Scabies and other ectoparasites
Human scabies is a parasitic infestation caused by Sarcoptes scabiei var hominis. The microscopic mite burrows into the skin and lays eggs, eventually triggering a host immune response that leads to intense itching and rash. Scabies infestation may be complicated by bacterial infection, leading to the development of skin sores that, in turn, may lead to the development of more serious consequences such as septicaemia, heart disease and chronic kidney disease. In 2017, scabies and other ectoparasites were included as Neglected Tropical Diseases (NTDs), in response to requests from Member States and the recommendations of the WHO Strategic and Technical Advisory Group for NTDs.
Fingers infected with scabies
Scabies is one of the commonest dermatological conditions, accounting for a substantial proportion of skin disease in developing countries. Globally, it is estimated to affect more than 200 million people at any time, although further efforts are needed to assess this burden. Prevalence estimates in the recent scabies-related literature range from 0.2% to 71%.
Scabies is endemic in many resource-poor tropical settings, with an estimated average prevalence of 5 – 10% in children. Recurrent infestations are common. The sheer burden of scabies infestation and its complications imposes a major cost on health-care systems. In high-income economies, cases are sporadic, yet outbreaks in health institutions and vulnerable communities contribute to significant economic cost in national health services.
In 2015, it was estimated that the direct effects of scabies infestation on the skin alone led 0.21% of disability-adjusted life-years (DALYs) from all conditions globally. The indirect health impact of scabies complications, including bacterial infection, renal and cardiovascular disease may be far greater.
Scabies occurs worldwide. However, it is the most vulnerable groups - young children and the elderly in resource-poor communities - who are especially susceptible to scabies and the secondary complications of infestation. The highest rates of infestation occur in countries with hot, tropical climates, especially in communities where overcrowding and poverty coexist, and where there is limited access to treatment.
Pathology and sequelae
Scabies mites burrow into the top layer of the epidermis where the adult female lays eggs. The eggs hatch in 3-4 days and develop into adult mites in 1-2 weeks. After 4–6 weeks the patient develops an allergic reaction to the presence of mite proteins and faeces in the scabies burrow, causing intense itch and rash. Most individuals are infected with 10-15 mites.
Immunosuppressed individuals, including people living with HIV/AIDS, may develop an uncommon manifestation called crusted (Norwegian) scabies. Crusted scabies is a hyper-infestation with thousands to millions of mites, producing widespread scale and crust, often without significant itching. This condition has a high mortality if untreated, due to secondary sepsis.
Mite effects on immunity, as well as the direct effects of scratching, can lead to inoculation of the skin with bacteria (particularly Staphylococcus aureus and Streptococcus pyogenes), leading to the development of impetigo (skin sores), especially in the tropics. Impetigo may become complicated by deeper skin infection such as abscesses or serious invasive disease, including septicaemia. In one highly endemic area in Fiji, 94% of diagnosed impetigo was attributable to scabies infestation. In tropical settings, scabies-associated skin infection is a common risk factor for immune-mediated complications of impetigo such as acute post-streptococcal glomerulonephritis (kidney disease) and possibly rheumatic heart disease. Evidence of acute renal damage can be found in up to 10% of children with scabies infestation in resource-poor settings and, in many, this persists for years following infection contributing to permanent kidney damage.
Scabies is usually transmitted person-to-person through close skin contact (e.g. living in the same residence) with an infested individual. The risk of transmission increases with the level of infestations, with highest risk due to contact with individuals with crusted scabies. Transmission due to contact with infested personal items (e.g. clothes and bed linens) is unlikely with common scabies, but may be important for individuals with crusted scabies. As there is an asymptomatic period of infestation, transmission may occur before the initially infested person develops symptoms.
Diagnosis of scabies infestation is based on clinical recognition of the typical features of infestation, supported when necessary by visual imaging techniques such as dermatoscopy or microscopy of skin scrapings from burrows. Patients typically present with severe itch, linear burrows and vesicles around the finger webs, wrists, upper and lower limbs and belt area. Infants and small children may have a more widespread rash, including involvement of the palms, soles of the feet, ankles, and sometimes the scalp. Inflammatory scabies nodules may be seen, particularly on the penis and scrotum of adult males and around the breasts of females. Because of the delay between initial infection and development of symptoms, burrows may be seen in close contacts that have yet developed itch.
Individuals with crusted scabies present with thick, exfoliating crusts that may be more widespread, including the face.
Primary management of affected individuals involves application of a topical scabicide such as 5% permethrin, 0,5% malathion in aqueous base, 10–25% benzyl benzoate emulsion or 5–10% sulphur ointment. Oral ivermectin is also highly effective, and is approved in several countries. Safety of ivermectin in pregnant women or children under 15 kg body weight has not been established, so ivermectin should not be used in these groups until more safety data are available. Itch commonly intensifies with effective treatment for 1-2 weeks, and treated individuals should to be informed about this.
Because people in the early stage of new infestation may be asymptomatic and because the treatments for scabies do not kill the parasite’s eggs, best results are obtained by treating the whole household at the same time and repeating treatment in the time frame appropriate for the chosen medication.
Secondary management involves prompt treatment of the complications of scabies, such as impetigo using appropriate antibiotics or antiseptics.
Patients with crusted scabies are important to identify as they are a significant source of reinfection to the rest of the surrounding community. Specialist management is required.
Control and elimination
Population control of scabies and its complications has been identified by a number of countries as a public health priority, though a definitive control strategy is still being developed.
Mass drug administration (MDA) strategies appear promising in achieving community control, with a number of studies showing that MDA of a scabicide can substantially reduce prevalence of scabies, with concomitant reductions in impetigo. A comparative trial of oral ivermectin MDA, topical permethrin MDA and clinic based-treatment of infected individuals with permethrin found that a single round of MDA using oral ivermectin, followed by repeat treatment 7-14 days later for individuals with scabies, was able to reduce the prevalence of scabies and impetigo at 12 months by 94% and 67% respectively in small island communities in Fiji. The permethrin MDA arm and the standard care arm also reduced prevalence, but to a lesser degree. Additional operational research will be needed to confirm these findings and to develop a feasible control strategy.
Outbreaks of scabies can occur in either closed, institutional settings (such as hospitals, boarding schools or long-term care facilities) or open community settings. Refugee or internally displaced person camps are at particularly high risk, as the overcrowding typically present in the camps increases skin to skin contact between infested and uninfested individuals. Outbreaks can be extended and difficult to control. The general principles include surveillance in high-risk settings, early confirmation of an outbreak, and involvement public health experts. Mass treatment strategies are usually recommended.
What will WHO do about scabies?
In response to interest expressed about development of a control strategy and an outbreak response plan, WHO will begin convening experts to review the available data and develop a control plan that includes recommendations for outbreak response. In order to develop a control strategy, WHO recognizes that the burden of disease and the risk of long-term sequelae need to be better defined. As details of the strategy are specified, WHO will identify those components that can be integrated with existing activities in order to facilitate rapid, cost-effective uptake of the strategy.
It will be important to ensure access to effective treatments for any strategy to succeed. WHO is already working with partners to have ivermectin added to the WHO Model List of Essentials Medicines when it is next updated. Additionally, it is working to ensure that quality, effective medications will be accessible to the countries that need them.
External links to websites of some organizations with information on scabies:
NB: Site under construction for "other ectoparasites"