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Publishing in the Journal of the Royal Society Interface a group of scientists have used a model to analyse the use of personal protection techniques, such as netting and insect repellents, for the avoidance of diseases spread by carriers like mosquitoes. From this model they suggested some circumstances under which such devices might increase disease in the unprotected portion of the population.
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Dr David Weetman, Department of Vector Biology, Liverpool School of Tropical Medicine, said:
“The phenomenon described in the modelling study by Miller et al. is theoretically viable, and should indeed be carefully considered as a possibility that might occur in some specific scenarios. However inclusive review of the available field literature suggests little reason to panic about use of well-tested personal protection methods. Similar initial concerns about insecticide-treated nets proved unfounded, with no evidence of diversion to nearby non-users observed in real field experiments1.
“Furthermore, numerous field studies have shown that even untreated bed nets provide a considerable overall benefit to non-users 2, because while their share of remaining biting burden is increased, the overall number of infectious mosquitoes in the population is dramatically reduced3,4. Even in the extreme case of partial coverage of an intervention with little killing power, as is the case for some repellents, mosquitoes still must work harder and search for longer to obtain their blood meal, reducing their chances of surviving long enough to incubate these pathogens all the way through to their infectious forms. This expectation is not disease-specific: all of the world’s most important mosquito-borne viruses and parasites need several days to develop inside mosquitoes before they mature and become infectious to subsequent human victims.
“Furthermore, if the intervention providing personal protection has killing capabilities such as walls sprayed, or nets treated with insecticides, the chances of mosquito survival to an age at which a previously-ingested parasite is infective, or a virus has reached a sufficient level of viremia to be transmissible, becomes yet slimmer because mosquitoes typically feed every 2 to 3 days, making at least one encounter with a protected person highly probable during the pathogen’s incubation period. For treated bednets, this benefit of even imperfect coverage is so great that unprotected individuals living as far away as 600m from an area with high net coverage can enjoy measurably reduced rates of malaria, anemia and mortality5.
“Given the proven, measurable and massive contributions that insecticidal nets, and indoor sprays have achieved6, we strongly caution against an over-reaction to these theoretical analyses.
“In the current climate of focus on the widespread arboviruses such as Zika and dengue, which are transmitted by day-biting Aedes mosquitoes, perhaps the greatest concerns arising from the paper relate specifically to topical repellents. Such compounds only temporarily irritate mosquitoes upon direct physical contact and might cause redirection to an unprotected host nearby7,8. However, to our knowledge the impacts on mosquito population sizes and infection rates remain to be assessed in sufficiently large-scale studies.
“Moreover, topical repellents should not be regarded as a monotherapy against arboviruses; other methods of bite-avoidance should also be employed, and methods of delivering insecticides such as IRS should be explored further, along with continuing investigation of spatial insecticides with both repellent and killing properties. More data are clearly warranted and probably the clearest message from the current paper, and one noted by the authors, is the need for more research into the impact of insecticides or repellents on mosquito behaviour and long term fitness rather than simply immediate knock down or deterrence. Such conceptually basic, but actually quite technically-demanding studies, will help to provide the data capable of determining whether the parameter values creating the sometimes disturbing predictions of the model may be realistic.
“Thus until proven otherwise by data, available products for personal protection should remain key pillars of global vector control strategy and those most at risk of malaria, dengue, chikungunya, Zika or any other mosquito-borne infection, should take full advantage of these life-saving measures whenever they can.”
1.Hewitt S, et al. The effect of bednets on unprotected people: open-air studies in an Afghan refugee village. Bull Entomol Res. 1997; 87: 455-9
2.Lengeler C. Insecticide-treated bed nets and curtains for preventing malaria. Cochrane Database of Systematic Reviews. 2004; 2: CD000363
3.Killeen GF, et al. Target product profile choices for intra-domiciliary malaria vector control pesticide products: repel or kill? Malar J. 2011; 10: 207
4.Killeen GF, et al. Preventing childhood malaria in Africa by protecting adults from mosquitoes with insecticide-treated nets. PLoS Med. 2007; 4: e229
5.Hawley WA, et al. Community-wide effects of permethrin-treated bednets on child mortality and malaria morbidity in western Kenya. Am J Trop Med Hyg. 2003; 68 (Supplement 4): 121-7
7.Moore SJ, et al. Are mosquitoes diverted from repellent-using individuals to non-users? Results of a field study in Bolivia. Trop Med Int Health. 2007; 12: 532-9
8.Maia MF, et al. Do topical repellents divert mosquitoes within a community? Health equity implications of topical repellents as a mosquito bite prevention tool. PLoS One. 2013; 8: e84875
Dr James Logan, Senior Lecturer and Director of arctec, Department of Disease Control, London School of Hygiene & Tropical Medicine, said:
“We have known for some time that mosquitoes may be diverted away from people who are wearing repellents to those who are not. However, they are an important method of protecting an individual against mosquito bites and should be used by travellers to disease endemic counties.”
Prof. Azra Ghani, Professor of Infectious Disease Epidemiology, Imperial College London, said:
“Aedes aegypti are day-time biters and don’t move a great distance, so this study really isn’t of much relevance to Zika.
“This theoretical modelling study identifies some sets of parameters in which an increase in disease burden could be expected. However, the model does not realistically capture any individual vector-borne disease, nor is it validated against any specific datasets. I would therefore be very cautious in interpreting these results as relevant to current efforts to reduce vector-borne disease.
“Perhaps the best documented impact of vector control to date has been the impact of distribution of long-lasting insecticide treated nets against malaria. According to the WHO World Malaria Report 2015, the proportion of the population at risk from malaria who slept under a bed net on the previous night in sub-Saharan Africa has risen from less than 2% in 2000 to 55% in 2015. Over the same time period, malaria case incidence globally has been estimated to fall by 37% and mortality by 60%. A detailed analysis of these patterns reported by Bhatt et al. in Nature in 2015 shows that the bed nets averted 68% of the total cases in sub-Saharan Africa over this period. In all of the countries bed net usage is partial.
“The empirical evidence from that Nature study and numerous other local studies, including the Cochrane review of bed net trials against malaria, therefore demonstrate a consistent positive effect of bed nets across a wide range of different mosquito ecologies even when protection is partial. Bed nets therefore remain one of the most cost-effective ways to reduce malaria transmission and are likely to be effective for vector-borne diseases caused by mosquito species that bite primarily at night.”
References:
World Malaria Report 2015 (http://apps.who.int/iris/bitstream/10665/200018/1/9789241565158_eng.pdf?ua=1)
Bhatt S et al. 2015 Nature 526 207-211 (http://www.nature.com/nature/journal/v526/n7572/full/nature15535.html)
‘The risk of incomplete personal protection coverage in vector-borne disease’ by Ezer Miller et al. published in the Journal of the Royal Society Interface on Wednesday 24 February 2016.
Declared interests
Dr David Weetman: “employee of LSTM
Dr James Logan: “Dr Logan is a Senior Lecturer in Medical Entomology at the London School of Hygiene and Tropical Medicine with grant funding to research personal protection against disease transmission. He is also the Director of arctec – a centre at the London School which evaluates vector control technologies.”
Prof. Azra Ghani:
My comments are in a personal capacity and don’t necessarily represent the views of any of the above.”