Life-cycle considerations, resistance, medication safety point to malathion tx

October 1, 2007

The efficacy of anti-lice treatments depends on the prevalence of louse resistance, whether the agent is pediculicidal only or also has ovicidal activity, and when it is applied relative to the louse life cycle. Those considerations together with its favorable safety profile point to malathion as the treatment of choice for head lice.

National report - Evaluation of head lice therapy must take into account the medication's activity and recommended administration regimen relative to the parasite's life cycle, prevalence of resistance and safety. Based on these parameters, malathion 0.5 percent formulated with isopropyl alcohol and terpineol (Ovide,TaroPharma) should be considered as the treatment of choice, says Jake Levitt,M.D.

Dr.Levitt is a clinical instructor in the department of dermatology, Mount Sinai School of Medicine, New York City, and a vice president of Taro Pharmaceuticals,Hawthorne, N.Y.

Discordance found

Underlying that discordance is the fact that only malathion has potent ovicidal and pediculicidal activity - the other agents are only reliably pediculicidal - and its predicted consequence is better efficacy for malathion when the products are used according to the label directions.

Malathion is the only anti-lice agent in the United States with no documented reports of louse resistance, which underscores its therapeutic advantage.

"Current guidelines on the treatment of head lice from the American Academy of Pediatrics (AAP) recommend permethrin 1 percent as the first-line treatment. Those guidelines were issued in 2002 and are due for a revision.

"The FDAhas intended malathion for first-line use in head lice,and available evidence considering the louse life cycle, resistance, safety considerations, and medication mechanism of action suggest that when updated, the AAP guidelines should list malathion as the treatment of first choice," Dr. Levitt tells Dermatology Times.

Understanding the life cycle

Understanding the life cycle of the head louse is the foundation for designing rational and effective dosing regimens for treating head lice, according to John Marshall Clark, Ph.D., University of Massachusetts at Amherst.

Dr. Clark developed a system for rearing lice in vitro and described the minimum and maximum duration of the various stages in the head louse life cycle.

The cycle begins with a newly laid egg (nit) without an eyespot (indicating absence of a nervous system) that is invulnerable to medications that act on the louse nervous system.Over a period of up to four days, the egg develops its nervous system (eyespot), which is vulnerable to some nervous system agents. It takes seven to 12 days fromwhen the egg is laid to when it hatches.Hatchlings are susceptible to medications with pediculicidal activity and take 8.5 to 11 days to lay new eggs, completing the life cycle.

"Rational planning of therapy needs to take into account whether an anti-lice agent is solely pediculicidal or also has ovicidal activity and whether the timing of application is coordinated with the louse life cycle," Dr. Levitt says.

Over-the-counter formulations of permethrin 1 percent (Nix) or pyrethrins 0.33 percent plus piperonyl butoxide 4 percent (Pronto Plus, Rid, A-200) are recommended to be applied on day zero and then repeated seven to 10 days later.Those agents are pediculicidal only, so the initial application eradicates susceptible adult lice but not any of the eggs, with or without eyespots.

"In theory, these agents should be used a third time after another week, once all of the eggs have had a chance to hatch,but before they develop into adults. Although no studies have been performed to verify that an additional treatment would enhance efficacy,"Dr. Levitt says.

Comparing treatment options

Lindane is pediculicidal and has inconsistent ovicidal activity. In theory, its optimal use would also require three treatments. However, because of its neurotoxic potential, lindane is recommended only to be applied one time.

Malathion with isopropyl alcohol and terpineol is labeled for application on day zero and again at day seven, if needed.As it is ovicidal and pediculicidal, it would be expected to kill the nits and adult lice with the first use, while the treatment at day seven would eradicate the eggs without an eyespot that have developed a nervous system or hatched.