Diagnosis is based on history-taking and recognition of the clinical signs associated with scabies.

The cardinal symptom is intense pruritus, typically sparing the head and face and worsening in the evening hours.

The most specific clinical sign is the burrow—visible to the naked eye—a 2mm x 15mm thin, sinuous linear trajectory. Although it is the pathognomonic lesion, it is often hidden by excoriation or superinfection. Other types of lesions, such as papules (figure 1a) and genital nodules in cases of sexual transmission (figure 1c), are more common.

Figure 1.

Clinical and microscopic images of scabies; a) classical form in a young adult with skin type 3 exhibiting the characteristic burrows (arrows) and papules (arrowheads) evident at typical locations on the hand; b) crusted or Norwegian form (hyperinfestation) in an elderly bedridden woman with skin type 2, nail abnormalities, and discreet crusted lesions on her pinky finger, posing a challenge for diagnosis; c) genital nodules and erosions (arrow) and burrow (arrowhead) in a young man with skin type 5; d) infantile acropustulosis, pustules on the sole of a 3-month-old baby with skin type 6; e) atypical form with disseminated eczematous lesions (abdomen shown) in a middle-aged woman with skin type 3; f) classic impetiginized form, pustules in typical scabies location on the hand of an 8-year-old girl with skin type 6; g) blistering atypical form, with blisters on the side of the hand (arrow) and a burrow on the thumb (arrowhead) in a young woman with skin type 3; h) image of the burrow obtained using a manual dermatoscope. At one end, the dark triangle consistent with the anterior section of the mite can be seen (arrowhead). The sinuous linear path starting here ends in a triangular image (arrows). These structures make up the “delta wing sign”; i) image of the mite (arrow) and its eggs (arrowheads) obtained through optical microscopy of scraping of one of the burrows.

(0.69MB).

Clinical presentation shares signs and symptoms with other dermatopathies and manifests differently in infants and individuals with a deficient immune response to the mite.55,56 Various clinical presentations are shown in Table 1. Figures 1a-i illustrate several clinical and microscopic images.

Bacterial superinfection, favored by scratching, is the most common complication. The most widely reported bacteria involved in the infection are Staphylococcus aureus and Streptococcuspyogenes, and the usual clinical presentation is impetigo (figure 1f). In untreated or weakened patients, it can progress into deep soft tissue infections such as cellulitis and sepsis.55,57

Pruritus prevents rest and, along with the stigma associated with the disease, causes significant psychological distress.58

The IACS criteria (Table 2), which include typical lesions and symptoms, and epidemiological data, allow for standardized diagnosis with varying levels of rigor. This includes confirmed (level A), clinical (level B), or suspected scabies (level C). The utility of the IACS criteria is limited in clinical forms other than the classical presentation.59

The diagnostic gold standard is the visualization of the mite under an optical microscope. The 10x magnification handheld dermatoscope—a more accessible technique—allows for the visualization of different structures and non-invasive confirmation diagnosis (figure 1i). A brown triangular figure at its distal end is consistent with the anterior section of the mite. This figure (delta), along with the triangular image at the opposite end and the sinuous path that connects them (trail), is the “delta wing sign.”60 (figure 1h).

Various diagnostic aid techniques have been described61 (Table 3 and supplementary data. Annex 6).

Table 3.

Diagnostic aid and confirmation techniques.

	Technique	Facilitates viewing the burrow	Allows visualization of the mite	Non-invasive technique	In vivo visualization of the mite	Accessibility/availability	Price	Learning difficulty
Transillumination	Light source applied to the palmar surface of the interdigital fold
Ink stain	Apply ink to the burrow and clean the excess
UV light	Illuminate with LED 365 nm61,68
Dermatoscope	Manual device with 10x magnification and illumination
Videodermatoscope	Devices of different complexity and magnifications						a
Confocal microscopy	Microscopy with high-resolution visualization of the superficial layers of the skin. Resolution similar to histopathologyy
Optical coherence tomography	Uses light sources (interferometry), obtaining non-invasive high-resolution imaging of the superficial structures of skin and mite
Optical microscopy	Visualization under optical microscope of samples obtained with adhesive tape (adhesive tape test), scraping (Müller test), or biopsy					a	a	a
Serological and molecular tests	Detection of mite targets enabling simple and rapid point-of-care diagnosis					Currently not available

, yes;

, no; ↑, high; ↑↑, very high; ↑↑↑, extremely high; ↓, low; ↓↓, too low; ±, medium.

Green color: positive data; red color: difficulty.

a

Differing for the different techniques available.

To determine whether there is an outbreak and establish control measures, it is necessary to know if we are dealing with one or multiple cases, classical or hyperinfestation forms, and recognize if vulnerable groups are affected, such as displaced or institutionalized populations (schools, hospitals, refugee camps, nursing homes, penitentiaries...) 12,17,21,62 (figure 2).

Figure 2.

Diagnostic algorithm of scabies.

(0.66MB).

Cure is defined by the absence of new lesions and the absence of parasites in 2 different tests performed on the remaining lesions over 2 consecutive weeks.12,13

After parasitological cure, burrows may still be visible, and dermatitis and itching may still go on for weeks. Topical scabicides are irritants and may contribute to this persistence.63

Figure 2 illustrates the diagnostic algorithm of scabies.

Several macrocyclic lactones are in the pipeline. Moxidectin, which has a good safety profile, has been approved by the U.S. Food and Drug Administration (FDA) to treat onchocerciasis in humans. Compared to ivermectin, moxidectin has greater in vitro scabicidal activity, cutaneous bioavailability, and half-life (20 to 40 days, which is longer than the parasite's life cycle), which would allow for single-dose treatment.64,65

Spinosad, in a 0.9% topical suspension, is an insecticide that has been approved in the United States to treat scabies. It has a high safety profile.66

Malathion, crotamiton, and lindane are no longer available in Spain, the latter having been withdrawn due to its neurological toxicity.

The combination of topical permethrin plus oral ivermectin in a single dose of 200μg/kg was more effective than monotherapy to treat classical scabies. However, the methodology of the only study in which it has been evaluated does not allow us to determine the level of evidence or grade of recommendation of this combination.67

For hyperinfestation, a combination of topical and oral scabicides is advised. Topical treatment is repeated daily for a week and then every 48hours until definitive cure. Oral ivermectin is administered on days 1, 2, and 8 in mild cases and on days 1, 2, 8, 9, 15, 22, and 29 or until a definitive cure has been achieved in most severe cases (level of evidence 4, grade of recommendation C).10 Additionally, in the presence of crusts or hyperkeratosis, keratolytics should be added to the scabicides.12,14,68

In high-risk patients, such as immunocompromised individuals with classical scabies, combined treatment from the beginning may be considered.67,69 (level of evidence 5, grade of recommendation D).

In recent years, therapeutic failures have been described with all scabicides.3,70–72 Contributing factors may include drug resistance generation,73 inadequate cutaneous bioavailability of the drug,74 incorrect treatment compliance, and reinfection due to insufficient contact control.7,75 Poor control of fomites does not seem to be a plausible cause, given the low frequency of this source of contagion and the demonstrated efficacy in community campaigns that do not involve environmental mite elimination.76,77

In vitro tolerance of the mite to permethrin and ivermectin has increased since they started being used for scabies.71,78,79 In addition, mutations related to permethrin resistance have been identified. However, the few available clinical studies still report high efficacy rates.80,81

It is difficult to prove the cause of failures for several reasons: there is no experimental culture medium for the mite, and in vitro studies lack standardization; confirmation of correct treatment compliance is not always possible, and, eventually, the higher the prevalence the more difficult it becomes to access to all individuals who need treatment as direct contacts.

In case of therapeutic failure, due to absence of cure criteria 4 weeks into treatment, the complete regimen should be repeated. After confirming compliance with all necessary measures—personal, contacts, and environmental—the health care worker must decide whether to repeat the previous regimen, extend the skin contact time with the topical scabicide, increase the number of cycles of oral or topical treatment, prescribe the association of permethrin and ivermectin, or recommend an alternative treatment, among those currently considered second-line therapies. This decision should take into account the preferences and possible contraindications of the affected individual and his/her contacts (figure 3).

After mite elimination, skin lesions, itching, and bacterial complications may persist for 2 to 4 weeks.

Itching: oral antihistamines can alleviate it (level of evidence 5, grade of recommendation D). The topical application of medium-potency corticosteroids and tacrolimus has proven effective in reducing itching (level of evidence 1b, grade of recommendation A).83 Before starting the application, the absence of bacterial superinfection and parasitological cure should be confirmed to avoid masking infection and persistent scabies.

Skin lesions: after parasitological cure, topical corticosteroids can help resolve the inflammatory process (level of evidence 5, grade of recommendation D).

Bacterial infections: they should be treated with topical or oral antibiotics based on their severity and, eventually, they should be antibiogram-guided (level of evidence 2c, grade of recommendation B).84

In cases of classical scabies, all close primary contacts of the affected individual from 1 to 2 months before the onset of symptoms, should be treated, regardless of the presence of symptoms, including sexual partners and individuals who share a bed or have close household or institutional contact (schools, shelters, nursing homes, etc.).

For secondary contacts—those who have had close contact with a primary contact of a case—follow-up is advised.

The sexual partners in the 2 months prior to symptom onset should be examined and treated (level of evidence 5, grade of recommendation D). Close contact should be avoided until the affected individual and his/her partners have been treated (level of evidence 5, grade of recommendation D).10,11

In cases of hyperinfestation, all primary contacts should be treated, even if they have not been in intimate or prolonged contact. Secondary contacts should also be monitored, and treatment should be considered.12,62,81

For classical scabies, there is no evidence that fomite treatment is helpful.38 However, based on studies that found mites in the environment, 38 most clinical practice guidelines recommend eliminating mites on fomites that have come into contact with the affected individual's skin (level of evidence 5, grade of recommendation D).10

Mites can be completely eliminated by exposing clothes to 50°C temperatures for 10minutes (in washing machine, tumble dryer or water), keeping them in sealed bags for 3 (moderate ambient temperature and humidity) to 8 days (high ambient humidity and low temperatures), or keeping them in the freezer at -10°C for 5hours.97 Surfaces should be cleaned with soap, water, and a vacuum cleaner.17

To avoid reinfections, these measures should be implemented the same day as the treatment for affected individuals and contacts.

For crusted scabies, where patients harbor a very high number of mites, all guidelines recommend rigorous environmental decontamination.