The revenge of the resistant lice

To get rid of head lice, people have used for decades special preparations for covering hair containing insecticides from a group of substances named Pyrethrins, extracted from plants, or from their synthetic substitutes called Pyrethroids. When they first appeared on the market, these materials worked well. But continuous use of them caused the blood-sucking parasites to develop resistance to them. A recent US study of lice resistance by the pesticide toxicologist John Marshall Clark from the University of Massachusetts in Amherst and his colleagues, revealed that between two-thirds and three-quarters of the lice population are resistant to the effects of pesticides. They became "super lice."

This high prevalence of resistance means that most people waste their money when they buy anti-lice shampoos, whose prices range from 30 to 120 shekels per bottle, and are still the weapon of choice for those who fight lice. The use of the types of anti-lice shampoos may even prolong the suffering of the patients, as it takes at least a week to discover that the treatment has failed. The problem especially bothers children, the most frequent victims of lice, since in the US there are kindergartens and schools that require the children to stay at home until their heads are completely free of lice and eggs.

This resistance problem spurred scientists to try to find other methods of treating lice. In Europe, some non-insecticides have had success. In the US, doctors have recently added several new prescription drugs to the pool of treatment options. But scientists warn against careless use of the new drugs, to prevent the development of resistance to them in the lice population as well.

A common parasite everywhere

Lice are more common than we think, or notice at first glance. In a 2001 study they conducted Costa Momchoglu and his colleagues at the Hebrew University in Jerusalem, they examined the scalps of 280 Israeli children. One group of researchers looked for lice that could be spotted just by looking. They found an infection rate of more than 5%. Another group returned and tested the same children using lice combs. In the more rigorous examination, lice were found on the heads of children in a proportion four times greater than the previous examination. Researchers have reported a high rate of infection in other countries, including Turkey and England. According to Momchoglu, the findings show that, on average, one child out of five in developed countries is infected with lice.

Head lice are mainly spread when there is contact between people's heads. Children are especially vulnerable because when they play, they are in closer physical contact than adults. Sokoloff, whose chain of kinematics doctors employs workers in 40 states in the US, says that in her company they notice a noticeable increase in infection when the children return home from summer camps, and also after the long winter vacations, when they sleep together with friends or relatives.

The level of personal hygiene is probably not the determining factor of who will be infected. Long hair hides the lice very easily, making it difficult to comb and remove the eggs.

Head lice and body lice belong to the same biological species, The human louse. But body lice are capable of spreading diseases such as typhus, Excavation fever And even a word, while head lice have never been accused of massively spreading such diseases. The difference between the two types of lice probably lies in the differences in their immune response to bacteria. Clark and his colleagues found that when they infected both types of lice with the bacterium that causes burrowing fever, the head lice responded with a much stronger immune response compared to the response of the body lice. Head lice may not transmit the disease because their immune system suppresses the bacteria before it can be transmitted to humans.

However, head lice are annoying and quite a nuisance. In cases of really severe infection, one that affects the homeless or people living in high density, the severe scratching of the scalp may allow the penetration of bacteria and lead to severe infections.

Increasing durability

The beginning of the resistance to pesticides can be found in the past, during the Second World War. In the period immediately after the end of the war, millions of people displaced from their homes and living in displaced persons camps were treated with large quantities of an insecticide, which was then new, DDT, in order to prevent the spread of body lice and the plagues they transmit. Its use to combat body lice and other parasitic insects continued in some parts of Europe and Asia until the 80s; In the USA, its use was gradually stopped in the 20s due to concerns about the safety of its use. But the multiple use has left a long-lasting legacy among the insects.

When pyrethroids were introduced for use in Israel in the early 90s, a single treatment would eliminate the adult lice on a person's head within a few weeks. The substance remains on the scalp long enough to also kill the eggs, which usually hatch about 10 days or more after laying. However, within two or three years from the beginning of their increased use, the treatments with pyrethroid-based preparations lost their effectiveness and most of the lice were not harmed by them.

The use made years earlier of DDT triggered in the lice the ability to develop this resistance. The substance works by damaging the nervous system of insects. The cell membranes of nerve cells have tiny pores that allow the passage of sodium ions into and out of the cell. The entry and exit of sodium ions into a nerve cell regulate the transmission of nerve impulses by the cell. DDT causes these pores to remain permanently open, allowing an uncontrolled flow of sodium ions into the cells. The constant flow of sodium ions in causes the cells to transmit nerve impulses continuously and non-stop, and this leads to the distortion of the insect's body, paralysis and eventually death. Decades of exposure to the substance caused the appearance of mutations in certain strains of lice that neutralized the effect of DDT. The mutations multiplied and became common until the majority of the lice population became resistant to the substance. Pyrethrins and pyrethroids also act by disrupting the action of sodium channels. As a result, the lice had no trouble developing resistance to these substances through the same mutations that protected them from DDT.

Mutations conferring resistance to head lice have spread and reached high prevalence. the researcher Kyung Soop Yoon, an entomologist at Southern Illinois University in Edwardsville, collected lice from 2013 states in the US in 2015-48. He and his colleagues, including Clark, found that 132 of the 138 populations tested carried mutations for complete resistance, due to which the lice's nervous system became completely insensitive to pyrethroids.

New tactics

Humans have given head lice an advantage by repeatedly exposing them to the same substances. Clark explains that such repeated exposure leads to the development of resistance not only to the substance used for treatment, but to all preparations that work in the same way, or with similar mechanisms of action. In this situation, the only way to defeat the lice is to use a completely new approach, one that the parasites have no previous adaptations to which equip them to deal with you. To this end, in recent years researchers have developed a few preparations based on mechanisms of action completely different from that of pyrethroids. In the US, the Food and Drug Administration has approved three prescriptions for the treatment since 2009: the substance ulesfia, which contains alcohol in a high concentration that actually kills the lice by suffocation; Soda, which causes hyperexcitation of nerve cells by stimulation Acetylcholine-nicotinic receptors; and-Slides, which inhibits nerve impulses by activating Glutamate chloride channels in cell membranes.

In order for these preparations to work long-term, those who prescribe them to patients must switch between them and give a different preparation in each round of treatment, in order to prevent overexposure of the lice to one preparation, and in this way prevent the development of resistance. But there are no official medical guidelines for such a rotation of the preparations. The doctors can prescribe any preparation they prefer, and usually they are not aware of the benefit of using different preparations alternately.

The payment to be paid to the doctor, in addition to the price of the new preparations which are not always covered by the medical insurance, all these burden the patients and reduce their access to these preparations. Therefore, despite the decrease in their effectiveness, over-the-counter shampoos continue to be the first treatment recommended by doctors, and even by the American Academy of Pediatrics. The method of combing with a lice comb, according to Clark and Yoon, which many parents and professional lice experts use, is theoretically effective, but in practice it is difficult to perform properly.

In Europe the situation is completely different. There they completely abandoned pyrethroids, and in fact all insecticides already a decade ago, according to Ian Burgess, president of the International Association of Lice Researchers. Instead, in Europe it is common to use silicone and synthetic oils as a means of eliminating head lice. The oils wrap the lice in an impermeable shell and prevent them from excreting water. As fluids accumulate in the louse's body, its internal organs begin to collapse due to the effort to pump and evacuate water from the body to the outside. The louse dies from exhaustion resulting from the effort, or its intestine bursts from excess fluid.

Burgess explains that in Europe, the synthetic oils are considered medicinal preparations but not drugs, so they are less subject to regulation; In the US they are considered drugs that have not passed the regulatory hurdles. Burgess who also works for a research company that develops measures against lice. In general, Burgess believes that the European approach to fighting lice is working well. Today, when he checks children in schools, he finds the same infection rate as twenty years ago, but each child has fewer lice.

Still, despite the progress, it seems that the lice intend to keep researchers, us and our children, scratching our heads for a long time to come.