Practical Tips for Approaching a Scabies Diagnosis
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Much has been written about scabies, which holds a seminal place in the history of medicine, being the first ever described infectious disease.1 Meaningful clinical data on scabies, however, is hard to come by because scabies is difficult to study experimentally in humans, and animal models of the disease are rather limited. Dermatology texts teach that scabies is caused by the itch mite Sarcoptes scabiei, which burrows in the uppermost layer of the epidermis, laying eggs and causing a highly pruritic rash, often with nocturnal accentuation. Scabies burrows tend to localize to specific body sites, such as web spaces, wrists, feet, and genitalia, and it is widely held that the number of mites harbored by the average infected individual is less than a dozen. In a hard-to-study disease like scabies, it is worth asking, “How do we know what we know about scabies?”
Amazingly, much of our knowledge regarding clinical scabies comes from transmission studies performed by the British entomologist Kenneth Mellanby on conscientious objectors during World War II.2 Medical ectoparasites and their secondary infectious complications were responsible for much morbidity amongst fighting forces in the grueling trench warfare of the First World War. As events in Europe later made a second great conflict seem inevitable, Mellanby set out to study scabies to aid the British war effort. Mellanby procured a villa in the central English industrial town of Sheffield, where he recruited conscientious objectors to mandatory military conscription who had been granted the right to participate in noncombatant service. In a series of experiments and observation studies, Mellanby meticulously detailed where mites resided on the infected, in what numbers, and through what means they could be spread.
How did Mellanby go about detecting scabies? He detailed his procedure as follows: “The patient lies naked on a couch in a good light in a warm room and the surface of the body is inspected, using a watchmaker’s eyeglass; the mites are extracted with a mounted needle. With a little experience it is possible to detect the mites in the skin before removal. Each patient is carefully examined at least two, and often more, times, and the accuracy of the results has been ensured by keeping some patients from whom the mites have been removed in this way under observation for periods of weeks to ensure that none has been missed.”3
More than 80 years later, the diagnosis of scabies continues to rest on visual inspection. As dermatologists, we are uniquely positioned to conclusively find and document the presence of scabies mites on our patients, a process which is essentially 100% specific for rendering a diagnosis. Optimize your chances of finding a mite by employing good lighting and optimal magnification, as well as utilizing dermoscopy and knowing where to look. Patients are highly appreciative of a dermatologist’s ability to make a definitive determination of what is causing their itchy rash, rather than just what might be causing it.
“At Once the Easiest and Most Difficult Diagnosis in Dermatology”4
Scabies mites are located at the leading edge of burrows, and textbook cases are not difficult to diagnose. When burrows are scraped, and a female, male, or immature mite is microscopically detected, a definite diagnosis can be rendered. The presence of ova, eggshells, or scybala (fecal pellets) is equally conclusive. Assessment of burrows is aided immensely by dermoscopy, under which triangular pigmentation can be noted at the leading edge of the burrow, known as the delta-winged jet sign5(Figure 1). This corresponds to the pigmented head and legs of the scabies mite. Scrape this exact spot for ready microscopic detection. Once you have done it multiple times, and are comfortable with the clinicodermatoscopic correlation, then dermoscopy alone for classic burrows is sufficient to make the diagnosis of scabies (Figure 2). This can be a helpful time-saving tip, especially in a busy clinical practice.
However, obvious burrows are not always forthcoming, and highly excoriated patients often lack the primary morphology that would make a diagnosis straightforward. Based on Mellanby’s observations, 85% of scabies patients will have burrows on the fingers or wrists, so here is an excellent place to start the search. When a positive diagnosis is not forthcoming, a full skin examination is valuable, with particular attention to other sites of predilection, specifically the elbows, axilla, feet, buttocks, areola (in women), and penis and scrotum (in men). Keep in mind that the most common locations for burrows are not necessarily where the patient is experiencing itch or displaying rash.
When female mites cannot be identified by localizing burrows, sometimes immature mites (nymphs or larvae) can be detected by scraping what appear to be inflamed hair follicles. If lucky, these areas represent larval papules, or the sites where immature mites molt and transform into adults.6 Immature mite forms are easily identifiable as they have only 6 legs rather than 8, and I have confirmed a diagnosis of scabies on multiple occasions using this method when traditional burrows were not detectable.
Consensus Criteria for the Diagnosis of Scabies
Unfortunately, there will be times when positive microscopic evidence of scabies simply cannot be obtained. This can be for a variety of reasons: the number of mites on an individual is sparse, burrows have been scratched away, or perhaps the pruritic rash is something besides scabies after all. In this scenario, helpful consensus criteria have been published to standardize the diagnosis of scabies and involve classifying the certainty that any given case is scabies. The International Alliance for the Control of Scabies has put forth 3 main categories that are useful in this regard: confirmed scabies, clinical scabies, and suspected scabies.7
Confirmed scabies, as discussed, is defined as cases in which the scabies mite, its eggs, or feces have been definitively detected. Clinical scabies is defined as cases in which the scabies mite is not caught red-handed, but where the otherwise characteristic features of scabies are present, either burrows where positive microscopic confirmation could not be obtained or inflammatory papules on the penis or scrotum, which are considered diagnostic for scabies unless proven otherwise. In addition, a patient is considered to have clinical scabies if they have “typical lesions in a typical distribution” and have the 2 major clinical features of scabies: they must be itchy and they must have close contact with someone else who has a similar itchy presentation. Lastly, suspected scabies is a category applied to cases in which there are some features of scabies that are not better explained by another disease process. This enables the clinician to maintain an index of suspicion in the absence of key defining features. Suspected scabies is defined as “typical lesions in a typical distribution” with only one major clinical feature (itch or close contacts having a similar presentation), or “atypical lesions in an atypical distribution” with both major clinical features.
Treatment of Close Contacts
For the treatment of scabies, my preference is to utilize permethrin first, although starting with oral ivermectin is reasonable and may be advantageous in some circumstances. More important is that the patient’s close contacts need to be identified and treated. This also applies to asymptomatic close contacts who could potentially be in the early (sensitization) phase of scabies and not yet be symptomatic. Up to a 2-month delay is possible between catching scabies and developing itch. Failure to treat close contacts is a common reason for treatment failure, much more so than drug resistance, at least in 2024.
To treat close contacts, it is imperative that dermatologists actively inquire about the patient’s social and living circumstances. In my experience, it is not uncommon for this step to be skipped. One must remember that scabies is not so much a disease of the individual but rather a disease of the individual and their immediate environment or community. We must actively recommend that close contacts be treated and prescribe appropriate quantities of medications for these individuals, even if we have not seen them personally. This is a practice that dermatology as a specialty should actively strive to perform. If necessary, “expedited partner therapy,” as originally advocated by the Centers for Disease Control and Prevention to limit the spread of chlamydia and gonorrhea, can be invoked equally as well for scabies.8,9 Given the excellent safety profile of both permethrin and ivermectin, the real issue in effectively treating close contacts may be the potential cost of these medications.
Importance of Fomites Is Proportional to Parasite Burden
Mellanby commenced his studies assuming that contaminated blankets or bedding were the medium by which scabies is spread. However, to his surprise he found it exceedingly difficult to transmit scabies to uninfected patients via inanimate objects. Subjects who slept naked amongst the blankets of scabies-infested soldiers did not catch scabies at all. Subsequently, only 2 out of 32 subjects who donned the warm, recently worn underwear of known scabietic patients contracted scabies. To his surprise, Mellanby found out that scabies was considerably less easy to catch than he had assumed. Ultimately, Mellanby concluded that prolonged close and intimate contact was required to spread scabies, and that military resources need not be diverted to the sterilization of fomites on inanimate objects, such as bedding and clothing.
It is, however, widely recognized that a small subset of scabies cases, “crusted” or so-called Norwegian scabies, can be explosively contagious. Crusted scabies is essentially the human presentation of mange, and quite different than that of classical scabies. Patients with crusted scabies present with widespread crusting and hyperkeratosis.10 Their skin can often have an unkempt or extreme unwashed appearance. Within these crusts and hyperkeratotic skin teem tens or even hundreds of thousands of mites. In group settings, such as prisons, hospitals, and retirement homes, these cases can be highly contagious and an epidemiologic nightmare. The role of fomites in garden-variety cases of scabies is less clear, and here there exists a divergence of opinion in the dermatology community.11 While various practitioners may have an opinion based on their empiric experience, in my opinion it would make the most sense to look at the data, which again takes us back to Mellanby’s transmission studies.
In the beginning of his experiments when he found scabies difficult to spread, Mellanby was only successfully able to infect a few of his subjects and there were no accidental cases of transmission to his institute staff. Mellanby noted that during this period in which no patients harbored more than 50 mites, correspondingly no clinic workers accidentally obtained scabies in the discharging of their duties. However, once he started dealing with patients having 200 mites or more, medical workers began to accidentally contract scabies outside of the experimental protocol. Thus, Mellanby proposed that the contagiousness of fomites in scabies is proportional to the overall pathogen burden. Mellanby’s data showed that the average mite burden was 11.3 female mites per patient, and 52% of patients harbored 5 mites or less. On the flipside, he also noted that 3.9% of patients harbored over 50 mites, and one patient had as many as 511. It seemed that rare patients harbored a high mite burden, and these patients were more able to spread scabies through indirect means, presumably through fomites.
Based on Mellanby’s and other work, there are some simple recommendations that can be put forth to aid dermatologists. It seems like a practical, common-sense approach to consider parasite or mite burden when considering the role of fomites in scabies. This would be analogous to the use of viral burden as a clinical parameter in various viral infections. In patients with classical scabies who harbor less than a dozen mites, the role of fomites in contagion is minimal. On the other extreme, in crusted scabies where the pathogen burden is heavy, the role of fomites becomes outsized.
Thus, careful decontamination measures should be performed in all cases of crusted scabies. In noncrusted scabies, when 50 burrows or less are present, the only decontamination measure that is sensible is the laundering of bedding where feasible. This recommendation is based on experiments that have shown that mites from bedding can experimentally be transferred to and infect lab animals or human volunteers, albeit at a very low rate.12,13 Clinicians should feel justified in not recommending more thorough decontamination measures provided that the estimated burrow count (EBC) is less than 50. When the EBC is greater than 50, extra attention to fomites should be considered, including the laundering of clothing, towels, and other personal items, as well as isolating for 48 hours items that cannot be laundered. In the clinic setting, exam rooms of scabietic patients with less than 50 burrows do not need to be closed and subject to decontamination protocols.
Lastly, it should be noted that the above applies only to temperate scabies and not tropical scabies, which behaves in a rather different fashion. In such cases, the mite often eschews burrows or chooses to reside superficially in or around hair follicles due to different environmental conditions, particularly warmth and humidity. Because the environmental conditions in tropical scabies are more conducive to mite survival, and because the mite is located more superficially on the skin, fomite spread in tropical scabies is more likely to be a routine method of disease transmission. Thus, in tropical scabies, EBC as a clinical parameter may be less useful. Recommendations for decontamination protocols that consider the ambient temperature, humidity, and availability of natural resources, including electricity and water, have been put forth.14
Conclusion
In summary, there are no other specialists better poised than dermatologists to diagnose and treat scabies. To render a conclusive diagnosis, mites should be identified by localizing burrows at sites of high predilection. The use of dermoscopy has high utility and dermoscopic findings interpreted by the experienced user can be pathognomonic. Treatment involves not just dispensing medication to the patient, but also identifying close contacts and prescribing them medication as well. Most cases of scabies are not highly contagious in a casual setting. However, in the unusual instance that a patient harbors more than 50 mites, the decontamination of fomites is recommended. These recommendations apply to cases of temperate scabies. More studies need to be conducted to define the clinical characteristics and optimal treatment recommendations for scabies in tropical settings.
Disclosure: The author reports no relevant financial relationships.
References
1. Jackson S. Skin Disease and the History of Dermatology. CRC Press; 2023.
2. Mellanby K. Human Guinea Pigs. Merlin Press; 1945, 1973. Reprinted as Rasmussen LM (ed). Human Guinea Pigs, by Kenneth Mellanby, A Reprint with Commentaries. Springer Nature; 2020.
3. Johnson CG, Mellanby K. The parasitology of human scabies. Parasitology. 1942;34(3- 4):285-290. doi:10.1017/S0031182000016279
4. Stokes JH. Scabies among the well-to-do: some principles illustrated by the elite. JAMA. 1936;106(9):674-678. doi:10.1001/jama.1936.02770090010003
5. Argenziano G, Fabbrocini G, Delfino M. Epiluminescence microscopy. a new approach to in vivo detection of Sarcoptes scabiei. Arch Dermatol. 1997;133(6):751-753. doi:10.1001/archderm.133.6.751
6. Shelley WB, Wood MG. Larval papule as a sign of scabies. JAMA. 1976;236(10):1144-1145.
7. Engelman D, Yoshizumi J, Hay RJ, et al. The 2020 International Alliance for the Control of Scabies: consensus criteria for the diagnosis of scabies. Br J Dermatol. 2020;183(5):808-820. doi:10.1111/bjd.18943
8. Stoff BK, Introcaso C, Kinlaw K, Swerlick RA. Expedited partner therapy for scabies: legal and ethical considerations. J Am Acad Dermatol. 2013;69(5):795- 798. doi:10.1016/j.jaad.2013.05.021
9. Lester J, Sbicca JA, Bercovitch L. Ethical dilemmas and legal pitfalls in prescribing for the uninsured indigent patient: reconciling the irreconcilable. J Am Acad Dermatol. 2013;68(4):672-674. doi:10.1016/j.jaad.2012.05.046
10. Skayem C, Askour M, Gary C, et al. Severe scabies: a French multi-centre study involving 95 patients with crusted and profuse disease and review of the literature. Acta Derm Venereol. 2023;103:adv00878. doi:10.2340/actadv.v103.5351
11. Wolf R, Davidovici B. Treatment of scabies and pediculosis: facts and controversies. Clin Dermatol. 2010;28(5):511-518. doi:10.1016/j.clindermatol.2010.03.008
12. Mellanby K. The development of symptoms, parasitic infection and immunity in human scabies. Parasitology. 1944;35(4):197-206. doi10.1017/S0031182000021612
13. Arlian LG, Runyan RA, Estes SA. Survival and infestivity of Sarcoptes scabiei var. canis and var. hominis. J Am Acad Dermatol. 1984;11(2 Pt 1):210-215. doi:10.1016/ s0190-9622(84)70151-4
14. Bernigaud C, Fernando DD, Lu H, et al. How to eliminate scabies parasites from fomites: a high-throughput ex vivo experimental study. J Am Acad Dermatol. 2020;83(1):241-245. doi:10.1016/j.jaad.2019.11.069
