a skin scraping is performed to see if there are any mites left. We do skin scrapings every two weeks until the dog has two negative scrapings in a row. Only when the dog has two negative scrapings in a row is it safe to stop ivermectin therapy.
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