Crusted scabies, previously referred to as Norwegian scabies, was first described in 1848 by Danielssen and Boeck in Norwegian patients with leprosy. Crusted scabies is characterized by hyperkeratotic and scaly plaques caused by overwhelming infection with Sarcoptes scabiei mites.[,] Following the original description in patients with leprosy, subsequent reports have described crusted scabies in patients with chronic neurologic disorders, lymphoreticular disorders, organ transplantation recipients, and HIV-infected individuals.[,,,,,,,,] Crusted scabies does not result from a more inherently virulent strain of the scabies mite. Rather, this more intense infestation develops because of the inability of the host cutaneous immune response to control the proliferation of the scabies mite. Recent data suggest that patients with crusted scabies have a markedly imbalanced dermal inflammatory response, with an abundance of CD8 T-lymphocytes, minimal CD4 lymphocytes, and an absence of B-lymphocytes. Most reports of crusted scabies in HIV-infected persons have involved patients with advanced immunosuppression and CD4 counts less than 100 ;cells/mm3.[,,,] Because of the very high mite load that develops in patients with crusted scabies, these individuals can readily transmit mites to others, including health care workers.
The clinical features of patients with crusted scabies differ markedly from the typical findings seen in patients with ordinary (or simple) scabies. Patients with ordinary scabies typically have pruritic, localized, papules, often with linear or serpentine burrow formation. The lesions are often located between the fingers, flexural surfaces of the wrists and elbows, periaxillary areas, and waistline and groin, including the genitals.[,] The scalp, face, and neck are typically spared. In contrast, patients with crusted scabies characteristically develop red, scaly, patches that evolve into yellow-grey, crusted, scaly and hyperkeratotic plaques (Figure 1) and (Figure 2).[,]. These plaques have aptly been described as "heaps of beige sand". Often the plaques eventually take on a warty appearance and may develop fissures. Crusted scabies can develop as localized or generalized lesions and can manifest anywhere on the body. About 50% of patients with crusted scabies have itching. Reports in HIV-infected patients have included a wide range of clinical presentations, including papular dermatitis, generalized plaque formation, and localized plaques located on the scalp, elbow, feet,[,] penis, and vulva. In HIV-infected persons, clinicians have misdiagnosed crusted scabies as nonspecific eczema, psoriasis, or Darier's disease. One case report described a patient with crusted-scabies-associated immune reconstitution inflammatory syndrome; this patient developed an aggressive form of crusted scabies that manifested as rapid progression of lesions after starting antiretroviral therapy.
With ordinary scabies, patients typically are infested with less than 50 body mites, whereas patients with crusted scabies have thousands (and up to millions) of mites. The definitive diagnosis of scabies is made by examining a skin scraping with a light microscope under low power and identifying scabies mites, eggs, eggshell fragments, or fecal pellets (scybala) (Figure 3).[,] Specimens should be viewed with a drop of mineral oil placed on the slide. The adult scabies mites are approximately 0.2 to 0.5 mm long and too small to see without magnification (Figure 4).[,] The adult scabies mites are identified by their eight legs, distinct gnatosoma (head with mouthparts), and the absence of an anatomic division between their cephalothorax and abdomen.[,] Scabies larvae have six legs and are smaller than the adults. Potassium hydroxide is not recommended for a scabies prep, in contrast to examination for a dermatophyte infection, as the reagent can dissolve diagnostic material for confirming scabies infestation. Although detecting mites or their products may be elusive with ordinary scabies, the diagnosis of crusted scabies is usually not difficult, given the high mite burden. Skin biopsy is not needed to make a diagnosis of crusted scabies. Patients with crusted scabies almost always have increased serum IgE levels and most haveeosinophilia.[,]
Patients with ordinary scabies are most often treated with the topical agent permethrin 5% cream (Elimite). The non-prescription 1% permethrin cream (Nix) does not have adequate potency to treat scabies. Crotamiton (Eurax) is as an alternative topical agent. In many countries, topical benzyl benzoate is used and is effective, although skin irritation is a common side effect. Lindane (Kwell) has been designated as a second-line agent because of neurotoxicity associated with the use of this agent. In recent years, oral therapy with ivermectin (Stromectol) has become more widely used to treat ordinary scabies, mainly because of the ease of administration. Ivermectin is a semisynthetic macrocyclic lactone that kills S. scabiei by suppressing nerve conduction and paralyzing the mites.
Treatment of crusted scabies presents several challenges, mainly because of the very high mite burden and the protective barrier provided to the mites by the hyperkeratotic lesions. No randomized, controlled therapeutic trials have been performed in patients with crusted scabies, and data specific to treatment of crusted scabies in HIV-infected persons remains limited. Based on small open-label studies and case reports,[,,,] ivermectin has become the drug of choice for the treatment of crusted scabies.[,,] Several reports, however, have documented clinical failure with single-dose ivermectin in patients with crusted scabies.[,] The Centers for Disease Control and Prevention (CDC) recommends treating crusted scabies using ivermectin and a topical scabicide. The CDC recommended ivermectin dosing is 200 mcg/kg given on days 1, 2, 8, 9, and 15, with potential additional treatment possibly on days 22 and 29 for the treatment of severe scabies. The topical scabicide should consist of either 5% topical benzoly benzoate or 5% topical permethrin, with a full body application given daily for 7 days then twice weekly until the crusted scabies has resolved. Resistance of S. scabiei to ivermectin has been reported. If topical permethrin is used alone for crusted scabies, patients will require repeated applications, but even then, failure rates are high. Some expert also add a keratolytic agent (20% urea ointment or 12% lactic acid lotion) applied twice daily until the hyperkeratotic lesions resolve. Secondary bacterial infection should be treated promptly with appropriate antimicrobials. Oral antihistamines, such as hydroxyzine (Vistaril), given at a dose of 25 to 50 mg orally every 6 hours can be helpful if pruritus is prominent.
If crusted scabies is suspected, strict contact isolation techniques are warranted, including use of gloves and gowns with patient contact. In addition, it is essential to decontaminate patient bedding and clothing by machine washing (at 60°C) and machine drying (using the hot cycle). These items should include those with which the patient had direct contact during the prior 72 hours.[,] If the clothing or material cannot undergo machine washing and drying, then these materials should be sealed for 72 hours. In an organized facility, such as a hospital, nursing home, or group home, the diagnosis of crusted scabies warrants an extensive evaluation to identify all persons who have come into direct contact with the infected patient. Those undergoing evaluation should include patient care staff, support staff (including cleaning staff and laundry employees), other patients at the facility, and all visitors. Because transmission of mites occurs easily with crusted scabies. Most experts would recommend prophylactic treatment of all staff, patients, and visitors who have experienced direct contact with the patient. Because of its ease of administration, ivermectin is an attractive therapy to use when treatment of large numbers of persons in a facility is required.