Ivermectin Kills Head Lice Resistant to Standard Treatments

Head lice affect an estimated 12 million people in the United States 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 used to treat intestinal worms and plant parasites, 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.

Most products used to treat head lice contain the botanical 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. This has created a demand for new treatments that are effective and safe to use on children. Additional researchers include Joseph Strycharz and Kyong Sup Yoon of veterinary and animal sciences.

Clark and his team turned to ivermectin, a compound produced by soil-dwelling bacteria that is widely used to kill intestinal worms, mites and plant pests 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.

The effectiveness of ivermectin could save children from multiple applications of toxic chemicals. Since most people find head lice intolerable, they often repeatedly apply insecticides without realizing their potential for harm if overused or misapplied, says Clark. This typically impacts children due to their small size and high sensitivity to these toxic chemicals.

Another advantage is that ivermectin is not well-absorbed through the skin, which makes it suitable for products that are used externally. Ivermectin is also less toxic than lindane, an ingredient in prescription medications. 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, who has completed Phase I for FDA approval. Full FDA approval will probably take an additional two to three years.

Source: University of Massachusetts Amherst