When Standard Lice Medications Fail: Ivermectin Dosing and Other Options for Dogs

A Reader's Struggle With Persistent Lice After Importing a Dog

Marta wrote to us describing a frustrating situation: she brought a female dog in from Europe, and that dog carried biting lice to the rest of her animals. Despite treating with Revolution and Frontline, the infestation persisted. She had already steam-cleaned her home and replaced all the bedding. At the time of writing, she was attempting to use goat drench ivermectin and wanted guidance on a safe daily dose. She also expressed a strong preference for avoiding chemicals on her dogs and asked for any suggestions about these stubborn parasites.

Why Resistance May Be Developing

The fact that these medications are not working is somewhat unexpected, given how well they typically perform. For this kind of infection, Revolution is generally considered the first-choice product, and Frontline tends to be nearly as reliable.

Research has demonstrated that selamectin, the active component of Revolution, eliminated biting lice in every treated animal in one study, with no remaining clinical signs and no lice detected afterward. (1) Fipronil, the ingredient in Frontline, has likewise been shown to be almost as effective.

Both products have been on the market for over two decades, though. Because they have been widely used for flea control for so long, there are increasing anecdotal reports of them failing in some cases.

One possible explanation is that lower doses were used in kennel settings in Europe, the region Marta's dog came from, which could have allowed the lice to develop resistance.

Ivermectin Dosing Guidance

If neither selamectin nor fipronil has resolved the problem and Marta plans to try the sheep drench formulation of ivermectin, the amount used is typically the same as what is given for sarcoptic mange.

When injectable ivermectin was tested at 200 micrograms per kilogram administered every 14 days, it proved highly effective. (2) In that study of nearly 300 dogs, every single one was free of infection by day 28, which is 14 days after the second dose. The same outcome would be expected in Marta's dogs.

Oral Use of Sheep Drench

A sheep drench labeled at 0.08 percent ivermectin provides 800 micrograms per milliliter. If the product on hand has a different concentration printed on the label, the dose below would need to be recalculated. Readers in that situation can submit another question for help with the math.

The typical amount for dogs is roughly 0.25 ml per kilogram. For Japanese Chins weighing around 3 kilograms, that works out to 0.75 ml, or three-quarters of a cc.
Administer the first dose immediately, then repeat two weeks later.
Should the lice still be present after the second treatment, a third dose two weeks after that would be reasonable.

Safety at the Correct Dose

At this dosage level, ivermectin is not toxic and does not cause issues in most dogs. There is an inherited genetic sensitivity to ivermectin in herding breeds, German Shepherd Dogs, and Skye Terriers, but Japanese Chins have no reported problems of this kind.

In studies involving demodectic mange, a particularly stubborn mite, oral ivermectin was given at 0.6 mg/kg for two to six weeks. (3) That dose is three times higher than the one used for sarcoptic mange and was administered daily for several months rather than every two weeks, yet the dogs experienced no adverse effects.

This infestation should resolve without much difficulty. If it does not, older approaches such as insecticide shampoos and lime-sulfur dips remain available.

Non-Chemical Measures That May Help

Since Marta wants to limit harsh chemicals, several less potent options have shown some effectiveness and can be used alongside the ivermectin.

Lice comb: Combing will not eliminate every louse, but it removes the majority of the eggs.
Shaving: This is a bigger commitment, but once the coat is shaved, nits have no hair to cling to and adult lice are far easier to kill. The dog's appearance will change for several months, though the infestation can be brought to an end.
Bathing and oiling: Wash the dog to remove live lice, then apply an oil to smother any remaining parasites. Nearly any shampoo works if the dog is shaved or has very short hair. For long-haired dogs, a medicated shampoo left on for 5 to 10 minutes is needed so it can penetrate. After bathing, olive oil or coconut oil can be applied to suffocate lice the shampoo missed. This is best done outdoors, since the oil will create a mess indoors.

References

(1) Shanks DJ, Gautier P, McTier TL, Evans NA, Pengo G, Rowan TG. Efficacy of selamectin against biting lice on dogs and cats. Vet Rec. 2003 Feb 22;152(8):234-7. doi: 10.1136/vr.152.8.234. PMID: 12625538. i.nlm.nih.gov/12625538/
(2) Scheidt VJ, Medleau L, Seward RL, Schwartzman RM. An evaluation of ivermectin in the treatment of sarcoptic mange in dogs. Am J Vet Res. 1984 Jun;45(6):1201-2. PMID: 6377991. i.nlm.nih.gov/6377991/
(3) Ristic Z, Medleau L, Paradis M, White-Weithers NE. Ivermectin for treatment of generalized demodicosis in dogs. J Am Vet Med Assoc. 1995 Nov 15;207(10):1308-10. PMID: 7591924. i.nlm.nih.gov/7591924/

This article is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from your veterinarian. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.