Oral Ivermectin Therapy for Phthiriasis Palpebrum

Pediculosis pubis, as well as phthiriasis palpebrum, is caused by the cosmopolitan crab or Pthirus pubis. This wingless, blood-sucking insect not only infests eyelashes and pubic regions, but occasionally inhabits the hair of the face, axillae, chest, and rectal regions. Unlike the head louse, the pubic crab has a serrated surface on its first tarsal claws, allowing this organism traction on flat, hairless surfaces.¹ Thus, this louse species can navigate over the entire skin surface. Although docile when exposed to light, P pubis can be rather mobile in the dark. These characteristics explain the prominent role that fomite transmission plays with crab lice, as well as its occasional migration to the eyelids.

Pubic lice have been challenging to eradicate, as they often inhabit several hairy areas on individual patients. Lice need host hair to cling to when sleeping, and for attachment of their eggs. In children, the tendency to inhabit eyelashes relates to specific temperature and moisture requirements, as well as the lack of terminal hairs on most body regions in children of prepubetal age.² Thus, topical therapy may fail if one does not apply insecticide to all hairy areas, including the perirectal region.

Numerous insecticide formulations in aqueous vehicles and applied with cotton sticks have been suggested for phthiriasis palpebrum, including lindane,³ malathion,⁴ and pyrethrin.⁵ Additionally, various other reported treatments for pediculosis ciliaris include mechanical removal with fine forceps,⁶ yellow mercuric oxide eye ointment,⁷ cryotherapy,⁸ 20% fluorescein solution,⁹ physostigmine ointment,¹⁰ petroleum jelly,¹¹ and argon laser phototherapy.¹² Alcoholic formulations should be avoided owing to ocular irritation, and allergic reactions can occur in people sensitized to excipient components. Of note, delayed corneal damage has been reported with insecticidal use.¹³

Oral ivermectin (Stromectol; Merck and Co Inc, Whitehouse Station, NJ) has been purported to be the solution to pubic lice,¹⁴ but to our knowledge no case reports of its use for phthiriasis palpebrum have been reported. For this reason, we report our experience with 4 cases of phthiriasis palpebrum.

Four patients, ranging in age from 3 to 10 years, had complaints of itching and irritation of the eyes for several weeks. On clinical examination, all revealed reddened, crusty lid margins with grayish discoloration at their bases. On closer examination, numerous nits and motile parasites were noted on the eyelashes. All 4 patients had bilateral involvement, with 1 additionally having infestation with crab lice in the scalp hair. Three of the 4 patients had been previously unsuccessfully treated with 1% gamma benzene hexachloride shampoo and topical physostigmine. All patients were treated with two 200-µg/kg doses of oral ivermectin (Stromectol) given a week apart. No topical treatments were applied to the eyelids.

All 4 patients were cured with this therapy. All adult lice were eradicated within 2 days, while the nits remained attached to the eyelashes but dissipated during the following month as the eyelid hairs grew out.

Ivermectin is an anthelmintic agent that has proved to be an excellent antiparasitic drug for veterinary medicine since its introduction in 1981. Approved by the Food and Drug Administration for human use for onchocerciasis and strongyloidiasis in 1996, ivermectin has also been effective for loiasis, bancroftian filariasis, cutaneous larva migrans, scabies, and lice.^14,15 Ivermectin acts by blocking chemical transmission across nerve synapses that use glutamate or γ-aminobutyric acid (GABA). Invertebrates, like lice, are selectively paralyzed by ivermectin, as glutamate and GABA are the neurotransmitters for peripheral motor function. The neuromuscular system in humans does not operate via glutamate or GABA, and thus is not affected by the presence of this drug.

As ivermectin has a half-life of 16 hours, a single dose of ivermectin does not allow sufficient drug levels in the bloodstream to kill nymphs as they hatch from their nit capsule 1 week later.¹⁶ Pubic lice feed on human blood several times during each day and young lice require a blood meal soon after hatching from the egg.² Ivermectin has no ovicidal activity, and requires the louse to obtain the drug via blood meals. Similar to our experience with pubic lice in the groin, 2 doses of ivermectin, given a week apart, are required to eradicate phthiriasis palpebrum. There are 2 caveats with ivermectin. First, one needs to avoid treating persons weighing less than 15 kg, and to use caution in use in pregnant or breastfeeding women.^16,17 This restriction is based on the drug's ability to potentially cross poorly developed blood-brain barriers. Second, there has been one report of possibly increased incidence of death in elderly patients.¹⁸ However, there have been more than 10 million people treated worldwide for onchocerciasis and strongyloidiasis without any reported ramifications in older populations.¹⁷