Putting the burden of skin diseases on the global map

Skin diseases are the fourth most common cause of all human disease, affecting almost one-third of the world’s population, yet their burden is often underestimated, despite their visibility.1, 2 Burden of disease is a concept that was first developed in the 1990s by the Harvard School of Public Health, the World Bank and the World Health Organization (WHO) to describe death and loss of health due to diseases, injuries and risk factors for all regions of the world. This was based, to a large extent, on the disability-adjusted life-year (DALY). The DALY aggregates the time lost because of premature death and the time spent in a limiting health state. Consequently, the DALYs for a defined disease or health condition are calculated as the sum of the years lost due to specific premature mortality and the years lost due to disability for incident cases. While DALYs conveniently allow direct comparisons between diseases and disease groups, this approach is narrow. Epidemiological and public health research on disease burden therefore typically encompasses not only DALYs but also disease prevalence and incidence, morbidity (such as quality-of-life measures) and mortality, as well as the associated cost to health services and patients. As treatments are becoming more costly, comprehensive health economic evaluations become increasingly important for finitely resourced healthcare systems.3 It is not only the individual patient who carries the burden of their disease, but this burden often extends to their partners and society. A holistic understanding of the burden of skin diseases is key to the development of a concerted and sustained global response towards reducing their burden.4 The burden of skin diseases includes their high prevalence and the associated morbidity over time, including severe itching, for instance in the case of atopic dermatitis and urticaria, or disfigurement, such as in leprosy. Chronic inflammatory skin diseases, for example psoriasis, are common, and novel treatments, such as biologics, are costly. The high prevalence of skin cancer and associated treatment costs can be an economic threat to some healthcare systems. This is likely to worsen as the average life expectancy of the world’s population increases.5 In this special issue of the BJD, contributions from international author groups have been brought together in order to discuss the burden of selected skin diseases from different angles, for instance by using the Global Burden of Disease (GBD) database (http://www.healthdata.org/gbd/about) or by undertaking a systematic literature review to study the incidence and prevalence of skin disease or the impact on quality of life and cost. This includes both common and rarer disease entities, chronic inflammatory conditions, neglected tropical diseases (NTDs) and skin cancer. For the first time, Laughter et al.6 present data from the GBD dataset on atopic dermatitis. The GBD study captures disease mortality and morbidity data for more than 350 diseases and injuries in 195 countries, by age and sex, from 1990 to the present, allowing comparisons over time, across age groups, and between world regions. By analysing trends in the worldwide burden of atopic dermatitis between 1990 and 2017, Laughter et al. show that atopic dermatitis ranks 15th among all nonfatal diseases, with the highest disease burden among skin diseases, measured by DALYs. There was significant geographical variation, but the global burden of atopic dermatitis changed little over the study period, with the highest peak seen in early childhood and a second peak in middle-aged and older populations. There was also a moderate correlation between a country’s gross domestic product and the disease burden. Acne is another very common inflammatory dermatosis, and is reviewed by Layton et al.7 Recent studies suggest an increase in prevalence among adolescents and women. Key aspects of the burden of acne include its high visibility, often with associated scarring, its psychosocial impact and its treatment cost. Topical and oral antibiotics are commonly used as treatments, and there is an increasing concern about the risk of antibiotic resistance. Intervention studies are hampered by the diversity in outcome measures, now addressed by the Acne Core Outcomes Research Network (https://sites.psu.edu/acnecoreoutcomes). The global prevalence and incidence of psoriasis are examined by Iskandar et al.8 in a systematic review of population-based studies with a focus on differences in relation to age and gender, including studies from 22 countries. The incidence data confirmed a bimodal age pattern in psoriasis onset with peaks at 30–39 and 60–69 years of age, with an overall increase in disease prevalence over time. The study highlighted the methodological diversity in study designs, making direct comparisons and data pooling challenging. While chronic urticaria is less common than atopic dermatitis, psoriasis and acne, its impact on patients’ quality of life is often profound, mainly through severe itch and resulting sleep loss. Gonçalo et al.9 found that the overall annual cost of chronic urticaria can reach up to 15 550 purchasing power parity dollars, and more than 30% of people with urticaria have a history of anxiety or depression, sexual dysfunction and interference with life activities. Moving from chronic inflammatory skin diseases to skin cancer, Loney et al.10 bring together data from over 15 000 individuals recruited into 19 studies from 12 countries in the Americas, Europe and Oceania looking at the association between occupational exposure to solar ultraviolet radiation and the development of keratinocyte cancers, with a specific focus on geographical location and skin type. Eighteen of the 19 studies report an increased risk in basal cell carcinoma and squamous cell carcinoma among outdoor workers, although the association was only significant in 11 of these cohorts. Further well-designed and well-powered studies with careful adjustment for confounding factors are clearly needed. As Cox et al.11 show, scabies is one of the most common skin disorders. Comparisons between countries and with other diseases have been difficult because of a lack of clear diagnostic criteria until very recently.12 The impact on mental wellbeing and the association with nephritis, arising from secondary bacterial skin infections, have been neglected. Health economic estimates of the cost burden of scabies are also lacking. However, there is hope that the inclusion of scabies on the WHO’s list of NTDs may generate change. Last but not least in this special issue, Murdoch13 maps the global burden of onchocercal skin disease, an NTD caused by the nematode parasite Onchocerca volvulus, transmitted by Simulium blackflies. Originally, research efforts focused on the risk of blindness caused by the parasite. Mapping of the burden of skin manifestations of onchocerciasis was facilitated by the development of a clinical classification and grading system, allowing comparison of data from different geographical settings, a key element of the African Programme for Onchocerciasis Control across 20 countries. There is now real hope that the disease can be eradicated. Where do we go from here? Naturally, the selection of skin diseases for this special BJD issue left out many other conditions with a high burden on patients, health systems and societies at large, including genetic skin diseases, bacterial and fungal skin infections and skin ulcers. Back in 2016, the International League of Dermatological Societies launched the Grand Challenges initiative to put skin diseases on the world map.5 Building on the momentum from this initiative, promising steps have been made since, including the WHO resolution on psoriasis and the recognition of scabies as an NTD, the new designation of strategic targets as skin NTDs and the inclusion of more dermatological medications on the Essential Medicines List. Only a concerted effort to foster international advocacy, research and education campaigns will ensure that the voices of those with skin disease are heard at the global level. Carsten Flohr: Conceptualization (equal); Writing-original draft (lead). Rod Hay: Conceptualization (equal); Writing-review & editing (lead).