Fingers infected with scabies
Human scabies is a parasitic infestation caused by Sarcoptes scabiei var hominis. The mite, barely visible to the naked eye, burrows into the epidermis and lays eggs, triggering a host immune response that leads to intense itching in response to just a few mites. Scabies infestation is frequently complicated by bacterial infection, leading to the development of skin sores that, in turn, can cause more serious consequences such as septicaemia, heart disease and chronic kidney disease.
Scabies is one of the commonest dermatological conditions, accounting for a substantial proportion of skin disease in developing countries. Globally, it affects more than 130 million people at any time. Rates of scabies occurrence vary in the recent literature from 0.3% to 46%. In the developed world, outbreaks in health institutions and vulnerable communities contribute to significant economic cost in national health services. However, in resource-poor tropical settings, the sheer burden of scabies infestation, as well as their complications, imposes a major cost on health-care systems. In 2010, it was estimated that the direct effects of scabies infestation on the skin alone led to more than 1.5.million YLDS (years lived with disability), and the indirect effects of complications on renal and cardiovascular function are far greater.
Scabies affects people from every country. However, it is the most vulnerable, young children and the elderly in resource-poor communities who are especially susceptible to scabies as well as to the secondary complications of infestation. The highest rates occur in countries with hot, tropical climates, where infestation is endemic, especially in communities where overcrowding and poverty coexist.
Pathology and sequelae
Scabies mites burrow into the top layer of the skin where the adult female lays eggs. After 4–6 weeks, the patient develops an allergic reaction to the presence of mite proteins and faeces, causing an intense itch. 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 can, in turn, be complicated by deeper skin infection such as abscesses, as well as serious invasive disease and sepsis in infants. In tropical settings, scabies-associated skin infection is a common risk factor for immune-mediated complications such as acute post-streptococcal glomerulonephritis (kidney disease) and possibly rheumatic heart disease. Evidence of renal damage can be found in up to 10% of children with infected scabies in resource-poor settings and, in many, this persists for years following infection contributing to permanent kidney damage. Recurrent infestations are common.
Diagnosis of scabies is based on clinical recognition of the typical features. These comprise an itchy patient with linear burrows and vesicles around the wrists and especially finger webs, on the soles of the feet and ankles, and sometimes on the head in infants. Prolonged itching leads to the development of scabies nodules, which in adults are often found on the genital area, especially the penis and scrotum as well in areas around the breast. Additionally, asymptomatic family members may also have burrows in the finger webs. Itching occurs only if the individual reacts to the presence of the mite.
An uncommon but important clinical variant is “crusted scabies”. This condition occurs particularly in some immuno-suppressed patients, including those with HIV/AIDS, and is characterized by hyper-infestation with millions of mites, producing widespread scale and crust, often without significant itching. Patients with crusted scabies are important to identify as they are a significant source of reinfection to the rest of the surrounding community.
Primary management of affected individuals involves application of a topical scabicide such as permethrin 5% (caution in children aged under 6 months), 5% malathion in aqueous base, 10–25% benzyl benzoate emulsion or 5–10% sulphur ointment applied all over the body. In addition, there is increasing interest in the use of oral ivermectin (safety in pregnant women or children under 15 kg body weight has not been established). Best results are obtained by treating the whole household at the same time.
Secondary management involves prompt treatment of the complications of scabies, such as impetigo using appropriate antibiotics or antiseptics.
Control and elimination
Population control of scabies and its complications has been identified by some countries as a public health priority and an International Alliance for the Control of Scabies (IACS) is now working as a global network committed to this goal. Treatment of individuals with scabies and their contacts is unlikely to achieve this goal, and so there is increasing interest in implementing a mass drug administration (MDA) strategy. Large studies of MDA using oral ivermectin versus topical treatment are under way. An important aspect of control and elimination programmes is their integration into existing clinical and public health programmes and systems.