According to new research, North American head lice have become almost 100 percent resistant to permethrin, rendering the medication ineffective and outdated. Several new products have recently come on the market as alternative therapies.
The study, co-authored by John Clark, professor of environmental toxicity and chemistry at the University of Massachusetts, Amherst, was published in the March issue of Journal of Medical Entomology. Clark notes that the findings are not surprising or controversial. Indeed, scientists have been aware of the developing problem for about 20 years.
The modern drug resistant louse contains a mutant gene that makes its nervous system impervious to the neurotoxic effects of pyrethoid compounds, which include permethrin. The resistance has progressed as a direct result of treating head lice infestations consistently, over the years, with a single, now outdated, medication.
The head louse, Pediculus humanus capitis, is a tiny parasitic insect measuring about two to three millimeters long that resides on head and facial hair, including eyelashes and eyebrows, and feeds on human blood. It spreads among the population when people lean their heads together. Head lice typically infest elementary school children and preschool children who attend day care. The infestations can then spread to other family members in the home. The small insects do not transmit disease but can cause intense itching resulting in missed school days for about 10 percent of children in the U.S. According to the CDC, the infestation affects six to 12 million U.S. children, between five and 11 years old, annually.
Head lice have accompanied human beings throughout the evolution of mankind. Head louse eggs have been found on human skulls and mummies dating back as far as 8,000 B.C. In fact, the relationship between man and louse has become so intimate that the species can only survive on homo sapiens. Household pets do not play a role in transmitting the parasite.
Although the human head serves as the natural environment for Pediculus humanus capitis, other species of lice exist that inhabit the body and spread serious and sometimes life-threatening diseases. Body lice can infect humans with Rickettsia prowazekii, the bacterium responsible for typhus (not to be confused with typhoid fever). Typhus is a deadly disease that has periodically spread devastation throughout modern history. The plague of Athens during the Peloponnesian Wars may have been due to typhus, the same disease that wiped out much of Napoleon’s army in 1812, preventing the conquest of Russia.
Despite the nuisance that the insect presents to the public, some authors have postulated that humans and head lice shared a symbiotic relationship throughout history, where the lice received sustenance and the human host developed immunity to the deleterious sequelae of body lice infestation. There is also evidence that the head louse has helped modulate the human immune system over the long course of this relationship.
So, although the itching and scratching has led to the development of treatments to eradicate the creature, the louse may be determined to stay with us. During World War II, DDT was employed to wipe out the infestation for head and body lice among soldiers. But as its use increased, lice began to develop resistance to the chemical. Permethrin became the next weapon. And now new products such as Ulesfia, Sklice and Natroba are becoming available, replacing the outdated drug as head lice continue to adapt.
By Robert Wisnewski