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Advocates Urge Prevention Despite New Pesticide for Head Lice

(Beyond Pesticides, March 10, 2008) Head lice affect an estimated 12 million people in the U.S. each year, and are rapidly becoming resistant to over-the-counter and prescription medications. Researchers at the University of Massachusetts Amherst have found that ivermectin, a compound produced by soil-dwelling bacteria, was 100 percent effective in killing head lice resistant to many standard treatments. Results were published in the January 2008 edition of the Journal of Medical Entomology. Although ivermectin is not well-absorbed through the skin, some public health advocates are concerned about its use on humans for lice and scabies.

The National Pediculosis Association (NPA), a non profit agency, directs parents, health care professionals and child care providers to safer head lice control options via a standardized prevention approach focusing on routine screening, early detection and thorough combing and manual removal of lice and nits. NPA promotes this as a rational strategy over chasing lice with pesticides that offer more risk than benefit and have a well-documented history of lice resistance and failure.Most products used to treat head lice contain the insecticide pyrethrum, or its synthetic cousin permethrin, as the active ingredient. Over the past two decades, resistance to these chemicals has become a serious worldwide problem, causing a crisis in the chemical management of head lice. Some studies have found that lice are also becoming resistant to malathion, a pesticide used in prescription treatments that is more toxic than pyrethrum.“The arsenal of medications used to treat head lice is limited and shrinking, and health providers are spending an increasing amount of time and resources dealing with infestations,” says J. Marshall Clark of the veterinary and animal sciences department.

Dr. Clark and his team turned to ivermectin, a compound used to treat intestinal worms and plant parasites by targeting their nervous systems and muscles. Ivermectin was mixed in a base of water, olive oil, shea butter and several other ingredients to make a topical preparation designed to be applied to the skin or scalp, and tested on a strain of permethrin-resistant head lice collected from school children in southern Florida.

Formulations containing 1.0, 0.5 and 0.25 percent ivermectin were found to be 100 percent effective in killing newly hatched lice following 10 minutes of exposure. The topical formulation was also more effective than 0.5 percent ivermectin alone, indicating that the mixture may allow the ivermectin to penetrate more easily into the lice.

“Since most people find head lice intolerable, they often repeatedly apply insecticides without realizing their potential for harm if overused or misapplied,” says Dr. Clark. “This typically impacts children due to their small size and high sensitivity to these toxic chemicals.”

Ivermectin is thought to be less toxic than lindane an ingredient in prescription medications. Yet, ivermectin appears risky given the number of adverse event reports related to its use, especially among the elderly where deaths associated with ivermectin prompted a 1997 warning in the medical literature that it not be used at all.

NPA says prevention is not just about stopping head lice. It is also about protecting children from unnecessary and potentially harmful exposures to toxic pesticides such as lindane or malathion.

Future research will determine whether the ivermectin formulation has any effect on the eggs of head lice or the developing embryos. The testing was performed for Topaz Pharmaceuticals, which has completed Phase I for FDA approval. Full FDA approval will probably take an additional two to three years.

For information on controlling head lice without toxic chemicals, see Beyond Pesticides’ Head Lice Factsheet or Getting Nit Picky About Head Lice.

Source: University of Massachusetts Amherst


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