We’ve all heard about the worrying increase in antibiotic-resistant bacteria, but did you know that around the world head lice are also becoming resistant to many common treatments? The ugly blood-suckers have been feeding on us for thousands of years,1 yet despite our best efforts to break things off they show no signs of ending this very one-sided relationship. An article in The Telegraph reports that parents are fighting a losing battle against ‘mutated, indestructible super lice’,2 and while it’s not quite as dramatic as all that, it is an issue you should keep in mind when looking for head lice treatment. In this article, I’ll look at the growing trends of head lice resistance and which treatments offer the best hope of putting an end to an infestation.
A worldwide problem
The first truly effective head lice treatment was thought to have been discovered by the Chinese around 100 A.D. when they extracted pyrethrum powder from a species of chrysanthemum.3 But it wasn’t until the 1940s that the first modern commercial treatment appeared, containing the more refined active ingredient pyrethrin.3 Around 40 years later, its synthetic cousin, permethrin, was introduced, but by the 1990s resistance to both of these insecticides was already being reported.4 Malathion, another chemical treatment, has been shown to be more effective than pyrethrins and permethrin in the US,5 but in the UK, where malathion has been available for several decades, resistance has been reported.6 In Australia, there is evidence of head lice strains completely resistant to malathion in one school and others becoming resistant to pyrethrums and permethrin with kill rates down to 50% or less.7
Where do resistant head lice come from?
Resistant head lice come about in a similar way to antibiotic-resistant bacteria. Let’s say you have a population of 1,000 head lice in a school and every child is treated with a permethrin product. Of those 1,000 lice, 999 are killed, but one female louse survives because it had a random mutation in its genes that made it slightly harder to kill. One such mutation we know about is a change in the knockdown resistance gene (kdr), which gives lice reduced nerve sensitivity.8 If that single surviving louse goes on to reproduce, the following generation could all have the resistance gene making them nearly impossible to kill with permethrin. Lingering chemical residues at a sub-lethal dose after a failed treatment are also thought to contribute to resistance.9 Since the head lice life-cycle only takes around 3 weeks, resistance can develop rapidly in a population once it takes hold.
How to treat resistant head lice
One of the most common causes of treatment failure is not the presence of resistant lice, but from not completing the full course of treatment. With most types of head lice treatment, multiple applications are needed to break the head lice life cycle. This is because the eggs, known as nits, are much harder to kill and can sometimes survive the first treatment. Even if you can’t see any remaining lice or nits after a single treatment, there may be a few lingering survivors just waiting to restart the infestation as soon as your back is turned, so it’s important to follow the directions to the letter.
The good news is that even though head lice are getting tougher, we still have plenty of reliable ways to kill them. Two types of treatment for which there has been no reported resistance are:
Natural essential oils
Natural essential oils have now been clinically proven to kill head lice and eggs by interfering with the biochemical mechanisms of the louse, effectively poisoning them.10,11 Furthermore, there is no known resistance to these types of treatments. We recommend trying MOOV Head Lice Solution or MOOV Head Lice Shampoo.
Another way to circumvent the problem of resistance is to use a suffocant.9 These treatments contain ingredients that coat the surface of the louse, blocking its respiratory spiracles and causing death by suffocation. We recommend MOOV Head Lice Sensitive.
ALWAYS READ THE LABEL. FOLLOW THE DIRECTIONS FOR USE. IF SYMPTOMS PERSIST, WORSEN OR CHANGE UNEXPECTEDLY, TALK TO YOUR HEALTHCARE PROFESSIONAL.
1. Araújo A, Ferreira LF, Guidon N, Freire NM da S, Reinhard KJ, Dittmar K. Ten thousand years of head lice infection. Parasitol Today 2000;16(7).
2. Boyle D. ‘Indestructible’ head lice now resistant to over-the-counter remedies, researchers warn [Internet]. The Telegraph2016 [cited 2019 Feb 21];Available from: https://www.telegraph.co.uk/news/2016/08/03/indestructible-head-lice-now-resistant-to-over-the-counter-remed/
3. Eddowes J. History of Lice | Lice Clinics of America [Internet]. 2016 [cited 2019 Feb 25];Available from: https://www.liceclinicsofamerica.com/a-brief-history-of-lice/
4. Devore CD, Schutze GE. Head Lice. Pediatrics 2015;135(5):e1355–65.
5. Meinking TL, Serrano L, Hard B, Entzel P, Lemard G, Rivera E, et al. Comparative In Vitro Pediculicidal Efficacy of Treatments in a Resistant Head Lice Population in the United States. Arch Dermatol 2002;138(2):220–4.
6. Downs AMR, Stafford KA, Harvey I, Coles GC. Evidence for double resistance to permethrin and malathion in head lice. Br J Dermatol 1999;141(3):508–11.
7. Hunter JA, Barker SC. Susceptibility of head lice (Pediculus humanus capitis) to pediculicides in Australia. Parasitol Res 2003;90(6):476–8.
8. Yoon KS, Previte DJ, Hodgdon HE, Poole BC, Ho Kwon D, Abo El-Ghar GE, et al. Knockdown Resistance Allele Frequencies in North American Head Louse (Anoplura: Pediculidae) Populations. J Med Entomol 2014;51(2):450–7.
9. Greive K A, Lui AH, Barnes TM, Oppenheim VMJ. A randomized, assessor-blind, parallel-group, multicentre, phase IV comparative trial of a suffocant compared with malathion in the treatment of head lice in children. Australas J Dermatol 2010;51(3):175–182.
10. Greive KA, Altman PM, Rowe JS, Staton JA, Oppenheim JVM. A randomised, double-blind, comparative efficacy trial of three head lice treatment options: malathion, pyrethrins with piperonyl butoxide and MOOV Head Lice Solution. Aust Pharm 2007;26(9):738–743.
11. Greive K a, Lui AH, Barnes TM, Oppenheim VMJ. Safety and efficacy of a non-pesticide-based head lice treatment: Results of a randomised comparative trial in children. Australas J Dermatol 2012;53(4):255–263.