Epidermal parasitic skin diseases: a neglected category of poverty-associated plagues.
By Hermann Feldmeiera & Jorg Heukelbachb .
a. Institute of Microbiology and Hygiene, Charité University Medicine Berlin, Campus Benjamin Franklin, Berlin, Germany.
b. School of Medicine, Federal University of Ceará, Fortaleza, Brazil.
Epidermal parasitic skin diseases (EPSD) occur worldwide and have been known since ancient times. Despite the considerable burden caused by EPSD, this category of parasitic diseases has been widely neglected by the scientific community and health-care providers. This is illustrated by the fact that in the recent edition of The Communicable disease control handbook, a reference manual for public health interventions, only one EPSD (scabies) is mentioned.1 EPSD fulfil the criteria defined by Ehrenberg & Ault (2005) for neglected diseases of neglected populations, but are not listed on national or international agendas concerning disease control priorities.2,3 This probably explains why efforts to control EPSD at the community level have very rarely been undertaken.4
Six EPSD are of particular importance: scabies, pediculosis (head lice, body lice and pubic lice infestation), tungiasis (sand flea disease) and hookworm-related cutaneous larva migrans (HrCLM). They are either prevalent in resource-poor settings or are associated with important morbidity. In this paper we focus on these diseases, summarize the existing knowledge on the epidemiology and the morbidity in resource-poor settings and focus on the interactions between EPSD and poverty.
We use the term “underprivileged population” to designate a typical resource-poor setting in low-income countries, in contrast to the socioeconomic characteristics of affluent communities in high-income countries. The expressions “hot-climate country” and “cold-climate country” are used when we refer to climatic restrictions on the occurrence of EPSD.
Searches of PubMed and LILACS using keywords “parasitic skin disease”, “scabies”, “pediculosis”, “tungiasis”, “cutaneous larva migrans” and their synonyms were used as a source of references. Searches were made without time limitations. In addition, we used references retrieved by the authors during previous work on EPSD. Articles in English, French, Portuguese and Spanish were reviewed and analysed where quantitative data were provided, the study design was sound and the study had been performed in a resource-poor setting in a low-income country. Of 95 articles identified by these criteria, 50 were selected and cited in the reference list.
The six major EPSD differ considerably in their biological and epidemiological characteristics and life cycles (Table 1). Scabies is caused by a mite (Sarcoptes scabiei), pediculosis by lice, tungiasis by sand fleas (Tunga penetrans) and HrCLM by nematode larvae. Although HrCLM and tungiasis are self-limiting diseases, the parasites may persist for months and can cause long-lasting sequels. S. scabiei and lice propagate continuously and cause persisting symptoms if the infestation remains untreated.5
In EPSD, host-parasite interactions are restricted to the stratum corneum, the upper layer of the epidermis, which is where the ectoparasites complete their life-cycles, in part or entirely. In other parasitic skin diseases, such as leishmaniasis, loiasis or onchocerciasis, other layers of the dermis are also affected. Whereas S. scabiei and lice accomplish their life-cycle within or on top of the epidermis, T. penetrans needs the host only for the production of eggs and completes its other developmental stages off-host. In contrast, animal hookworm larvae that have penetrated into the epidermis find themselves at a biological impasse and cannot develop further.
Scabies, pediculosis capitis and pediculosis pubis occur worldwide but pediculosis corporis is restricted to cold-climate countries and is virtually absent in the tropics (Table 1). HrCLM is very rare in industrialized parts of the world but is ubiquitously present in developing countries.6 Tungiasis is geographically restricted to the Caribbean, sub-Saharan Africa and South America.7
Except in epidemic circumstances, data on EPSD are not recorded so there is no reliable information available on global disease occurrence, changes in incidence over time, and spatial distribution in endemic areas. Hengge et al.8 suggested that 300 million cases of scabies exist worldwide, with many more individuals being at risk at any point in time. Similarly, in resource–poor settings, virtually all individuals are permanently at risk for head-lice infestation, i.e. several billion people globally. As tungiasis and HrCLM are climatically and spatially restricted, the number of people at risk is lower, although still sufficient to merit attention.
The distribution of EPSD is irregular, and incidence and prevalence vary in relation to area and population studied. A study in a resource-poor community in urban Bangladesh, for example, showed that virtually all children aged less than 6 years developed scabies within a period of 12 months.9 In a rural village in the United Republic of Tanzania, the overall prevalence was 6%, in rural and urban Brazil 8–10%, and in rural India 13%.10–12 In Egyptian children, the prevalence was estimated to be 5% but in Australian Aboriginal communities the prevalence in this age group approached 50%.13,14 Of 5–9-year-olds children living in a displacement camp in Sierra Leone, 86% were found to be infested with S. scabiei.15
In some native populations in the Amazon lowland, head-lice infestation is present in virtually all inhabitants, while it is quite rare among adults in affluent societies.16 In an urban slum in Fortaleza, Brazil, girls experienced 19 new head-lice infestations per year, and boys 15 (authors’ unpublished data, 2008). In contrast, in Germany the incidence was estimated at 1500 per 10 000 children per year.16
During peak transmission, the prevalence of tungiasis in children living in resource-poor rural and urban communities in Brazil and Nigeria reached more than 60%.7,17 In contrast, in high-income communities in these same countries, tungiasis is restricted to single cases that typically occur when people visit local beaches.18 The situation is similar for HrCLM, with prevalence in children as high as 15% during the rainy season and an incidence of 1.840 cases per 10 000 individuals per year.19
EPSD usually show considerable seasonal variation of disease occurrence (Table 1).20–23 In the tropics, the cyclical changes are particularly evident in tungiasis and HrCLM; prevalence of tungiasis is highest in the dry season and of HrCLM in the rainy season.19,20
The factors responsible for the high burden of EPSD in resource-poor communities are complex and have not been clarified. It has been suggested that crowding, sharing of beds, frequent population movements, poor hygiene, lack of access to health care, inadequate treatment, malnutrition and social attitudes contribute to the high burden of scabies in these settings.24 It is difficult to disentangle the relative importance of economic, environmental and behavioural factors, since they frequently coexist.25 There is, however, circumstantial evidence that extreme poverty and its economic and social consequences play a pivotal role (Fig. 1).9,25
See complete article at World Health Organization.