Human scabies is caused by infestation with the mite Sarcoptes scabiei var hominis. The mite, barely visible to the naked eye, burrows into the epidermis and lays eggs and triggers a host immune response, leading to intense itching following infestation with just a few mites. Scabies infestation is frequently complicated by bacterial infection leading to the development of skin sores which in turn can cause more serious consequences such as septicaemia, heart 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. Scabies occurrence rates 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 the complications of scabies infestation, 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 than1.5.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 are seen in countries with hot, tropical climates, where infestation is endemic, especially in communities where overcrowding and poverty co-exist.
Pathology & 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 tropical settings scabies-associated skin infection is the 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 to10% of children with infected scabies in resource poor settings and, in many, this persists for many 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 in adults and on the soles of the feet and ankles and sometimes the head in infants. Prolonged itching leads to the development of scabies nodules which in adults are often found on genital area, especially the penis and scrotum as well 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 occurs particularly in immuno-suppressed patients and is characterised by hyper-infestation with millions of mites, producing widespread scale and crust. Patients with crusted scabies are important to identify as they are a significant source of reinfection to the surrounding community.
Primary management of affected individuals involves application of a topical scabicide such as permethrin 5% (caution in children 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 15kg 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: treating impetigo with appropriate antibiotics / antiseptics.
Control and Elimination
Population control of scabies and its complications has been identified by some countries as a public health priority. Treatment of individuals with scabies and their contacts is unlikely to achieve this goal, and so there is increasing interest in a mass drug administration strategy. Large studies of mass drug administration using oral ivermectin versus topical treatment are currently underway. An important aspect of control and elimination programs is the integration of these programs into existing clinical and public health programs and systems.