By Gary M. White, MD
Three classic burrows made by 3 mites, seen here on the side of the foot. The lower right circle of scale is where one of the mites was a week ago. Above and hooked over making a U-turn is where that mite is now (small black dot at the end of the burrow).
Scabies is the infestation of the skin by the scabies mite. Intense itching caused an allergic contact dermatitis to the mite, its eggs and feces ensues.
The patient presents with intense itching. It is often worse at night, but this may be because there is nothing to distract the patient from the itch. Other members of the family are often affected. The best place to see the burrows, which appear as 3-10 mm thread-like lines, is between the fingers in the web spaces and along the sides of the feet. Red papules on the penis are classic and are highly suggestive. Diffuse itching and red 2-4 mm bumps on the penis are almost always scabies.
For the experienced clinician, seeing even one classic burrow establishes the diagnosis. In uncertain cases or in order to confirm the diagnosis, a KOH or mineral oil examination is typically done. When a black speck can be seen at the end of a burrow, some clinicians use a needle to extract the mite and examine under the scope. Others have used the dermatoscope to view the mite in the skin.
Permethrin cream is first-line and more effective than oral ivermectin.
Give an amount sufficient for all people in the household with the following instructions: Apply from the neck down and leave on overnight. Apply to the scalp in children under 3 and anyone with an itchy scalp. Every nook and cranny of the skin must be covered. If one patient misses one spot, it could all come back. Wash all clothing in warm or hot water and then dry in the dryer. Cloth items which cannot be laundered may be stored for ten days. In the AM, wash all sheets and take a shower. A medium-potency topical steroid (e.g., triamcinolone 0.1%) may be given for 2 weeks as the itching takes a while to subside. Alternatively, some give IM triamcinolone, e.g., 40 mg. A scabies handout is available.
Ivermectin is usually given 200 ug/kg as a single dose and repeated in 14 days. This is CDC-recommended but not FDA-approved. Ivermectin alone is frequently ineffective in this author's experience and should be combined with topical permethrin. It shouldn't be used in pregnancy, in those breastfeeding, or in those less than 15 kg (where the blood-brain barrier is less effective and the risk of seizures is higher).
If the patient calls, saying the scabies is back, consider the following:
Permethrin, topical sulfur, benzyl benzoate, and crotamiton are all considered safe during pregnancy.
Elimite is not FDA-approved for infants less than 2 months of age. Precipitated sulfur 6 or 10% compounded in petrolatum is the treatment of choice here. It is applied all over including the face and scalp but avoiding the eyes and mouth for 3 consecutive nights. It is also safe in pregnancy.
In an interesting study in the Fijian Islands, mass administration of oral ivermectin reduced the prevalence of scabies over the next 12 months from 32.1% to 1.9% [N Engl J Med 2015; 373:2305-2313].
The penis, web spaces, inner wrists, and sides of the feet are the favored places. A careful examination will show the classic burrow. Often it is atop an inflammatory papule. Marking with black ink and then wiping off the surface ink can dramatically highlight the burrow.
Scabies may mimic a hand dermatitis.
A mite with an egg in side.
The mite on the left and 4 eggs on the right.