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A systematic review of scabies transmission models and data to evaluate the cost-effectiveness of scabies interventions

  • Naomi van der Linden ,

    Roles Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Visualization, Writing – original draft, Writing – review & editing

    Naomi.vanderLinden@astrazeneca.com.

    Current address: AstraZeneca, The Hague, The Netherlands.

    Affiliation Centre for Health Economics Research and Evaluation, University of Technology Sydney, Sydney, Australia

  • Kees van Gool,

    Roles Conceptualization, Funding acquisition, Supervision, Writing – review & editing

    Affiliation Centre for Health Economics Research and Evaluation, University of Technology Sydney, Sydney, Australia

  • Karen Gardner,

    Roles Conceptualization, Funding acquisition, Project administration, Supervision, Validation, Writing – review & editing

    Affiliation Public Service Research Group, School of Business UNSW Canberra, Canberra, Australia

  • Helen Dickinson,

    Roles Conceptualization, Funding acquisition, Validation, Writing – review & editing

    Affiliation Public Service Research Group, School of Business UNSW Canberra, Canberra, Australia

  • Jason Agostino,

    Roles Conceptualization, Validation, Writing – review & editing

    Affiliation Academic Unit of General Practice, Australian National University, Canberra, Australia

  • David G. Regan,

    Roles Validation, Writing – review & editing

    Affiliation The Kirby Institute, UNSW Sydney, Sydney, Australia

  • Michelle Dowden,

    Roles Conceptualization, Funding acquisition, Validation, Writing – review & editing

    Affiliation One Disease, Darwin, Australia

  • Rosalie Viney

    Roles Conceptualization, Supervision, Writing – review & editing

    Affiliation Centre for Health Economics Research and Evaluation, University of Technology Sydney, Sydney, Australia

A systematic review of scabies transmission models and data to evaluate the cost-effectiveness of scabies interventions

  • Naomi van der Linden, 
  • Kees van Gool, 
  • Karen Gardner, 
  • Helen Dickinson, 
  • Jason Agostino, 
  • David G. Regan, 
  • Michelle Dowden, 
  • Rosalie Viney
PLOS
x

Abstract

Background

Scabies is a common dermatological condition, affecting more than 130 million people at any time. To evaluate and/or predict the effectiveness and cost-effectiveness of scabies interventions, disease transmission modelling can be used.

Objective

To review published scabies models and data to inform the design of a comprehensive scabies transmission modelling framework to evaluate the cost-effectiveness of scabies interventions.

Methods

Systematic literature search in PubMed, Medline, Embase, CINAHL, and the Cochrane Library identified scabies studies published since the year 2000. Selected papers included modelling studies and studies on the life cycle of scabies mites, patient quality of life and resource use. Reference lists of reviews were used to identify any papers missed through the search strategy. Strengths and limitations of identified scabies models were evaluated and used to design a modelling framework. Potential model inputs were identified and discussed.

Findings

Four scabies models were published: a Markov decision tree, two compartmental models, and an agent-based, network-dependent Monte Carlo model. None of the models specifically addressed crusted scabies, which is associated with high morbidity, mortality, and increased transmission. There is a lack of reliable, comprehensive information about scabies biology and the impact this disease has on patients and society.

Discussion

Clinicians and health economists working in the field of scabies are encouraged to use the current review to inform disease transmission modelling and economic evaluations on interventions against scabies.

Author summary

Scabies is a neglected tropical disease affecting more than 130 million people, with major costs on health care systems worldwide. While effective treatments exist, it is unknown which treatment strategies result in the best outcomes against the lowest costs, and to what extent this differs between communities. Health economic modelling can help answer these questions, but has rarely been used in this disease area. This review discusses all available scabies transmission models (n = 4), and uses them to create a new, comprehensive modelling framework. This framework can be used as aid for creating a scabies transmission model, the details of which will be determined by the context (population) and the question being addressed. The current paper also reviews the data that is needed to inform scabies modelling: on scabies biology, quality of life and resource use. Unfortunately, available data is limited and particularly data on crusted scabies (associated with high morbidity and mortality rates) is rare. With this review, we hope to assist researchers and policy makers to predict and/or evaluate the cost-effectiveness of interventions against scabies in their population(s) of interest. To tackle scabies, it is key to use effective treatment strategies in a cost-effective and sustainable way. The models and data described in this review, may help researchers, clinicians and funding bodies to facilitate this.

Introduction

Scabies is a common dermatological conditions [1], affecting more than 130 million people at any time [2]. It is a neglected disease caused by the mite Sarcoptes Scabiei [3]. Scabies often results in severe itching, and in some patients, including those with compromised immunity, it may progress to “crusted scabies” (CS). The fissures associated with scabies provide a portal of entry for bacteria, potentially resulting in secondary infections, sepsis, indirect effects on renal and cardiovascular function, and death due to complications [4]. Secondary bacterial superinfections are uncommon in Western countries [5].

Worldwide, scabies is responsible for 0.07% of the total burden of disease [6]. Compared to its disease burden, scabies research is severely underfunded [7, 8], even though it imposes major costs on healthcare systems [2]. Various countries and organisations have identified scabies control as a public health priority and the World Health Organisation Strategic and Technical Advisory Group for Neglected Tropical Diseases recently recommended that scabies be included in the Neglected Tropical Disease profile in category A [2, 911].

Elimination of scabies is difficult, as cured patients often get re-infected. Treatment strategies range from treating individuals and their contacts, to mass drug administration (MDA) strategies [1219], which involves treating whole communities at once. Drugs include oral ivermectin as well as a range of topical treatment options. It is unknown which (combination of) treatment strategies results in the best health outcomes against the lowest costs, and to what extent this differs between communities. Health-economic modelling may help answer such questions. To determine the cost-effectiveness of interventions against scabies, it is crucial to take into account it’s infectious nature since the extent to which interventions impact transmission will (to a large part) determine their cost-effectiveness. While such disease transmission approaches have successfully been applied to guide interventions against other infectious diseases like Ebola and influenza [20], few attempts have been made to use modelling to aid decision-making about scabies intervention strategies.

Scabies interventions include efforts aimed at access and coordination of services, scabies detection, primary care, acute care, specialised care, social work, follow-up, or combinations of the above. Given the multifaceted nature of interventions required to combat scabies, health-economic evaluation requires a comprehensive modelling approach. For this article, a systematic review of existing scabies models was conducted to inform the development of a proposed modelling framework which can be adjusted to different situations/communities. The proposed modelling framework aims to determine long-term effects of alternative interventions on the incidence, prevalence, quality of life (QoL), resource use and costs associated with scabies and CS. It can be used as aid for creating a scabies transmission model, the details of which will be determined by the context (population) and the question being addressed.

For models to be of use for decision-makers, a range of clinical and economic inputs is required. However, up to now, no systematic overview of evidence-based information on these inputs, including evidence on the biology of scabies, patient QoL and resource use has been published. This systematic literature review fills this gap by providing an overview of published information that can be used to inform scabies modelling in human populations, and improve decision-making about scabies interventions in affected communities.

After discussing the search strategy, this paper will first discuss characteristics of published scabies models and a proposed, comprehensive scabies modelling framework. Secondly, the paper will discuss potential model inputs, consecutively: the life cycle of scabies mites, patient QoL, and resource use associated with scabies and CS.

Methods

In order to inform our proposed modelling framework design, a systematic literature search was performed on 26 and 27 July 2017, searching the databases PubMed, Medline, Embase, CINAHL, and the Cochrane Library. Search terms related to the disease ("scabies" OR "sarcoptes") were combined with search terms identifying the type of information required ("model" OR “modeling” OR “modelling” OR "transmission" OR "utility" OR “quality of life” OR “economics” OR “economic” OR “cost-effectiveness” OR “cost-utility” OR “cost” OR “cost-of-illness” OR “cost-consequence” OR “cost-consequences” OR “efficacy” OR “effectiveness” or “impact”).

Articles were limited to humans only, had to be published in English and after the year 2000. Studies from before the year 2000 were only included if they were cited in a more recent source (post 2000), confirming their continued relevance. Studies that considered scabies but did not present any models or model inputs on scabies biology, QoL or resource use were excluded. Articles were also excluded when they presented a case study of a single patient or an outbreak of scabies in a single institution, and if they simply provided a discussion of scabies guidelines or protocols in a particular country or institution. Those studies that mentioned scabies as one of a number of diseases/indications/causes/comorbidities were also excluded. Reference lists of reviews were used to identify any papers missed through the search strategy.

Data extractions were performed by the primary author, per topic area: (1) models, (2) biology of scabies, (3) patient QoL and (4) resource use. Data items were not predefined, meaning that all data on the various topic areas (e.g. QoL) was included and presented, independent of reported outcome measures (e.g. which type of QoL questionnaire was used). After the data was extracted per topic area, it was evaluated which data per topic addressed simple scabies, CS, and which were based on populations including both simple scabies and CS patients. Results were not meta-analysed.

The validity and reliability of disease models and inputs is highly dependent on the research question they try to answer, the population of interest, and how the models are informed/how the inputs are used. Therefore, this review did not include a quantitative risk of bias assessment, but a qualitative description of the limitations of the various models and input parameters. All scabies models were evaluated on their main characteristics, and strengths and limitations were identified. Strengths of the various models were combined to design a new, proposed modelling framework. Relevant data from papers on the biology of scabies, patient QoL and resource use was extracted and described.

A review protocol has not been published. A completed PRISMA checklist is provided as supplementary material.

Results

The literature search identified 821 articles, 30 of which were included. Fig 1 provides information on article selection, reasons for exclusion, and categorisation of included articles.

Published scabies models

Four scabies models were identified from the literature (Table 1). One model was a Markov decision tree, two were compartmental models, and one was an agent-based, network-dependent Monte Carlo model (see Table 2 for a description of these model types). None of the models identified specifically addressed CS.

Bachewar et al. [21] published the Markov decision tree as part of a randomised clinical trial comparing three alternative treatment regimens. As opposed to the other models, this model does not consider scabies epidemiology, transmission or population dynamics. A Markov decision tree is provided, which is used to calculate the cost-effectiveness of alternative treatments, using efficacy data from the trial. The model serves as a mechanism to calculate and compare costs for a range of interventions, without considering the biology or transmission of scabies. This limits its use compared to the other identified models.

Gilmore [22] published an agent-based Monte Carlo model, using a variety of small-world network architectures to gain insight into scabies dynamics and the effect of alternative treatment strategies. This study focused on childhood scabies and found that in the absence of an effective vaccine, and with scabies continually imported to communities from non-local contacts, eradication is impossible and open-ended treatment regimens are required. A crucial advantage of Gilmore’s model is that it allows for non-random mixing patterns [25], since it is likely that the contacts between individuals that result in scabies infestation, do not occur at random [26]. Mixing patterns are a characteristic of a network (e.g. community) referring to the extent to which nodes (e.g. people) connect (e.g. are in close enough contact to result in infection).

Bhunu et al. [23] published a deterministic, compartmental model, using Descartes’ rule of signs and numerical simulations to show endemic equilibria and determine whether the current treatment regime is sufficient to control scabies infection, or whether a vaccine is required. Assumed values for key model parameters were not substantiated, and it is not clear what the model is calibrated to. This means it is impossible to evaluate the reliability of the model and any of its results. The model focussed on predicting the potential impact scabies vaccination might have in case a vaccine would become available. It should likely be viewed as a theoretical exercise rather than one that provides actual insights into scabies epidemiology or into the effectiveness of any available intervention.

Lydeamore et al. [24] recently published another compartmental model to explore the impact of alternative MDA treatment strategies. As opposed to the other models, this model aimed to capture the mite’s life cycle in relation to the host. The authors considered this critical, as the parasite’s life state (e.g. eggs versus living mites) can interact critically with treatment success or failure. This is a valuable model characteristic when studying the effectiveness of different treatment types (e.g., ovicidal versus non-ovicidal). In contrast to the model by Gilmore at al., homogeneous mixing is assumed.

None of the models included QoL. An advantage of including QoL is that it can be used to quantify the impact of a wide range of conditions and that (under certain requirements) it can be multiplied with duration of life to obtain QALYs (quality-adjusted life years). By measuring the impact of interventions on QALYs, outcomes can be compared not only between different (types of) interventions but also across different disease areas. This is needed to inform decision-making, particularly when funds need to be distributed over interventions/programs in a range of different areas. In models, QoL can be used as outcome measure, for example by weighting health states (e.g. “CS grade 1”) by their associated utility value. Furthermore, disutilities can be attached to complications as well as treatment-related adverse events. The same is true for costs, which can be attached to the various health states and events in cost-effectiveness models.

Only the model by Bachewar et al. included a cost-effectiveness analysis, but it did not take into account transmission dynamics.

Proposed scabies modelling framework

Combining the strengths of the abovementioned models, Fig 2 provides a proposed modelling framework to inform cost-effectiveness analyses. This framework can be used as aid for creating a scabies transmission model, the details of which will be determined by the context (population) and the question being addressed. Like the model by Gilmore (2011), it allows for modelling networks and mixing patterns. This is a valuable attribute in case the model will be used for evaluating the cost-effectiveness of interventions which aim, for example, to prevent reinfection through household level interventions (e.g. ensuring treated CS patients return to scabies-free homes), or community interventions to reduce the prevalence of scabies. Mixing patterns can be based on assumptions or, preferably, appropriate data collection. For example, to inform scabies transmission modelling in indigenous communities in Australia, information is currently being collected on the number of infections and reinfections in the various communities, living conditions (e.g. number of persons per household), and the extent to which scabies-free zones are being established when CS patients return from the hospital. Network size and structure will be dependent on the type of community that is being modelled, including age structure since children tend to have a higher probability of acquiring scabies [27]. For more information on challenges when modelling contact networks, see Eames et al. 2015 [28].

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Fig 2. Scabies transmission model.

Abbreviations: CI & DP, case identification and diagnostic processes; CSd, crusted scabies diagnosed; CSu, crusted scabies undiagnosed; CSt, crusted scabies treatment; I, infectious; R, recovered; S1, susceptible (never has been scabies infected); S2, susceptible (after prior scabies infection); SSd, simple scabies diagnosed; SSu, simple scabies undiagnosed; SSt, simple scabies treatment. The figure shows a susceptible population (S1) of individuals who can be infected (I) with scabies. The probability of infection depends on the interaction between individuals with other, infected, members of their household or community reflected by the network diagrams. Infected patients develop simple scabies (ISS), which is initially undiagnosed (SSu). Case identification and diagnostic processes (CI & DP) determine the probability that a patient becomes diagnosed (SSd). Subsequently, there is a probability that a patient takes up treatment and moves to SSt. Treatment has a probability of success, dependent on treatment efficacy and compliance. In this case, patients are cured and susceptible for reinfection (S2), the probability and infectiousness of which can differ from the probability and infectiousness in individuals who never had scabies before. Re-infection or sustained infection after unsuccessful treatment can go undiagnosed if a patient does not receive appropriate follow-up. A proportion of patients with simple scabies will develop CS. Grade 1 CS (CS1) can progress to grade 2 CS (CS2) and grade 3 CS (CS3). While both simple scabies and CS can result in complications, the probability of this is higher in patients with CS and increases with higher grade CS. Complications can result in death or can resolve. “Background mortality” reflects people who die from other causes than CS, which can differ between the susceptible and infected populations due to differences in comorbidities. In case the model is used to inform an economic evaluation, each of the health states (S1, SSu, SSd, SSt, CSu (grades 1–3), CSd (grades 1–3), CSt (grades 1–3) and S2) is associated with a cost and a utility. Diagnostics, treatments and complications can be associated with additional costs and/or (dis)utilities. The model can be used to simulate a community and how individuals move through the various health states over time, accruing costs, life-years and utility based on the time they spend in each of the health states. Results can be expressed in terms of costs per quality-adjusted life year (QALY) gained, or any other outcome captured in the model (e.g. cost per reinfection prevented or cost per life year gained).

https://doi.org/10.1371/journal.pntd.0007182.g002

The proposed modelling framework also aims to capture the biology and natural history of scabies transmission in humans, in particular the life-cycle of the mites as this can impact treatment success rates [24]. Depending on the modelling question at hand, it may be possible to simplify the proposed modelling framework or use other types of modelling methods. For example, when the difference between ovicidal versus non-ovicidal treatments is irrelevant to the question at hand, it may not be needed to track life-cycle stages of mites in individual patients. As a rule, simple models should be preferred over complex ones when the decision problem allows, since they are easier to understand, less prone to inaccuracies, and quicker to develop and run [29]. On the other hand, oversimplification may result in unreliable or invalid results when relevant risk-factors or dependencies between modelled states or agents are not taken into account. While it may be possible to simplify the model for some questions, others might require additional health states, for example to allow modelling of long-term complications (e.g. chronic renal failure) and their effects. For more information on different types of modelling methods, see Siettos and Russo 2013 [30].

Within the proposed modelling framework, model time should be counted in days, to account for processes like infection, the scabies life-cycle, and treatment effects. Other processes may take substantially longer, such as processes related to certain complications, and impacts on life expectancy. For most modelling questions, a life time horizon will be sufficient.

None of the identified models explicitly included CS. This is a shortcoming, since CS is associated with high infectivity, morbidity and mortality compared to simple scabies, and has often been overlooked in scabies program design. The proposed modelling framework incorporates a probability of moving from simple scabies to CS, which can be dependent on (amongst other factors) immune status of the patient.

While other infectious disease models often include some notion of seasonality, this has not been the case for scabies models. Although scabies mites show increased mite movement and increased transmission in a warm environment, a study in Malawi found that scabies was more prevalent during the cold, dry season, possibly due to close interpersonal contact in crowded indoor environments [31]. Other studies, however, show no obvious seasonal variation at all [3133]. In the absence of evidence to the contrary, the proposed modelling framework does not accommodate seasonality.

The following sections of this paper discuss input parameters that can be used to inform the suggested modelling framework, or other newly developed scabies models.

Life cycle of scabies mites

Following Lydeamore et al. [24], it is crucial to account for the life cycle of the scabies mites to model treatment effectiveness, especially when aiming to discriminate between ovicidal and non-ovicidal treatments. The literature review identified 14 articles presenting information on the life cycle of scabies mites (see Table 3). Since there is often a lack of Sarcoptes scabiei var. hominis mites, many studies have relied on animal strains of scabies mites and a host animal model such as rabbits or pigs [34]. The studies obtained through the search strategy did not provide any other information specifically on the life cycle of Sarcoptes scabiei var. hominis apart from “survival away from host”. However, based on direct comparisons, Sarcoptes scabiei var. canis seems to be a suitable model for sarcoptes scabiei var. hominis [34, 35].

Quality of life

The literature review identified 3 QoL studies performed in scabies patients: 1 from China (= 96), 1 from Brazil (n = 105) and 1 from India (n = 102). None of these studies addressed the QoL of CS patients compared to the QoL associated with simple scabies, and two of the studies [48, 49] excluded CS patients. Jin-Gang et al. [48] used the Dermatology Life Quality Index (DLQI), and Worth [50] and Nair [49] used a modified version of that same questionnaire. Modifications made by Worth et al. included: 1) adapting the language to local culture and attitudes; 2) modifying questions to increase relevance for persons living in an urban slum in the tropics; and 3) changing questions that were not applicable in children. Nair et al. used the modified version from Worth et al., with slight modifications as per the requirements of the Indian population.

Table 4 shows QoL results from the three studies, as per category of effect from scabies on QoL. Note that categorisation was based on classifiers described in the studies (e.g. “small effect”), not the modified DLQI item scores, since the questionnaires differed slightly between studies. Jin-Gang et al. reported a mean DLQI score of 10.09 (sd 5.96), with most QoL impact of scabies due to symptoms, embarrassment, work or study and sexual difficulties. Most common categories of impairment according to Worth et al. were feelings of shame (77.2% in adults, 46.6% in children), the need to dress differently (35.1% in adults, 29.3% in children), restriction on leisure activities (24.6% in adults, 36.8% in children), stigmatisation at work/school (21.1% in adults, 25.0% in children), social exclusion (24.6% in adults, 17.9% in children), teasing (26.3% in children), and problems with sexual partners (10.9% in adults). Women/girls perceived more restrictions than men/boys.

A review of studies using the Children’s Dermatology Life Quality Index (CDLQI) questionnaire to measure QoL in skin conditions [51] found an overall estimated CDLQI score of 9.2 (95%CI: 0.0–20.3) associated with scabies. The review identified two studies that were not identified in our literature review: Balci et al. [52] and Lewis-Jones & Finlay [53]. Both included children with a wide range of skin diseases, including only few scabies patients (n = 9 and n = 6, respectively). Olsen et al. commented that while scabies might have a large effect on QoL at the time of completing the questionnaire, this may only be over a short time as it is curable. While also curable, the disutility of CS may be more substantial, given the severity of associated symptoms and complications.

Resource use

While simple scabies is relatively straightforward to treat, patients may not seek care, may wait a long time before doing so, and may be misdiagnosed. In Cameroon, Kouotou et al. [54] found that it takes 4 to 720 days between the onset of symptoms and the first consultation with a dermatologist, with a mean of 77.1 days (sd 63.7). At the first consultation with a dermatologist, 74.9% had already tried previous treatment, such as antibiotics, antifungals, antihistamines or plant-based medicines.

Based on claims data for the employer-sponsored privately-insured population in the United States, treating one episode of scabies costs on average 95 USD [55]. When selecting on episodes for which drug treatment was claimed, costs were 163 USD per episode. Given the incidence of scabies, this results in an overall annual economic burden of 10.4 million USD for treating scabies in this population, most of which (3.7 million USD) is for children ≤15 years. Costs between alternative treatment options differ substantially. For a cost-benefit analysis from a US perspective, we refer to Elgart [56].

In some populations, scabies-associated health resource use is substantial. In five remote communities of Northern Australia, only a few aboriginal children (16%) manage to reach their first birthday without having at least one documented episode of scabies and/or skin sores [57]. Here, the median number of presentations per child under 12 months due to scabies is 3 (IQR 1, 5). Of these children, 70.5% present more than once, and the average age of first presentation is 4 months (IQR 2–7). In another Australian study, Whitehall et al. [32] found that the mean duration of hospital admissions for children with scabies is 4.5 days. Scabies comprised 4.2% of the total number of admissions for all reasons, and 8.3% of all bed days. The minimum cost per admission was 9,584.07 AUD. In Australia, the estimated annual cost associated with the management of pediatric scabies and pyoderma per patient was 10,000 AUD in 2013 [58].

Resource use may differ substantially between locations/communities, depending on the healthcare system, funding, remoteness, and cultural differences, amongst other factors. Local data collection will generally be required to inform model inputs. Resource use or costs for treating CS have not been published. A cost-of-illness study from an Australian perspective is currently being performed by (part of) the authors of this paper. Note that the cost-effectiveness of interventions to prevent CS may be substantially impacted by their ability to prevent long-term complications (e.g. rheumatic fever and chronic valvular heart disease) which may increase costs and decrease patient life expectancy and quality of life.

Discussion

Based on a systematic literature review, this paper discusses published models and proposes a new, comprehensive modelling framework to develop cost-effectiveness analyses of treatments for scabies. Models should be informed by population, disease and treatment characteristics, which may differ between communities. Available information on required model inputs was systematically reviewed. Prior to this review, the literature lacked a good account of these inputs, including the life cycle of scabies mites, patient QoL, and resource use. This review resolves this problem and should be supplemented by locally specific data collections and expert opinion where required.

There is a lack of reliable, comprehensive information about scabies biology and the impacts this disease has on patients and society. This may be due to the limited amount of resources directed towards scabies research [7, 8], and its tendency to affect resource-poor populations. Given the efficacy of available treatments and the relatively low costs of these treatments (although still prohibitively expensive in some low-income settings), current high prevalence rates of scabies are unacceptable. Interventions should aim to reduce scabies incidence in a sustainable, cost-effective manner. In doing so, it may be worth focusing additional efforts on identifying and treating patients with CS, who can be “core transmitters” of the disease, while experiencing high morbidity and mortality rates [38]. The importance of targeting CS patients has often been overlooked in program design for simple scabies.

Scabies elimination efforts should be prioritised for communities that are worst affected, and with sustained intervention, this is a realistic goal [59]. Given that many of these communities are resource-poor, cost-effective use of resources is crucial and can be informed by health-economic modelling, taking into account community-specific resource constraints and expected budget impact of proposed interventions. Furthermore, careful data collection (for example, aided by making scabies a notifiable disease) may help guide funds to where they are most needed.

While the current article provides a comprehensive overview of key issues and a proposed modelling framework to aid future scabies modelling work, it is only a first step in this direction. Researchers and policy makers are encouraged to use and adjust this modelling framework to develop an economic evaluation predicting the (cost)-effectiveness of interventions against scabies in their population(s) of interest. Any input on the proposed modelling framework by external parties is welcomed.

As with all health economic models, model transparency and validation of the results is critical to its success and potential impact. The current modelling framework has not been validated and should only be used as an aid for model development. By using the current review and proposed modelling framework to substantiate the modelling approach and select appropriate inputs, transparency can be improved. Proper validation involves face validity, verification of internal validity, external validity, and predictive validity. Health economists and modellers working in the field of scabies are referred to the ISPOR report on model transparency and validation for recommendations on how to appropriately validate and report on their model and results in a population of interest [60].

For data inputs that are uncertain, real-world data collection may be crucial to ensure reliability of modelling outcomes. Furthermore, the impact of uncertain model inputs can be tested by using sensitivity analyses to determine how variation in modelling inputs impacts the results, both deterministically and probabilistically. Given the identified knowledge gaps, it is important to perform extensive sensitivity analysis in any scabies model that will be developed. Meanwhile, grant bodies are encouraged to invest in scabies research to address the knowledge gaps identified in this review regarding the biology, QoL and cost impact of simple scabies and CS.

As far as the authors are aware, transmission modelling has seldom been used to answer questions on scabies interventions. One reason for this may be the lack of readily available information to inform modelling work, which this review aims to (at least partially) address. Another reason may be unfamiliarity or scepticism on the side of authorities and funding bodies with respect to the value of theoretical results obtained from modelling. Health economists and other scientists can best illustrate the value of modelling by using evidence-based, validated approaches to tackle relevant, real-world questions which can directly inform clinical or governmental decision-making.

Supporting information

Acknowledgments

The authors thank Alexander John Waber for creating the figures for this article.

References

  1. 1. Swe PM, Christian LD, Lu HC, Sriprakash KS, Fischer K. Complement inhibition by Sarcoptes scabiei protects Streptococcus pyogenes—An in vitro study to unravel the molecular mechanisms behind the poorly understood predilection of S. pyogenes to infect mite-induced skin lesions. PLoS neglected tropical diseases. 2017;11(3):e0005437. Epub 2017/03/10. pmid:28278252; PubMed Central PMCID: PMCPMC5360341.
  2. 2. WHO. Scabies 2017 [Accesed 29/08/2017]. Available from: http://www.who.int/lymphatic_filariasis/epidemiology/scabies/en/.
  3. 3. Wong SS, Poon RW, Chau S, Wong SC, To KK, Cheng VC, et al. Development of Conventional and Real-Time Quantitative PCR Assays for Diagnosis and Monitoring of Scabies. Journal of clinical microbiology. 2015;53(7):2095–102. Epub 2015/04/24. pmid:25903566; PubMed Central PMCID: PMCPMC4473232.
  4. 4. Goldstein BGG, A.O. Scabies 2017 [Accessed 16/08/2017]. Available from: https://www-uptodate-com.ezproxy.lib.uts.edu.au/index.html#!/contents/scabies?source=search_result&search=scabies&selectedTitle=1~81.
  5. 5. Esposito L, Veraldi S. Skin bacterial colonizations and superinfections in immunocompetent patients with scabies. International journal of dermatology. 2018;57(10):1218–20. pmid:29974940
  6. 6. Karimkhani Aksut C, Dellavalle RP, Naghavi M. Global skin disease morbidity and mortality: An update from the Global Burden of Disease Study 2013. JAMA Dermatology. 2017;153(5):406–12. pmid:28249066
  7. 7. Hagstrom EL, Patel S, Karimkhani C, Boyers LN, Williams HC, Hay RJ, et al. Comparing cutaneous research funded by the US National Institutes of Health (NIH) with the US skin disease burden. Journal of the American Academy of Dermatology. 2015;73(3):383–91.e1. Epub 2015/06/09. pmid:26051697.
  8. 8. Karimkhani C, Boyers LN, Prescott L, Welch V, Delamere FM, Nasser M, et al. Global burden of skin disease as reflected in Cochrane Database of Systematic Reviews. JAMA Dermatol. 2014;150(9):945–51. Epub 2014/05/09. pmid:24807687.
  9. 9. IACS. Website of the International Alliance for the Control of Scabies 2017 [Accessed 29/08/2017]. Available from: http://www.controlscabies.org/.
  10. 10. OneDisease. Website from One Disease 2017 [Accessed 29/08/2017]. Available from: http://onedisease.org/.
  11. 11. WHO. Report of the Tenth Meeting of the WHO Strategic and Technical Advisory Group for Neglected Tropical Diseases. Geneva: 2017 29–30 March. Report No.
  12. 12. Kearns TM, Speare R, Cheng AC, McCarthy J, Carapetis JR, Holt DC, et al. Impact of an Ivermectin Mass Drug Administration on Scabies Prevalence in a Remote Australian Aboriginal Community. PLoS neglected tropical diseases. 2015;9(10):e0004151. Epub 2015/10/31. pmid:26516764; PubMed Central PMCID: PMCPMC4627839.
  13. 13. Mohammed KA, Deb RM, Stanton MC, Molyneux DH. Soil transmitted helminths and scabies in Zanzibar, Tanzania following mass drug administration for lymphatic filariasis—a rapid assessment methodology to assess impact. Parasites & vectors. 2012;5(299):No pagination. Epub 2012/12/25. pmid:23259465; PubMed Central PMCID: PMCPMC3543323.
  14. 14. Lawrence G, Leafasia J, Sheridan J, Hills S, Wate J, Wate C, et al. Control of scabies, skin sores and haematuria in children in the Solomon Islands: Another role for ivermectin. Bulletin of the World Health Organization. 2005;83(1):34–42. pmid:15682247.
  15. 15. Heukelbach J, Winter B, Wilcke T, Muehlen M, Albrecht S, de Oliveira FA, et al. Selective mass treatment with ivermectin to control intestinal helminthiases and parasitic skin diseases in a severely affected population. Bull World Health Organ. 2004;82(8):563–71. Epub 2004/09/18. pmid:15375445; PubMed Central PMCID: PMCPMC2622929.
  16. 16. Pruksachatkunakorn C, Wongthanee A, Kasiwat V. Scabies in Thai orphanages. Pediatrics international: official journal of the Japan Pediatric Society. 2003;45(6):724–7. Epub 2003/12/04. pmid:14651549.
  17. 17. Romani L, Whitfeld MJ, Koroivueta J, Kama M, Tikoduadua L, Tuicakau M, et al. Mass drug administration strategies to control scabies in a highly endemic population. American Journal of Tropical Medicine and Hygiene. 2015;93 (4 Supplement):162. pmid:613370162.
  18. 18. Haar K, Romani L, Filimone R, Kishore K, Tuicakau M, Koroivueta J, et al. Scabies community prevalence and mass drug administration in two Fijian villages. Int J Dermatol. 2014;53(6):739–45. Epub 2013/10/31. pmid:24168177; PubMed Central PMCID: PMCPMC4282484.
  19. 19. Abedin S, Narang M, Gandhi V, Narang S. Efficacy of permethrin cream and oral ivermectin in treatment of scabies. Indian journal of pediatrics. 2007;74(10):915–6. pmid:17978449
  20. 20. Heesterbeek H, Anderson RM, Andreasen V, Bansal S, De Angelis D, Dye C, et al. Modeling infectious disease dynamics in the complex landscape of global health. Science. 2015;347(6227):aaa4339. pmid:25766240
  21. 21. Bachewar N, Thawani V, Mali S, Gharpure K, Shingade V, Dakhale G. Comparison of safety, efficacy, and cost effectiveness of benzyl benzoate, permethrin, and ivermectin in patients of scabies. Indian journal of pharmacology [Internet]. 2009; 41(1):[9–14 pp.]. Available from: http://onlinelibrary.wiley.com/o/cochrane/clcentral/articles/078/CN-00754078/frame.html.
  22. 22. Gilmore SJ. Control strategies for endemic childhood scabies. PloS one. 2011;6(1):e15990. Epub 2011/02/02. pmid:21283575; PubMed Central PMCID: PMCPMC3026796.
  23. 23. Bhunu C, Mushayabasa S, Monera T. Assessing the Impact of Vaccination on Controlling the Spread of Human Scabies. ISRN Computational Biology. 2013;2013.
  24. 24. Lydeamore M, Campbell P, Regan D, Tong S, Andrews R, Steer A, et al. A biological model of scabies infection dynamics and treatment informs mass drug administration strategies to increase the likelihood of elimination. Mathematical Biosciences. 2018.
  25. 25. Grassly NC, Fraser C. Mathematical models of infectious disease transmission. Nature reviews Microbiology. 2008;6:477–87. pmid:18533288
  26. 26. Mossong J, Hens N, Jit M, Beutels P, Auranen K, Mikolajczyk R, et al. Social contacts and mixing patterns relevant to the spread of infectious diseases. PLoS medicine. 2008;5(3):e74. pmid:18366252
  27. 27. Romani L, Steer AC, Whitfeld MJ, Kaldor JM. Prevalence of scabies and impetigo worldwide: a systematic review. The Lancet infectious diseases. 2015;15(8):960–7. pmid:26088526
  28. 28. Eames K, Bansal S, Frost S, Riley S. Six challenges in measuring contact networks for use in modelling. Epidemics. 2015;10:72–7. pmid:25843388
  29. 29. Brennan A, Chick SE, Davies R. A taxonomy of model structures for economic evaluation of health technologies. Health economics. 2006;15(12):1295–310. pmid:16941543
  30. 30. Siettos CI, Russo L. Mathematical modeling of infectious disease dynamics. Virulence. 2013;4(4):295–306. pmid:23552814
  31. 31. Tasani M, Tong SY, Andrews RM, Holt DC, Currie BJ, Carapetis JR, et al. The Importance of Scabies Coinfection in the Treatment Considerations for Impetigo. The Pediatric infectious disease journal. 2016;35(4):374–8. Epub 2015/11/26. pmid:26599569.
  32. 32. Whitehall J, Kuzulugil D, Sheldrick K, Wood A. Burden of paediatric pyoderma and scabies in North West Queensland. Journal of paediatrics and child health. 2013;49(2):141–3. Epub 2013/01/26. pmid:23347222.
  33. 33. Feldmeier H, Jackson A, Ariza L, Calheiros CM, Soares Vde L, Oliveira FA, et al. The epidemiology of scabies in an impoverished community in rural Brazil: presence and severity of disease are associated with poor living conditions and illiteracy. Journal of the American Academy of Dermatology. 2009;60(3):436–43. Epub 2008/12/10. pmid:19064303.
  34. 34. Arlian LG, Morgan MS. A review of Sarcoptes scabiei: Past, present and future. Parasites and Vectors. 2017;10 (1) (no pagination)(297). pmid:616865735.
  35. 35. Arlian L, Runyan R, Achar S, Estes S. Survival and infestivity of Sarcoptes scabiei var. canis and var. hominis. Journal of the American Academy of Dermatology. 1984;11(2):210–5.
  36. 36. ICPMR. Scabies 2016 [Accessed 16/08/2017]. Available from: http://medent.usyd.edu.au/fact/scabies.html.
  37. 37. Heukelbach J, Feldmeier H. Scabies. Lancet. 2006;367(9524):1767–74. pmid:16731272.
  38. 38. Whybrew C. Treating scabies infestations in children and adults. Prescriber. 2017;28(5):15–20. pmid:616606636.
  39. 39. Friedman R. The story of scabies: Froben Press; 1948.
  40. 40. Currie BJ, McCarthy JS. Permethrin and ivermectin for scabies. New England Journal of Medicine. 2010;362(8):717–25. pmid:20181973.
  41. 41. Walton SF, McKinnon M, Pizzutto S, Dougall A, Williams E, Currie BJ. Acaricidal activity of Melaleuca alternifolia (tea tree) oil: in vitro sensitivity of sarcoptes scabiei var hominis to terpinen-4-ol. Archives of dermatology. 2004;140(5):563–6. Epub 2004/05/19. pmid:15148100.
  42. 42. Gould D. Prevention, control and treatment of scabies. Nursing Standard. 2010;25(9):42–6. pmid:21141167. Language: English. Entry Date: 20110121. Revision Date: 20150819. Publication Type: Journal Article.
  43. 43. Mellanby K. The development of symptoms, parasitic infection and immunity in human scabies. Parasitology. 1944;35(4):197–206.
  44. 44. Heilesen B. Studies on acarus scabiei and scabies by Bjørn Heilesen. Acta Dermato-Venereologica. 1946;26(14):370.
  45. 45. Munro J. Report of Scabies Investigation. Journal of the Royal Army Medical Corps. 1919;33(1):1–41.
  46. 46. Van Neste D, Mrena E, Marchal G. Life cycle of scabies mite (Sarcoptes scabiei var. hominis) studied by scanning electron microscopy (author's transl). Annales de dermatologie et de venereologie. 1981;108(4):355–61. pmid:6789755
  47. 47. Arlian L, Vyszenski-Moher D. Life cycle of Sarcoptes scabiei var. canis. The journal of Parasitology. 1988;74(3):427–30. pmid:3132547
  48. 48. Jin-gang A, Sheng-xiang X, Sheng-bin X, Jun-min W, Song-mei G, Ying-ying D, et al. Quality of life of patients with scabies. Journal of the European Academy of Dermatology and Venereology: JEADV. 2010;24(10):1187–91. Epub 2010/03/20. pmid:20236379.
  49. 49. Nair PA, Vora RV, Jivani NB, Gandhi SS. A study of clinical profile and quality of life in patients with scabies at a rural tertiary care centre. Journal of clinical and diagnostic research: JCDR. 2016;10(10):WC01. pmid:27891435
  50. 50. Worth C, Heukelbach J, Fengler G, Walter B, Liesenfeld O, Feldmeier H. Impaired quality of life in adults and children with scabies from an impoverished community in Brazil. Int J Dermatol. 2012;51(3):275–82. Epub 2012/02/22. pmid:22348561.
  51. 51. Olsen JR, Gallacher J, Finlay AY, Piguet V, Francis NA. Quality of life impact of childhood skin conditions measured using the Children's Dermatology Life Quality Index (CDLQI): a meta-analysis. The British journal of dermatology. 2016;174(4):853–61. Epub 2015/12/22. pmid:26686685.
  52. 52. Balcı DD, Sangün Ö, İnandı T. Cross validation of the turkish version of children’s dermatology life quality index. J Turk Acad Dermatol. 2007;1(4):71402a.
  53. 53. Lewis‐Jones M, Finlay A. The Children's Dermatology Life Quality Index (CDLQI): initial validation and practical use. British Journal of Dermatology. 1995;132(6):942–9. pmid:7662573
  54. 54. Kouotou EA, Nansseu JR, Sieleunou I, Defo D, Bissek AC, Ndam EC. Features of human scabies in resource-limited settings: the Cameroon case. BMC dermatology. 2015;15(12):No pagination. Epub 2015/07/24. pmid:26201604; PubMed Central PMCID: PMCPMC4512092.
  55. 55. Owusu-Edusei K Jr., Chesson HW, Gift TL. The economic burden of pediculosis pubis and scabies infections treated on an outpatient basis in the United States: evidence from private insurance claims data, 2001–2005. Sexually transmitted diseases. 2009;36(5):297–9. Epub 2009/03/20. pmid:19295471.
  56. 56. Elgart ML. Cost-benefit analysis of ivermectin, permethrin and benzyl benzoate in the management of infantile and childhood scabies. Expert opinion on pharmacotherapy. 2003;4(9):1521–4. Epub 2003/08/29. pmid:12943481.
  57. 57. Kearns T, Clucas D, Connors C, Currie BJ, Carapetis JR, Andrews RM. Clinic attendances during the first 12 months of life for Aboriginal children in five remote communities of northern Australia. PloS one. 2013;8(3):e58231. Epub 2013/03/08. pmid:23469270; PubMed Central PMCID: PMCPMC3585931.
  58. 58. Thomas J, Carson CF, Peterson GM, Walton SF, Hammer KA, Naunton M, et al. Therapeutic Potential of Tea Tree Oil for Scabies. The American journal of tropical medicine and hygiene. 2016;94(2):258–66. Epub 2016/01/21. pmid:26787146; PubMed Central PMCID: PMCPMC4751955.
  59. 59. McLean FE. The elimination of scabies: a task for our generation. Int J Dermatol. 2013;52(10):1215–23. Epub 2013/07/10. pmid:23834401.
  60. 60. Eddy DM, Hollingworth W, Caro JJ, Tsevat J, McDonald KM, Wong JB. Model transparency and validation: a report of the ISPOR-SMDM Modeling Good Research Practices Task Force–7. Medical Decision Making. 2012;32(5):733–43. pmid:22990088