Optical microscopic study on a novel morphological classification method of multiple diagnostic features of Sarcoptes scabiei var. hominis

Abstract

Optical microscopy is the gold standard technique used to confirm the diagnosis of scabies. Multiple diagnostic features of the pathogen Sarcoptes scabiei var. hominis (S. scabiei) can be identified under a microscope and classified into 3 categories: mites, eggs and fecal pellets. However, mite and eggshell fragments can also be observed, which have been ignored in the 2020 International Alliance for the Control of Scabies (IACS) Criteria and by most researchers. In this study, we propose a novel morphological classification method that classifies multiple diagnostic features into 5 categories and 7 subcategories. Our results revealed that 65.2% (1893 of 2896) of the positive cases were confirmed through the identification of mites, eggs or fecal pellets, whereas up to 34.6% (1003 of 2896) of the positive cases were confirmed through the identification of mite or eggshell fragments. Therefore, the important diagnostic values of mite and eggshell fragments should be emphasized. Importantly, for the first time, mite and eggshell fragments were classified into 7 subcategories, some of which are easily ignored or confused with contaminating artefacts. We believe that this novel morphological classification method will be beneficial for operator training in interpreting slides and in improving the 2020 IACS Criteria.

Keywords: Sarcoptes scabiei var. hominis; morphological classification; optical microscopy; scabies.

Graphical abstract

Figure 1.

Examples of mites and the anatomic structures. (A) Larvae; (B) nymphs; (C) adult male mites; (D) adult female mites (ventral aspect); (E) adult female mites (dorsal aspect); (F) a mite was cut into 2 parts; however, the gnathosoma and most idiosoma structures can be observed in a field; （G-J） larva with incomplete eggshells (red arrow). Complete body structures: a, gnathosoma; b-d, legs Ⅰ, Ⅱ, Ⅲ, and Ⅳ; f, long bristles; g, cuticular striations; h, cuticular spines; i, setae; j, ovum (magnification: 100).

Figure 2.

Eggs at different developmental levels (magnification: 100).

Figure 3.

Mite fragments. (A1–A4) Gnathosoma-type fragments; (B1–B4) idiosoma-type fragments; (C1–C4) leg-type fragments (magnification: 100).

Figure 4.

Eggshell fragments. (A1–A4) Double-leaf fragments; (B1–B4) single-leaf fragments; (C1–C4) 1-shaped fragments; (D1–D4) amorphous fragments (magnification: 100).

Figure 5.

Fecal pellets. (A and B) Fecal pellets in piles; (C) fecal pellets and a double-leaf fragment; (D) fecal pellets and a mite (magnification: 100).