A diagnosis of scabies is made with ease at scanning magnification when an adult mite or her progeny are spotted in a cornified layer. The organisms reside in a tunnel termed a “burrow,” and the female mite is spied easily at very low power because she is large, several times larger than the mite of Demodex folliculorum. The itch mite, Sarcoptes scabiei, prefers the cornified layer of surface epidermis for its home, whereas Demodex favors infundibular epidermis. Parenthetically, mites of scabies are not disposed to habitation in the skin of the face and scalp, the exception being infants.
All too often, however, neither the female mite nor her offspring can be spotted in the stratum corneum of lesions of scabies. In that circumstance, a histopathologist notes in the dermis a superficial and deep perivascular and interstitial mixed-cell infiltrate composed mostly of lymphocytes and eosinophils. Those changes often are indistinguishable from ones induced by “bites” of arthropods such as mosquitoes and bedbugs, and in response to the effects of creeping eruption and to organisms such as those that in aggregation form coral. How then, in the absence of the presence of a female mite, larvae, nymphs, or ova, can a diagnosis of scabies be made definitively? If, within the cornified layer, egg shells are spotted, a diagnosis of scabies can be made with certainty. Those chitinous rims of ova are not the same morphologically as they were when embryos were contained within them; with release from them of scabietic larvae, they collapse and their shell assumes the shapes of squiggles and curlicues. Another clue to diagnosis of scabies when no mites are present in sections is brown fecal nuggets within spaces in the stratum corneum lined by parakeratotic cells. Those brown nuggets represent excrement of adult female mites. Consider the circumstance of the poor female mite, confined as she is to a tunnel, a cul-de-sac at that, in a cornified layer. No lavoratory facility, no loo, no powder room, no outhouse is available to her. The female mite has no recourse in her long and solitary journey but to savor superficial components of the skin at meals spaced episodically, to extrude ova, and to defecate. How good it is of her to release brown fecal nuggets as she tunnels through the cornified layer; they enable histopathologists to infer her whereabouts, even though she cannot be seen directly. In short, those brown pellets are a scatologic clue to diagnosis of scabies.
As unsettling as is the tale of the female mite of scabies is the sadder story yet of the male mite. In brief, because it is brief, immediately after the male and female copulate, the female becoming pregnant, the male dies!