Answer to last month's Clinical Quiz

Question:

An elderly home resident complains of generalised itchy rash for three weeks. Topical steroid did not help the condition. Skin scraping at the patient's finger web is taken and examined under the microscope. The finding is shown in the picture.

Answer: C. Scabies

Scabies is caused by Sarcoptes scabies. The adult female mite is 0.4 mm long while the male mite is 0.2mm long. Copulation occurs in a small burrow excavated by the female mite. The fertilised female then enlarges the burrow and lays eggs. The mite shows a preference for certain sites to burrow. They tend to avoid area with a high density of pilosebaceous follicles. On the average, an infested patient harbours less than 20 adult female mites.

Transmission is by close physical contact such as sharing a bed or sexual contacts. Outbreaks occur in hospitals, old-age homes and other institutions. Outside the host, free adult mite and eggs can survive 36 hours and 10 days respectively. Symptoms occur three to four weeks after acquiring the infection. This latency period may not occur if an individual had a previous infestation and the onset of symptoms is much earlier. Itching is the most obvious symptom of scabies.

The pathognomonic sign of scabies is the burrow; it is a short, wavy, dirty-appearing line crossing skin lines. Burrows may appear on the wrists, the borders of hands, the sides of fingers, the finger webs, the feet particularly the instep and in male the genitalia. Pruritic papules, which accompany hypersensitivity reaction, occur around axillae, peri-areolar regions, peri-umbilical regions, buttock, thighs and external genitalia.

Absolute confirmation can only be made by microscopic examination. A burrow is gently scraped off the skin with a blunt scalpel, and the material is placed in a drop of mineral oil on a microscopic slide. The presence of mites, eggs or fragments of eggshells confirms the diagnosis. Other diagnostic tests include needle extraction of mite, epidermal shave biopsy and punch biopsy.

It is important that all members of the household and all close contacts should be treated simultaneously. The drugs that are commonly used include 0.5% malathion, 5% permethrin cream and 25% benzyl benzoate emulsion.