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Crusted scabies can appear all over the body and not be confined to specific sites, and it's not essential that the patient be pruritic to make a diagnosis.
Crusted scabies, a rare form of scabies, can present in a generalized manner, and affected patients may not necessarily endorse pruritus.
A recent case of a three-year-old girl with Down Syndrome, who was mistaken for having erythrodermic psoriasis, (Pediatric Dermatology. 2013 Oct 21), highlights that clinical presentation alone may not be sufficient to prompt a diagnosis of crusted scabies, explains Vanessa Gildenstern, a pediatrician at Phoenix Children's Hospital in Phoenix, AZ
"The patient had thick, crusted scabies lesions all over her body," explains Dr. Gildenstern, an assistant professor at the University of Arizona. "Our first thought was that she had erythrodermic psoriasis. That is how it looked, and the patient did not have pruritus."
The patient was initially treated with cyclosporine, but she was not responding as clinicians had hoped. She continued to have significant hyperkeratosis of the elbows, knees, and plantar surfaces five weeks after therapy was started, says Dr. Gildenstern.
Going back to the drawing board, Dr. Gildenstern says she and colleagues recognized that crusted scabies would be included in the differential diagnosis for this patient. A mineral oil preparation demonstrated scabies mites, eggs, and scybala. The patient's family members were not affected. The original source of contact was not identified.
If a mineral oil preparation does not confirm the diagnosis, performing a skin biopsy should be considered if a high degree of clinical suspicion for scabies remains.
"For children with crusted scabies and healthy immune systems, you would think they would be incredibly pruritic," says Dr. Gildenstern.
Crusted scabies occurs more often in patients who are immuno-compromised or neuro-compromised, notes Dr. Gildenstern, adding that their patient's neurological status may explain why she did not experience sensations of pruritus or did not respond to those sensations.
While there is not an immediate threat in terms of delayed diagnosis to a patient in the pediatric setting, a delay can result in co-morbid infection and become a public health issue, according to Dr. Gildenstern. There is a risk for superinfection, likely with staph or strep, if fissures or excoriations are present, says Dr. Gildenstern. "It's an opportunity for new infections," says Dr. Gildenstern.
"You can imagine a patient could be harboring tons of mites and infest classmates at school," explains Dr. Gildenstern. "It's a form of infestation, like lice."
Similarly, other members of the family are at risk of developing scabies. "You are obligated to make sure that everyone in the home is treated because it's very contagious," says Dr. Gildenstern.
In order for therapy to be effective, all members of the family are informed that they need to receive therapy at the same time. "Not only the patient, but family members should be treated on the exact same day and again a week later," says Dr. Gildenstern.
Standard therapy for crusted scabies is a topical scabicide, permethrin 5% cream. For an infant with scabies, it should be applied all over the body, including the face and scalp. Application should stay on overnight and be repeated a week later, adds Dr. Gildenstern. "We let families know that the chances of eradicating mites are highest if they follow our instructions."
In the reported case, clinicians used both topical and oral scabicidal therapy, with a goal to clear the infestation more rapidly, explains Dr. Gildenstern. "We were struck by the severity of her case and thought adding ivermectin would help hasten clearance of the condition," she says.
"At the time of our interaction with the patient, oral ivermectin was not approved for use in children younger than five years of age. Oral ivermectin now had indications for patients weighing more than 15 kilograms," says Dr. Gildenstern.
A possible complication arising from the use of ivermectin is allergy to the medication itself, which may present as urticaria, warns Dr. Gildenstern. "There is a rare incidence of pruritus as a side effect," she adds.
This particular case underscores to dermatologists how diverse the presentation of crusted scabies can be. "It can be as simple as a few pruritic papules present in classic locations like the wrists, webspaces, axillae, and waistline or it can present as a generalized, scaling, non-pruritic rash," explains Dr. Gildenstern. "It's important to be open-minded (about the presentation)."
Dr. Gildenstern had no relevant disclosures.