Scabies is a skin condition caused by infestation with the microscopic mite Sarcoptes scabiei var hominis . Common scabies causes severe itching, mite burrowing, and secondary skin lesions.
Scabies has a strong causal relationship with impetigo1 which can lead to more severe skin and soft tissue infections, invasive bacterial infections and post-streptococcal sequelae.2
Crusted scabies is a rare form, usually affecting immunosuppressed people, and is characterized by hyperkeratotic skin containing thousands or millions of mites.
The World Health Organization (WHO) adopted scabies as a neglected tropical disease (NTD) in 2017.3 This recognition has led to increased global awareness and efforts toward scabies control and even elimination as a public health problem.
The 2018 meeting of the WHO Strategic Technical Advisory Group on Monitoring and Evaluation of NTDs noted that "strong initial evidence is available for ivermectin-based mass administration of drugs (MAD) for the control of scabies in endemic populations." and simplified clinical case definitions for the field; however, there is currently no global strategy for scabies control."4
With an increasing global focus on scabies, it is timely to review recent advances in the understanding of scabies epidemiology, diagnosis, treatment, and public health control.
| Epidemiology of scabies |
Two recent studies have advanced the understanding of global epidemiology, also highlighting several gaps and problems. The Global Burden of Disease Study estimated the global prevalence of scabies in 2015 at approximately 200 million, and that scabies causes 71 disability-adjusted life years (DALYs) per 100,000 people, ranking 101 of the 246 conditions studied and contributing to 0.21% of global DALYs.5
This burden is comparable to that caused by Haemophilus influenzae type b meningitis (rank 100) and acute lymphoid leukemia (rank 103). A systematic review in 2015 described the available global data on the prevalence and distribution of scabies. 1
The prevalence of scabies ranged from 0.2% to 71.4%, which was significantly higher in children than in adolescents and adults and was highest in Latin American and Pacific regions. Most of the included studies were carried out in countries with a low or medium human development index.
These studies were unable to discern the distribution of scabies within countries and within populations, particularly in disadvantaged populations where overcrowding is more common. The high prevalence in indigenous communities within Australia, 1 and within refugee and displaced people populations, including recent arrivals to Europe, 6 highlights the highly inequitable distribution of scabies.
While the direct burden of scabies estimated by the Global Burden of Disease Study is high, this may be the "tip of the iceberg" of the burden mediated through secondary bacterial infection. In studies in the Pacific, the population’s attributable risk of impetigo due to scabies ranges from 41% to 93%.7,8
Complications resulting from impetigo include focal and systemic bacterial infections, post-streptococcal glomerulonephritis, and possibly rheumatic fever although the attributable risk of scabies is currently unknown. Furthermore, the considerable morbidity and mortality of crusted scabies was not included in the DALY calculations.
The quality of many studies included in these reviews was low, with variation in sampling and diagnosis. More rigorous studies are needed in a variety of settings and targeting disadvantaged populations to describe the global burden and distribution of scabies. In addition, population sampling methods and standardized diagnostic criteria are necessary.
| Diagnostic methods |
The clinical signs of scabies are papules, vesicles and linear tunnels with associated pruritus and scratch marks.
- In children and adolescents, injuries are more common in the interdigital spaces and wrists. Lesions are also frequently found in the armpits, belt line, legs, feet, and buttocks.
- In infants, lesions are commonly seen on the palms, soles, and ankles, but can be widespread, including involving the head and face.
The gold standard for diagnosis is a demonstration of scabies mite, eggs, or fecal material through microscopy examination of skin scrapings.
As scabies infestation typically involves only 10-15 mites, this method is highly operator-dependent, with low sensitivity, and is not feasible for most resource-limited clinical settings or field studies. Low-power direct visualization of tunnels in the skin using dermoscopy may be a useful aid to clinical examination.9
Advances in high-powered non-invasive methods including videomicroscopy, videodermoscopy, and reflectance confocal microscopy allow confirmation of the diagnosis through direct visualization of the mite.9
These methods, although sensitive, require considerable time for a complete examination and are dependent on expensive equipment and specialized personnel; therefore, they are more appropriate for use in clinics or research settings.
Serological and molecular techniques are under development but are not yet at a stage where they can be recommended for clinical or public health use. In most places, therefore, diagnosis depends on clinical evaluation of suggestive lesions in typical body distributions, supported by the presence of itching and affected close contacts.
A systematic review of diagnostic methods in therapeutic trials revealed wide variation with no predominant method.10 Most studies did not have well-defined diagnostic criteria. This variation complicates the interpretation and comparison of findings from epidemiological and therapeutic studies.
A standardized diagnostic approach is necessary to improve the burden of disease and determine the effectiveness of clinical therapies and public health interventions. An improved diagnostic approach will be needed considering feasibility in resource-poor settings while maintaining good sensitivity and specificity.
A recent consensus study led by The International Alliance for Scabies Control using the Delphi method has produced a set of diagnostic criteria for scabies. eleven
The criteria are organized into 3 levels according to the degree of diagnostic certainty. This approach allows versatility in the standards to be applied, taking into account the specific objectives and practical aspects of research and mapping projects.
While validation of these criteria in various research settings is required to determine diagnostic accuracy and legitimization of implementation, they represent a useful starting point to better describe the epidemiology of scabies.
| Treatments |
For all scabies medications, treatment of all household contacts of an index case is recommended. There are several topical treatment options for scabies, including permethrin (the most effective but also the most expensive topical agent), benzyl benzoate, crotamiton, lindane, sulfur compounds, and malathion.
Although these treatments are effective, adherence is compromised by skin irritation and inconvenience (they must be applied to the entire body for 8 hours or more). In resource-poor countries, the cost and limitations in the supply of medicines contribute to inadequate treatment of affected people and their household members.
Inadequate application or adherence is the main reason for treatment failure and continued transmission of mites. Permethrin resistance has not been confirmed for human scabies but has been observed in animal scabies and other ectoparasites.12
Ivermectin is the only effective oral treatment available. Ivermectin has no ovicidal activity, therefore a second dose is recommended after 7–14 days to kill new offspring. It has been approved for clinical use for scabies in several countries, including France, the Netherlands, Germany, Australia and New Zealand, and is used in many other places.
In addition to increased adherence for individual treatment, oral ivermectin has advantages as a treatment for household contacts and for community control using an AMD approach.12
Ivermectin is not recommended for pregnant women and children weighing less than 15 kg due to a lack of safety data. However, inadvertent use in pregnant women and young children for other indications has not shown additional risk in these populations.13,14
A 2018 Cochrane review compared oral ivermectin, topical ivermectin, and topical permethrin for scabies.15 There was little difference between oral ivermectin and topical permethrin in achieving complete resolution of the infestation by the second week after treatment.
The authors also found no difference in efficacy when comparing oral ivermectin with topical permethrin, topical ivermectin with topical permethrin, or topical ivermectin with oral ivermectin. There was also no significant difference in cure when comparing 1 versus 2 doses of ivermectin. However, the methodologies of the included studies limit confidence in these conclusions, and more studies are needed.
Moxidectin is an oral agent, recently approved for onchocerciasis, that shows promise as oral therapy for scabies. It is related to ivermectin, but with potential advantages due to having a substantially longer plasma half-life (up to 43 days, compared to less than 1 day for ivermectin) and greater lipophilicity allowing greater bioavailability in the skin.12 ,16,17 These properties could eliminate the need for a second dose of treatment and confer protection against reinfestation.
A study in pigs compared a single dose of moxidectin with 2 doses of ivermectin16 and found an efficacy of 100% versus 62%, respectively, when measured 47 days after treatment. Bioavailability and safety studies especially in young children will be important to establish moxidectin as a recognized treatment for scabies.
| Public health control |
Although available topical and oral medications provide effective individual treatment, people living in resource-limited settings with high prevalence are rapidly reinfested by household and community contacts.
An alternative strategy is to reduce community prevalence and therefore minimize transmission using AMD as demonstrated in the following studies. A series of single-arm studies in Panama, northern Australia and the Solomon Islands provided initial evidence to support this approach.
A more recent comparative study in Fiji demonstrated that ivermectin-based AMD was more effective than standard treatment (permethrin for affected individuals and household members) and permethrin-based AMD.18 The island group assigned to ivermectin-based AMD Ivermectin experienced a 94% relative reduction in scabies 12 months after AMD, with prevalence falling from 32% to 1.9%.
The standard care and permethrin-based AMD groups experienced a reduction in scabies prevalence of 49% and 62%, respectively.
Impetigo prevalence also decreased with the greatest reduction in impetigo prevalence (67%) in the ivermectin group compared to the standard care groups and the permethrin-based AMD groups (32% and 54%, respectively).
Another study investigated ivermectin-based AMD in a remote Australian Aboriginal community using a before and after study design of 2 rounds of ivermectin-based AMD, separated by 12 months.19 Prevalence decreased 6 months after each treatment but recovered quickly. There are several factors that may have led to contrasting results with the Fiji study.
1. First, administration occurred over 4 months, which may have allowed reinfestation, although acquisition rates were noted to be less than 1%–2% for a 6-month interval.
2. Second, there was considerable population mobility, with 34% of participants at 12-month follow-up not being present in the community at baseline. A broader distribution of AMD may be necessary to account for population movement.
3. Third, the increase in prevalence at 12 months was in a group of people linked to a person with crusted scabies, a condition that can serve as a source of ongoing transmission due to very high mite counts.
A recent study in the Netherlands evaluated ivermectin-based AMD to control scabies in newly arrived asylum seekers. The program actively screened arrivals from Ethiopia and Eritrea, and 65% of the 897 people were diagnosed with clinical scabies.
All individuals tested received ivermectin (apart from pregnant women and infants) and symptomatic individuals received a second dose after 2 weeks.
Evaluation of the program demonstrated a reduction in recurrent episodes of scabies from 42% to 27% after the program and a reduction in scabies complications from 12% to 5%.6
| Global responses to the need for scabies control |
The recognition of scabies as an NTD by the WHO and recommendations to establish guidelines and standards for public health interventions demonstrate motivation toward scabies control driven by multinational requests for guidance.
The development of a global strategy for scabies control will require standardized diagnosis, mapping and surveillance strategies, as well as guidance on the public health management of scabies.
Important operational research to evaluate the feasibility of AMD as a disease control option includes scalability testing, cost-effectiveness evaluation, evaluation of program acceptability, and investigation of whether AMD for scabies can translate into reductions in complications. serious bacterial and autoimmune causes of scabies. Establishing a viable and sustainable drug supply for scabies control is also crucial.
The current indications for ivermectin on the WHO Model List of Essential Medicines are for the treatment of filarial disease and intestinal helminths; The addition of scabies as an indication will be a crucial step for the implementation of any program.
Finally, global partnerships and meaningful collaboration, such as those fostered by the International Alliance for Scabies Control, 2 will be critical to progress toward the control and potentially even the elimination of scabies.
