Understanding Scabies and Lice
Drug-Resistant forms of scabies and lice have increased
In many parts of the world, including the U.S., drug-resistant forms of scabies and lice have been on the rise. Resistance is a concern with all available scabies and lice medications, even those more recently developed. The unpredictable nature of resistance and geographical variability further underscores the need for a variety of treatment alternatives to effectively manage these highly-contagious parasitic diseases—scabies, head lice and pubic lice (crabs).
Drug resistance makes scabies and lice medications less effective for some patients
The resistance of scabies and lice to available medications has increased in recent years, and has become an important cause of treatment failure.1–3 In some instances, cross-resistance has also been demonstrated, meaning resistance to one medication causes resistance to another.4 This further limits the number of viable treatment options for a given individual and even populations of people living in a particular area or setting. Click here to learn why second-line treatments are medically necessary for some patients.
Scabies and lice can develop drug resistance for a number of reasons:5
- Adaptability of the parasites themselves (i.e., “survival of the fittest”)
- Change in medication formulations
- Dilution of medication on wet hair
- Use of too little medication
- Use of too much medication
- Inappropriate use of medication as preventative therapy
Drug resistance patterns can vary from country to country, state to state, city to city and setting to setting, necessitating the need for a variety of treatment options.
Resistance has been reported for all commonly used scabies medications
- Studies in northern Australia show increasing resistance of scabies to permethrin3
- Scabies mites resistant to crotamiton have been reported6
- Scabies resistant to lindane treatment have been observed in North, Central, and South America, as well as in Asia7
- Recently, the first documented cases of ivermectin-resistant scabies were reported in patients with Norwegian (crusted) scabies8
Resistance is a concern with all commonly used lice medications
- A recent 2006 study using biochemical and molecular methods showed that 80% of head lice from a representative sampling of patients in Wales were resistant to pyrethroids (e.g., permethrin)9
- Head lice in the U.S. are significantly less susceptible to permethrin than those in South America10 or those in Borneo11
- Evidence of double resistance (e.g., resistance to both permethrin and malathion) has been documented12
- Cross-resistance between permethrin and pyrethrins (i.e., when resistance to one drug causes resistance to another due to their chemical similarities) has also been reported11,13
- An increase in the incidence of head lice in the U.K. since the late 1980s has been attributed to malathion resistance13
- Evidence of double resistance (e.g., malathion and permethrin) has also been documented12
- Malathion was unavailable in the U.S. for several years but was more recently reintroduced into the U.S. healthcare market14
- Note that the manufacturer of the version of malathion currently sold in the U.S. (Ovide®) states that: “there are no documented reports that U.S. head lice are resistant to Ovide® (malathion) lotion 0.5%” 15
- Nonetheless, development of resistance to malathion over time remains a concern14
- Pyrethrins with piperonyl butoxide
- Nash B. Extracts from “best treatment”: Treating head lice. British Med J. 2003;326:1256–1257.
- Hansen RC. Overview: the state of head lice management and control. Am J Manag Care. 2004;10(9 Suppl):S260–S263.
- McCarthy JS, Kemp DJ, Walton SF, et al. Scabies: more than just an irritation. Postgrad Med J. 2004;80:382–387.
- Roberts RJ. Clinical practice. Head lice. N Engl J Med. 2002;346(21):1645–50.
- Meinking TL. Clinical update of resistance and treatment of pediculosis capitis. Am J Manag Care. 2004;10:S264–268.
- Johnston G, Sladden M. Scabies: diagnosis and treatment. British Med J. 2005;331:619–622.
- Chosidow O. Scabies and pediculosis. Lancet. 2000;355:819–826.
- Currie BJ, Harumal P, McKinnon M, et al. First documentation of in vivo and in vitro ivermectin resistance in Sarcoptes scabiei. Clin Infect Dis. 2004;39:e8–e12.
- Thomas DR, McCarroll L, Roberts R, et al. Surveillance of insecticide resistance in head lice using biochemical and molecular methods. Arch Dis Child. 2006;91:777–778.
- Yoon KS, Gao JR, Lee SH, et al. Permethrin-resistant human head lice, Pediculus capitis, and their treatment. Arch Dermatol. 2003;139:994–1000.
- Pollack RJ, Kiszewski A, Armstrong P, et al. Differential permethrin susceptibility of head lice sampled in the United States and Borneo. Arch Pediatr Adolesc Med. 1999;153:969–973.
- Heukelbach J, Feldmeier H. Ectoparasites: the underestimated realm. Lancet. 2004;363:889–891.
- Downs AM. Managing head lice in an era of increasing resistance to insecticides. Am J Clin Dermatol. 2004;5(3):169–177.
- Ko CJ, Elston DM. Pediculosis. J Am Acad Dermatol. 2004;50(1):1–12; quiz 13–4.
- Ovide® (malathion) Lotion, 5% website. Available at: http://www.ovide4headlice.com/index_co.asp?siteID=257.
- Witkowski JA, Parish LC. Pediculosis and resistance: the perennial problem. Clin Dermatol. 2002;20(1):87–92.