Scabies: Refine your exam, avoid these diagnostic pitfalls

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Applied Evidence

Scabies: Refine your exam, avoid these diagnostic pitfalls

J Fam Pract. 2020 January;69(1):10-17

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References

1. Anderson KL, Strowd LC. Epidemiology, diagnosis, and treatment of scabies in a dermatology office. J Am Board Fam Med. 2017; 30:78-84.

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3. Arlian LG, Morgan MS. A review of Sarcoptes scabiei: past, present and future. Parasit Vectors. 2017; 10:297.

4. Carr PC, Brodell RT. Images in clinical medicine: scabies. N Engl J Med. 2016; 374:e13.

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6. Hengge UR, Currie BJ, Jäger G, et al. Scabies: a ubiquitous neglected skin disease. Lancet Infect Dis. 2006; 6:769-779.

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8. Heukelbach J, Feldmeier H. Scabies. Lancet. 2006; 367:1767-1774.

9. Karaca Ş, Kelekçi KH, Er O, et al. Scabies incognito presenting as a subcorneal pustular dermatosis-like eruption. Turkiye Parazitol Derg. 2015; 39:244-247.

10. Gutte RM. Bullous scabies in an adult: a case report with review of literature. Indian Dermatol Online J. 2013; 4:311-313.

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14. Dupuy E, Dehen L, Bourrat E, et al. Accuracy of standard dermoscopy for diagnosing scabies. J Am Acad Dermatol. 2007; 56:53-62.

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18. Fonseca V, Price HN, Jeffries M, et al. Crusted scabies misdiagnosed as erythrodermic psoriasis in a 3-year-old girl with Down syndrome. Pediatr Dermatol. 2014; 31:753-754.

19. Carr PC, Brodell RT. Images in clinical medicine: scabies. N Engl J Med. 2016; 374:e13.

20. Park JH, Kim CW, Kim SS. The diagnostic accuracy of dermoscopy for scabies. Ann Dermatol. 2012; 24:194-199.

21. Lallas A, Apalla Z, Lazaridou E, et al. Scabies escaping detection until dermoscopy was applied. Dermatol Pract Concept. 2017; 7:49-50.

22. Micali G, Lacarrubba F, Verzi AE, et al. Scabies: advances in noninvasive diagnosis. PLoS Negl Trop Dis. 2016; 10:e0004691.

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SIDEBAR
How fast do scabies mites move? Dermoscopy video answers that question

Richard Usatine, MD; Ashfaq Marghoob, MD
University of Texas Health San Antonio (Dr. Usatine); Memorial Sloan Kettering Skin Cancer Center, Hauppauge, NY (Dr. Marghoob).

Dr. Marghoob reported that he has received honoraria for speaking for 3GEN. Dr. Usatine reported no potential conflict of interest relevant to this article.

A 22-year-old man infected with human immunodeficiency virus presented to a clinic with a 6-month history of intense pruritus. Physical examination revealed scale crust on the hands and wrists (FIGURE) and in the pubic region.

Dermoscopic examination of the rash on the wrist revealed scabies mites actively crawling on the surface of the skin. A superficial skin scraping revealed the Sarcoptes scabiei mite, several eggs, and scybala under the microscope. The extensive infestation in this patient with crusted scabies was related to his immunocompromised state. (The patient was successfully treated with oral ivermectin and topical permethrin.)

Unprecedented documentation of mite speed a clue to infectivity. The 36-second VIDEO captured an adult mite travelling a distance of 3 mm, translating to a speed of 5 mm/ min. Six younger adults/larvae were moving more rapidly with a maximum distance of 6 mm in 36 seconds (1 cm/min). The movement of a mite within the epidermis is slower, as the mite consumes keratin in creating burrows. The more rapid rate of movement on the skin surface may help to explain the contagious nature of scabies.

Vidyard Video

In this case, the mites and larvae were viewed on the screen of a smartphone to which a dermatoscope was attached magnetically. The mites were first visualized in the standard photo mode. Video mode was then used to capture the motion using the maximum zoom feature of the phone, to a magnification factor of 13.3×.

Literature to date has been silent on mites’ rate of motion. A Medline search yielded only 3 papers that addressed the issue of in vivo movement of scabies mites. None viewed the mites other than in their burrows and none calculated a rate of motion.

In one study using videodermotoscopy, Micali stated that in most of the 16 cases identified, it was possible to observe the mites moving inside the burrows.1 No video images were published, and there was no mention of speed or characterization of the movement.¹
A second study used reflectance confocal microscopy (RCM) to examine a single patient with crusted scabies. The authors claim to have viewed the ectoparasite's motion within the human host but provided no details of that motion.²
In the third study, videodermoscopy showed a slightly higher sensitivity for scabies detection than RCM (95% vs. 92%).³ The authors did not mention visualization of movement of mites in their work but did quote the Micali paper for its mention of movements of the mite.

Applying digital dermoscopy in practice. It appears that this is the first published video documenting the movement of scabies mites and larvae in vivo using dermoscopy. This should pave the way for additional observations of scabies movement on and below the skin using dermoscopy with video. We recommend using the maximum zoom capability of the device along with the dermatoscope to view this movement. What has been surmised before—that the mite must move above the skin to infect human contacts—has now been captured in vivo using the power of dermoscopy.

CORRESPONDENCE
Richard P. Usatine, MD, 903 West Martin Street, San Antonio, TX 78207; usatine@uthscsa.edu.

REFERENCES
1. Micali G, Lacarrubba F, Lo Guzzo G. Scraping versus videodermatoscopy for the diagnosis of scabies: a comparative study. Acta Derm Venereol. 1999; 79:396.

2. Gürel MS, Turgut Erdemir AV, Tekin B. A case report of real-time in vivo demonstration of Sarcoptes scabiei. Turkiye Parazitol Derg. 2017; 41:229-232.

3. Cinotti E, Labeille B, Cambazard F, et al. Videodermoscopy compared to reflectance confocal microscopy for the diagnosis of scabies. J Eur Acad Dermatol Venereol. 2016; 30:1573-1577.

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