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Is head lice resistance rearing its head again?

Thanks to their mode of action, physical insecticides have been considered ‘resistance proof’ when it comes to treating head lice – but now evidence is emerging to suggest this may not be the case.

The main treatment options that can be recommended for head lice include products containing traditional chemical insecticides or newer physical insecticides, or a non-pharmacological, rigorous wet combing approach. 

Physical insecticides such as dimeticone, cyclomethicone and isopropyl myristate work by coating the surface of lice and their eggs and physically smothering them. These have become the cornerstone of modern head lice management and are found in many of today’s most popular OTC products – but resistance is becoming a growing concern. 

A recent research letter published in the British Journal of Dermatology has shone a spotlight on this issue, with authors from the Medical Entomology Centre at Insect Research & Development Limited in Cambridge, concluding: “At this stage non-responsive lice have only been found in a few households but, as with resistance to insecticides found in the early 1990s, this is likely [to be] only the tip of a more extensive problem within the wider community. 

“If head lice are being selected for greater tolerance of immersion in physically acting fluids, product manufacturers, clinicians and regulators need to take action now to establish not only which types of formulations and ‘active’ materials are losing their effect but also to develop alternatives to replace them before problems, similar to those encountered with insecticide use in the 1990s, become more widespread.”1

Limited evidence

Ian Burgess, director of the Medical Entomology Centre is lead author of this research. The evidence is limited at the moment because we have not had the opportunity to follow up many people, he explains. “The ones described in the letter are of children [where] we had a chance to carry out repeat follow-ups at roughly weekly intervals, a week being too short a period of time for lice emerging from eggs to become adults. 

“What we found was each of these individuals showed a full spectrum of louse development stages almost every week, which means that the treatment did not kill the adults or perhaps older nymphs that could then become adults. 

“Each of these people did not have infested family members and, in our follow-ups, we could not trace any contacts who had lice either. The failed treatments ranged across three completely different products, in one case a product that had not previously been used in the UK as far as we were aware at the time.”

“New evidence suggests that some treatment failures were because the product did not work as expected”

Growing problem?

Burgess says these findings could potentially signal increased resistance to physical insecticides as a growing problem in the UK. “We have always found a level of lack of success with physically acting products. Sometimes this has been due to not being able to kill all louse eggs, sometimes through a failure to get the fluid to thoroughly coat every hair, louse or egg, and sometimes because people get reinfested. 

“However, the new evidence suggests that maybe some of those failures were because the product did not work as expected. Lower efficacy has increasingly occurred over the years since the products were introduced but there was no way of knowing whether these were chance issues or something more substantial affecting efficacy.”

We do not yet know how or what is causing this failure, Burgess continues, although we can make some educated guesses. “In each case it is likely to either be some selected change in the proportion of different lipids in the waterproofing lipid coating of the louse, so they are not so easily disrupted. 

“Alternatively, some of the products that enter the respiratory system of the louse to block it and stop the insects excreting water may be less easily able to do this if some difference in the structure of the spiracle has been selected for, so the fluid does not go in.”

What to recommend?

In terms of first-line head lice treatment, both NICE and the NHS are reluctant to recommend any one specific product. NICE advises that choice of treatment be determined by the preference of the person and/or their parents/carers after considering the advantages and disadvantages of each treatment, what has been previously tried and the cost of the treatment.2 

Burgess agrees that there is no simple answer. “Until recently we would use Hedrin Once spray gel for families that contacted us when they were having problems with dealing with lice. However, since two of our reported ‘resistance’ cases had been using that, we have needed to use something else,” he explains. “For the present we are sticking with Hedrin Once spray gel as our starting point and then moving on from there if there is a problem at follow-up after one week.”

One of the main issues is the lack of conclusive clinical evidence supporting any specific treatment approach or product over another. “The problem is that very few products have actually been subjected to properly constructed clinical investigations. On some websites they say they have evidence from in vitro tests but nothing more,” Burgess says.

“As a result we are all reduced to the level of trying something, seeing if it works and, if not, trying something else – not an attractive option given the cost and the angst head lice cause.”


1. Burgess I & Brunton E. Head lice: evidence that resistance to physically acting treatments is developing. British Journal of Dermatology. 2023; 189(1):144-145

2. NICE Clinical Knowledge Summaries. Head lice. Revised November 2021. 

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