The mode of action of dimeticone 4% lotion against head lice, Pediculus capitis
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Treatment of head lice using physically acting preparations based on silicones is currently replacing insecticide use due to widespread resistance to neurotoxic agents. It has been postulated that some products act by asphyxiation, although the limited experimental evidence and the anatomy of the louse respiratory system suggest this is unlikely.
Observation over several hours of lice treated using 4% high molecular weight dimeticone in a volatile silicone base showed that, although rapidly immobilised initially, the insects still exhibited small movements of extremities and death was delayed. One common effect of treatment is inhibition of the louse's ability to excrete water by transpiration through the spiracles. Inability to excrete water that is ingested as part of the louse blood meal appears to subject the louse gut to osmotic stress resulting in rupture. Scanning electron microscopy coupled with X-ray microanalysis to detect silicon showed dimeticone lotion is deposited in the spiracles and distal region of the tracheae of lice and in some cases blocks the lumen or opening entirely.
This work raises doubts that lice treated using dimeticone preparations die from anoxia despite blockage of the outer respiratory tract because movements can be observed for hours after exposure. However, the blockage inhibits water excretion, which causes physiological stress that leads to death either through prolonged immobilisation or, in some cases, disruption of internal organs such as the gut.
KeywordsBlood Meal Head Louse Silicone Fluid Narrow Duct Head Louse Infestation
Resistance of head lice, Pediculus capitis, to neurotoxic insecticides such as the pyrethroids and malathion has become widespread in developed countries since the first reports in the early 1990s. [1, 2, 3] The impact of resistance on therapy has had several effects including: repeated exposure of children to insecticides with an increasing reluctance on the part of parents to use pesticide based products ; increased prevalence of lice in most communities ; and a plethora of products available through online purchase that claim efficacy but without clinical evidence.
Dimeticone 4% lotion is a recent new product development in treatment of head louse infestation.  From the product conception it was recognised that its mode of action was to kill the insects by physical mechanisms rather than by neurotoxicity. However, the mode of action of the product has been unclear and other investigators working with dimeticone preparations of different molecular weight, as well as those investigating more straightforward occlusive creams, have loosely referred to the mode of action of those products as "asphyxiation" or "suffocation" but without clear biological evidence in support of their claims. [7, 8]
The structure of the louse respiratory system minimises the risk of penetration by fluids so demonstration that a product is capable of entering the respiratory tract of a louse is not adequate evidence for anoxic asphyxiation.  Additionally it has long been recorded that lice are able to survive immersion in water for several hours, presumably tolerating long periods without oxygen, and specific experimental investigation of the potential for suffocation by various preparations has not been shown to be currently feasible. 
Dimeticone 4% lotion is a licensed medicinal product in the UK and a class I medical device in most other European countries. It contains 4% w/w 100000 centistokes (CSt) dimeticone and 96% w/w cyclomethicone D5. Dimeticone is a polydimethylsiloxane with the chemical formula http://en.wikipedia.org/wiki/Chemical_formula (CH3)3SiO [SiO(CH3)2]nSi(CH3)3, where n is the number of repeating monomer http://en.wikipedia.org/wiki/Monomer units [SiO(CH3)2], which is directly proportional to the molecular weight and viscosity. In the case of 100000 CSt dimeticone the average value of n is 1875, giving an average molecular weight of 139050.  Cyclomethicone D5 is the volatile cyclic silicone, decamethylcyclopentasiloxane.
Head lice feed exclusively on blood and take several feeds each day. However, they do not excrete liquid urine or faeces like other blood feeding insects. Evidence from our laboratory (unpublished observations) shows that most water ingested as blood is excreted via the spiracles and starved lice die from dehydration within a relatively short time. This appears to occur in two stages. During the first, the lice show little physical change, are able to feed if given the opportunity, but the abdomen may be seen to decrease in volume. At the second stage, which is reached at any time between 3 and 24 hours following their previous meal, the lice remain capable of walking, will fasten to the skin and attempt to suck blood, but are unable to feed, presumably because they are too dehydrated to produce the saliva needed to prevent haemostasis. Complete immobility and death may occur at any point after this. Other investigators have found a mean time to immobility of 21.3 ± 12.1 hours. [12, 13]
Early laboratory investigation of the effect of dimeticone 4% lotion on lice suggested it was capable of entering the spiracles and tracheae with resultant disruption of physiological mechanisms such as water management, resulting in gut rupture of recently fed lice.  However, other investigators, using a different commercially available mixture of low molecular weight dimeticone compounds, have questioned this interpretation, preferring to attribute the activity of silicone based lotions to suffocation, despite the high tolerance of lice for anoxic effects and the high oxygen permeability of silicones. . I have conducted an investigation of the physical distribution of high molecular weight dimeticone 4% lotion when applied to head lice, and of the physiological outcome of treatment, as a contribution towards understanding the activity of this type of product.
Insects and treatments
Head lice, Pediculus capitis, were obtained by combing infested children from whom assent and parental consent had been obtained under ethical approval from Cambridgeshire 3 (formerly Peterborough & Fenland) Research Ethics Committee.
Lice for observation of physiological effects were treated using silicone lotions, either by immersion or else by application of droplets to the cuticle surface using a micro-syringe and needle. Silicone lotions used in these experiments were Hedrin® 4% dimeticone lotion (Thornton & Ross Ltd, Huddersfield, UK), which contains 4% high molecular weight dimeticone, and Nyda® L (G. Pohl-Boskamp GmbH & Co. KG, Hohenlockstedt, Germany), an approximately 92% silicone content mixture of two low molecular weight dimeticones in equal proportions.
Lice for weight loss experiments were weighed individually using a Sartorius 4503 micro-balance (maximum tare 4.1 grams, d = 0.001 milligrams). Each louse was weighed, fed on the back of a volunteer hand, weighed again, and then at 15 minute intervals. Some fed lice were immersed in dimeticone 4% lotion for 10 seconds immediately after feeding then blotted on absorbent tissue to remove fluid before subsequent weighing.
Lice for electron microscopy were treated using dimeticone 4% lotion applied by immersion of the insects for 10 seconds followed by draining of excess fluid on Whatman No 1 filter paper.
Surface micrographs of the lice were obtained using a FEI/Philips XL30 ESEM environmental scanning electron microscope operating in wet mode i.e. using water vapour as an imaging gas.
The analysis of the chemical elements found in louse spiracles was achieved by using a Quanta 200 3D dual beam scanning electron microscope equipped with a Gatan Cryotransfer System and an Oxford Instruments INCA X-ray microanalysis package.
The specimens were attached to a sample stub using Tissue-Tek® embedding fluid, quickly frozen to preserve their integrity by plunging into liquid nitrogen slush, and then platinum coated and viewed at -180°Celsius.
Internal structures of spiracles were investigated by milling away tissue at the site of interest using a gallium, focussed ion beam (FIB). This allowed the chemical elements found within the structure to be qualitatively identified using the X-ray analysis system. The key marker element used for dimeticone 4% lotion was silicon as it is not found at any significant background levels in lice.
Physical and physiological effects of silicone treatment
Lice immersed in silicone lotions were observed to cease overt mobility within 60 seconds, as reported previously by different observers [6, 8, 14, 15]. Lice removed from the either of the silicone fluids exhibited no signs of recovery after blotting or if washed with shampoo and water to remove surface silicone. Lice left in the fluids for longer periods also remained motionless, with the exception of occasional peristaltic movements of the gut. However, after about 2 hours some lice immersed in low molecular weight dimeticone began to exhibit small movements of limbs, such as flexing of the tibiae or tarsae, as well as small movements of the antennae, indicating that lice immersed in the fluid had not suffered anoxic asphyxiation within that period of time. Movements of this type were seen to continue for more than 1 hour after first being noted.
Richling and Bökeler  reported rapid flow of the low molecular weight silicone mixture into the tracheae of immersed lice and entry of silicone fluid into tracheae when applied to the surface of the louse abdomen using a micro-syringe.  I attempted to repeat these experiments on several occasions, using more than 50 lice of different ages and development stages. Neither of the silicone lotions, when applied in this way, could be induced to spread through the tracheal system of a louse beyond the junction of the trachea and the spiracle.
Scanning electron microscopy
The external physical structures of both thoracic and abdominal spiracles, shown by SEM, and the internal structure of the abdominal spiracle, revealed by milling, were found to be essentially the same as described from light microscope histology by Webb . Thoracic and abdominal spiracles differ in external appearance and in details of internal structure but are essentially similar. We concentrated our investigation mainly on abdominal spiracles.
X-ray spectroscopy linked to scanning electron microscopy shows that a head louse treatment lotion product containing 4% high molecular weight dimeticone is able to enter and form a coating to the internal surfaces of the spiracles of lice. This coating occludes the internal structures of the spiracle and blocks the opening of the trachea. Lice soaked with dimeticone 4% lotion experience physical disruption of the gut several hours after treatment.
In commenting on the unusual structure of the spiracles of lice of the genus Pediculus, Webb  stated, "The passage of air from the atrium through the narrow duct of the spiracular gland in itself seems so difficult that any muscles designed to close this aperture further, even if the duct is not filled with secretion, would appear to be unnecessary. On the other hand it is possible that contraction of the muscle forces the trachea against the spiracular gland and causes the duct to gape thus enlarging the aperture between the atrium and the trachea. If this is so then contraction of the muscle opens the spiracle, a condition which appears to be in contrast to that obtaining in other Anoplura."
Webb's description and conclusions contradict the description given by Buxton in his monograph on human lice, "There are six pairs of abdominal spiracles, carried on segments 3 to 8. All spiracles (including that on the thorax) possess a closing apparatus, which appears to work by compressing or buckling the trachea a short distance below the surface of the body. It seems highly probable that the louse uses this in order to reduce the loss of water from within its respiratory system."  This statement, based on drawings by Hase,  has become the "received wisdom" about the mechanism of action of the human louse respiratory apparatus, in that it is often stated that when immersed in water they "close their spiracles" and enter a state of immobility. [18, 19] The actual mechanism operating is important because blockage of spiracles, or relaxation of muscles that close the tracheae, forms the basis of the proposed "asphyxiation" mode of action for several newly developed products that claim physical activity against lice. [6, 7, 20, 21]
If lice immersed in water "close" their respiratory tracts in order to exclude fluid it presumably requires considerable anaerobic metabolic energy expenditure to continuously contract the muscles closing the tracheae, which may not be sustainable for the length of time lice can remain viable while immersed in water. However, if the tracheae are normally "closed" and only "open" to allow occasional air intake, like many terrestrial insects, [22, 23] it would certainly explain the ability of lice to readily exclude water when immersed. However, as cyclic and discontinuous gas exchange patterns are also considered methods whereby insects limit water loss, the evidence we have from weighing experiments appears to suggest that, after feeding at least, lice expel water freely rather than conserve it, and the stress induced when water loss is prevented indicates that louse physiology is not like that of most other insects. In some ways louse respiratory behaviour is more consistent with the proposal that oxygen is relatively toxic for insects so opening of the spiracles is limited and mainly used for excretion of carbon dioxide. [24, 25]
If the normal anatomical and physiological operation of the louse respiratory structures is to have a closed trachea it is difficult to see how even a low surface tension fluid such as low viscosity silicone could penetrate deeply into the trachea. Richling and Böckler  applied small drops of fluid to the tip of the abdomen, which allowed silicone to systematically flow through the tracheal system and "vent" through untreated spiracles. When an insect is immersed in or coated with dimeticone lotion, the fluid would enter all spiracles equally and trap most of the air inside, in a similar manner to the way water blocks the tracheoles but does not enter the alveoli of the lungs in drowned mammals. However, it is not difficult to conceive how in lice the entrance to this set of structures can be physically blocked by high molecular weight dimeticone deposited from a low molecular weight volatile solvent, as demonstrated in this study. Furthermore, if lice live continuously with a low oxygen requirement it is unlikely that even if deep penetration of the tracheae by silicone fluids did occur it would result in anoxic asphyxiation as the primary cause of death, because silicone fluids have a high permeability to oxygen. Therefore, the most likely mode of action of dimeticone 4% lotion (Hedrin® 4% lotion) against head lice is to permanently physically block the outermost sections of the louse respiratory tract, preventing water excretion, which may lead to gut rupture in recently fed insects and irreversible immobilisation, leading ultimately to death, in others.
Silicone based products for treatment of head louse infestation show an immediate immobilisation effect on the insects that mimics immersion in water. However, because silicones are difficult to remove or wash off, lice do not recover as they would if removed from water but remain immobile until death.Contrary to the widespread opinion that physically acting pediculicides work by suffocation the evidence is that occlusion of spiracles and tracheal trunks by silicone deposits produces a barrier that inhibits excretion of water, which results in osmotic stress leading ultimately to gut rupture in fed insects.
This study was supported logistically by Dr Nigel Cooper and Steve Skilleter, Thornton & Ross Ltd, Huddersfield, UK, who sponsored the study. Thornton & Ross Ltd played no role in the design of the study, interpretation of the results, or writing of the manuscript, but was permitted to know the outcome of the study prior to submission for publication. Thanks especially to Jon Rickard and Trevor Fairhead, Sector of Biological and Soft Systems, Cavendish Laboratory, University of Cambridge, for scanning electron microscopy and spectrographic analyses, Mark Burgess, Medical Entomology Centre, for micro-video photography, and Simeon Wigmore, The Practice, for image overlays.
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