CORRECT DIAGNOSIS:

Crusted scabies

DISCUSSION:

The scabies mite, Sarcoptes scabiei var homiins, an obligate parasite, resides in burrowed tunnels within the stratum corneum (1). This same mite is also responsible for the more severe version known as crusted scabies. Crusted scabies was first described in Norway in 1849, and was formerly known as Norwegian scabies (2). Scabies is known to be a worldwide problem that can affect all races and socioeconomic groups, however, crusted scabies affects a more specific population. It is often found in patients with compromised immune systems such as the elderly and those with systemic illnesses including HIV, lymphoma and history of organ transplantation (3). It is also specifically seen in patient populations who are unable to scratch or sense itch, which allows the infestation to become more severe and often unrecognized until much later in its course (3,4). The degree of infestation and symptoms can be highly variable, but in general, patients with crusted scabies do not present with the severe pruritis seen in typical scabies. Patients will complain of a mild pruritic rash and sometimes no itch at all.

While traditional scabies has an average of 10-15 mites (5), in crusted scabies there can be thousands to millions of mites (4). The more extensive number of mites on the patients makes this condition significantly more contagious than typical scabies (4). While traditional scabies is contagious, it often requires extended periods of contact with an infected host. In contrast, crusted scabies can be transmitted through much shorter periods of direct contact or even by indirect contact such as by contacting the shed keratinous material or coming into contact with the bed linen the infected person is using (6).
Crusted scabies is often a clinical diagnosis. It typically presents as a psoriasiform dermatitis with intense hyperkeratotic crusts in acral areas due uncontrolled proliferation of mites in an immunocompromised host (7). To aid in diagnosis, skin scrapings of suspected burrows can be performed and examined under a light microscope with a mineral oil preparation (3). In crusted scabies, the hyperkeratotic material can be collected with pickups or ophthalmic scissors (6). The increased amount of scale in crusted scabies and increased amount of mites makes specimen collection easier than traditional scabies (8). Additionally, dermoscopic visualization of mites and eggs can aid in the diagnosis. Mites are typically found at the tip of burrows (6). Ultimately, diagnosis most often occurs through clinical evaluation and a positive response to appropriate antiscabietic management. (3).

TREATMENT:

The current standard in scabies treatment is 5% permethrin cream, applied from the neck down in two applications, spaced one week apart. Its mechanism of action involves the blockade of sodium channels, resulting in paralysis of the arthropod. In infants and the elderly, the face and scalp can be included in the treatment. Clothing and other potential fomites can be washed in hot water and dried in high heat. Patients should be reassured that “post-scabetic” pruritus is due to the body’s reaction to dead mites and does not necessarily indicate a treatment failure (3).
Crusted scabies is often difficult to treat and resistant to topical therapies. Topical permethrin may be effective as a single therapy; however, resolution of the infestation may be prolonged. The addition of oral ivermectin may be necessary (9). Although it is not FDA-indicated for the treatment of scabies, ivermectin is highly effective. It causes arthropod paralysis by blocking the transmission of glutamine or gamma-aminobutyric acid (GABA). Two doses of 200-400mcg/kg are given, separated by 1-2 weeks (3,9). Adverse effects of ivermectin are usually mild and may include gastrointestinal upset, dizziness, and/or an urticarial skin eruption (9).
Keratolytics such as salicylic acid or urea may be used to reduce hyperkeratosis associated with crusted scabies. This may enhance the efficacy of topical therapies. Care should be taken as patients may be colonized with or develop bacteremia due to S. aureus or S. pyogenes. Appropriate antibiotics should be given in such cases. Symptomatic treatments such as topical corticosteroids and oral antihistamines may also be used in the treatment of patients with crusted scabies. Finally, patients should be placed under contact isolation precautions and close contacts should be prophylactically treated with permethrin cream (9).

REFERENCES:

1. Karthikeyan K. Crusted scabies. Indian J Dermatol Venereol Leprol 2009;75:340-7.

2. Parish LC, Lomholt G. Crusted scabies alias Norwegian scabies. Int J Dermatol 1976;15:747-8.

3. Burkhart CN, Burkhart CG, Morrell DS. Chapter 84: Infestations. In: Bolognia J, Jorizzo JL, Schaffer JV. Dermatology. 3rd ed. Philadelphia: Elsevier Saunders; 2012:1423-6.

4. Siedelman, J, Garza RM, Smith CM, Fowler VG. More than a Mite Contagious: Crusted Scabies. Am J Med. 2017:1-3.

5. https://www.cdc.gov/parasites/scabies/gen_info/faqs.html

6. Executive Committee of Guideline for the Diagnosis and Treatment of Scabies. Guideline for the diagnosis and treatment of scabies in Japan (third edition): Executive Committee of Guideline for the Diagnosis and Treatment of Scabies. J Dermatol. 2017 May 31.

7. Hengge UR, Currie BJ, Jäger G, Lupi O, Schwartz RA. Scabies: a ubiquitous neglected skin disease. Lancet Infect Dis. 2006;6:769-779.

8. Hardy M, Engelman D, Steer A. Scabies: A clinical update. Aust Fam Physician. 2017;46(5):264-268.

9. Navi D, Koo J. Crusted Scabies: A Clinical Review. J Drugs Dermatol. 2006; 5(3) 221-227.