Treatment of Pediculus Humanus Var. Capitis Infestation in Cowlitz County, Washington, With Ivermectin and the Licemeister® Comb

Head lice have developed resistance to permethrin,1, 2 the most popular product for their treatment in the United States. Resistance probably occurred in Cowlitz County, WA, in the mid-1990s and was confirmed by an in vitro assay in late 1997 (unpublished observations). With the advent of ivermectin in the United States, a new treatment option became available. I began to treat patients with ivermectin in April, 1997, and here report my experience with it and with the LiceMeister® comb, which has 32 closely spaced, rigid stainless steel teeth in an array that is 47 mm wide and 37 mm long and is superior to plastic and aluminum nit removal combs (unpublished observations). Methods. Cowlitz County is a quasirural area located approximately 70 km north of Portland, OR. The population centers of Longview and Kelso are beside the Columbia River and have large mills. In the mid-1990s the Cowlitz County Health Department began to receive calls from frustrated families about their inability to delouse themselves. In April, 1997, I started to treat patients with ivermectin sporadically at the county health department. In late June, 1997, I began a clinic for treating with ivermectin. Patients were accepted to the clinic only if they had persistent infestation after standard treatments, mainly 1% permethrin creme rinse, but sometimes with lindane, pyrethrin plus piperonyl butoxide, olive oil, mayonnaise, petroleum jelly and other nostrums. Most patients/subjects had used many treatments for periods of weeks to months. I examined patients with a hand lens to confirm infestation. I generally tried to treat the whole family, but sometimes only some members chose to be treated because others, mainly adult men, thought they were not infested. Subjects paid for ivermectin and combs themselves or occasionally through third party payers. Thus cost was a deterrent to treatment for many families. The dosage of ivermectin was chosen according to the schedule used for onchocerciasis in the package insert. Generally the dosage was ∼0.2 mg/kg of body mass. I routinely prescribed only one dose, in increments of one-half tablet. For those who had moving lice 24 h after treatment, I prescribed a second dose. As LiceMeister® combs (National Pediculosis Association, Newton, MA; Patent DES353915) became available, I encouraged patients to buy and use them. Eventually I required patients to buy a LiceMeister® before I would prescribe ivermectin. Patient subjects bought the ivermectin at local pharmacies at a cost of ∼$US 9.00 to $15.00 per 6-mg tablet. LiceMeister® combs were also bought at local pharmacies or by mail order, at ∼$15 to $20. I defined cure by self-report via telephone. Results. I prescribed ivermectin for 128 persons. I could contact only 66 by telephone, of whom 13 had not purchased ivermectin for various reasons, including cost and reluctance to use an "experimental" treatment. One reported cure without ivermectin. I report the details of the remaining 52 evaluable patient subjects. The mean time from the clinic visit until the last telephone call or until relapse/reinfestation was 42 days, with a median of 14.5 days and a range of 2 to 219 days. One dose reportedly cured 38 (73%) of the patients. Another 4 reported cure by a second dose 1 or 2 days later. One was still infested with live lice after 2 doses. Seven relapsed or were reinfested and were difficult to evaluate. Most had not used LiceMeister® to remove nits. All 7 were initially seen in the first 2 weeks after institution of the weekly clinic. I compared characteristics of those who were apparently cured by one dose and of those who were not. Some data were not available at the time of review. I did not perform statistical tests because the cured and persistently or reinfested subjects were not randomly selected (Table 1). Those subjects who were cured did not differ in obvious ways from those who were not.TABLE 1: Comparison of dosages of ivermectin and ages of subjects by cure status after single dose Discussion. A single oral dose of ivermectin was generally effective for the treatment of head lice, especially when used in combination with a LiceMeister® comb and when the whole family was treated. About 15% of patients required a second dose 24 h after the first. Despite use of ivermectin to treat millions of persons for onchocerciasis in areas where head lice are common, only a few studies of its use as a pediculicide are available. Dunne et al.3 found little or no difference in the prevalence of lice during an observation period of as long as 2 months after treatment in Sierra Leone. This provided ample time for reexposure, and nit removal probably was inadequate. Glaziou et al.4 treated Tahitian children with a single dose and found only 27% to be free of nymphs and adults after 14 days, and 31% after 28 days. Reexposure was probable and nit removal was not described. Oddly none of the nymphs alive 14 days after treatment seem to have matured in the ensuing 14 days. Youssef et al.5 treated Egyptians with topical ivermectin and claimed complete killing of all stages, including nits. The optimal dosage for treatment of head lice with ivermectin requires more study. A second dose 10 days after the first has been recommended,6 but my experience shows that this is unnecessary for most patients and ill advised for others, whose surviving lice could infest other persons in the interim. Orally administered ivermectin affects lice only when they feed on their hosts, so nits must be removed mechanically. The duration of the effect of oral ivermectin on laboratory-adapted body lice fed on rabbits is at least 3 days, after which the effect wanes and disappears 6 days after the dose.7 A sublethal residue of a pediculicide may promote resistance.8 This can be minimized by total nit removal and treatment of intimate contacts who have subclinical infestation. Acknowledgment. This study received no commercial sponsorship. Thomas A. Bell, M.D., M.P.H.