Subungual and ungual scabies: avoiding severe presentation in high-risk patients

Abstract

Scabies is a pruritic disorder caused by Sarcoptes scabiei var. hominis infestation of the skin. Transferred by close body contact, scabies is endemic within nursing homes and among poor and overcrowded populations. Crusted scabies is a severe form of disease, characterised by a large, thick, crusted eruption with significant mite infestation. We report a patient hospitalised with crusted scabies that had massive nail involvement. A 79-year-old female patient with multiple comorbidities and several recent prior scabies diagnoses presented with agitation and dystrophic fingernails; scabies mites were found embedded in and below the nail keratin. Aggressive treatment resulted in complete resolution, with notable improvements in mental status. Crusted scabies with nail involvement is extremely rare and may be more likely to develop from initially subclinical infestation sites. It is important to consider this potential presentation, as standard topical treatments may prove ineffective when there is deep nail involvement.

Background

Scabies is a contagious, pruritic infestation caused by the parasitic mite Sarcoptes scabiei var. hominis.¹ The mite burrows within the stratum corneum of the epidermis and feeds on dissolved host tissue, causing intractably pruritic inflammatory papules.² Crusted scabies (also known as Norwegian scabies) is a severe form of the disease characterised by hyperkeratotic skin crusts.³ Crusted scabies is rare and may be seen in patients suffering from neurological disorders or immune suppression (ie, HIV/AIDS, Human T-Lymphotrophic Virus-1), as these conditions can allow for an exponentially larger mite burden.^4–7

The higher mite burden makes crusted scabies more infectious and difficult to treat, and outbreaks constitute a significant public health risk in assisted living environments and other close-contact communities.^{6 8} Ungual scabies occurs when the infestation spreads to the nail, and appears to be associated with poor outbreak control and treatment outcomes, perhaps due to diagnostic difficulty.^9–11 A PubMed search of ‘subungual scabies’ yields only three relevant articles, the most recent of which was published in 1985, while a search of ‘nail scabies’ yields 17 articles relevant to the diagnosis and treatment of scabies in the nail. The literature on this matter remains fairly sparse, highlighting the lack of clinical knowledge regarding aetiology and optimal management of this presentation.

Case presentation

A 79-year-old female patient of Moroccan and Jewish descent with myelodysplastic syndrome, thrombocytopaenia and congestive heart failure managed with low-dose prednisone and erythropoietin. In the 6 months prior to this admission, she had been hospitalised in the internal medicine department six times. She was hospitalised once due to stroke, once due to neurorehabilitation, three times due to fall and once due to pulmonary oedema. In two of these admissions, a secondary diagnosis of classic scabies was made due to infestation observed on the face. At each scabies diagnosis, the patient and all close contacts including family and day care nurses received a single dose of 5% topical permethrin, which is standard for classic scabies diagnosis. At the second diagnosis, more contacts were included for treatment, as the patient had returned with a recurrent infestation that had previously appeared to resolve, and it was believed that this may have been due to an untreated contact or improper treatment adherence.

On this last admission, the patient presented to the internal medicine department with severe facial swelling, believed to be due to recurrent scabies infestation or adverse reaction to treatment and was transferred to the dermatology department for further evaluation and treatment. In addition to facial infestation, the patient also presented with severe crusted scabies on both hands. On physical examination, the fingernails on both hands appeared thick, brittle, discoloured and dystrophic (figure 1).

Figure 1.

Left: left thumb showing thickened nail keratin (after biopsy). Right: dystrophic nails of the right hand.

Investigations

The patient appeared disoriented and agitated. Affected nails and surrounding skin were scraped and biopsied, revealing Sarcoptes scabiei var. hominis mites in the skin and nail, with mites penetrating deeply in the keratinised layer of the nail (figure 2).

Figure 2.

Histological findings. Sarcoptes scabiei var. hominis eggs embedded within keratinised layer of skin (top) and nail (bottom).

Differential diagnosis

Due to recent previous admissions for scabies treatment, scabies infestation and adverse reaction to treatment were the primary differential diagnoses, but toxic erythroderma was also considered.

Treatment

Oral ivermectin 200 μg/kg was administered on days 1, 2, 8, 9, 15, 22 and 29, along with topical 5% permethrin cream (applied once from head to toe and washed off after 18 hours) and topical 10% sulfur ointment. The patient, her close family members and her day care nurse all received 5% permethrin cream prophylaxis. Infested nails were removed from the patient using topical 30% salicylic acid (chemical onycholysis), and the exposed nail bed was then treated with the topical antiscabies regimen described above. The infestation resolved after 3 weeks, at which point the patient was thoroughly assessed for persistent foci of infestation.

Outcome and follow-up

Full recovery was seen at the 3-month follow-up visit, including improved mental status (likely due to resolution of the chronic pruritus).

Discussion

One of the most common dermatological conditions, scabies is endemic worldwide, and it is estimated that over 130 million people are infested at any given time.¹² Risk of infestation is heightened among impoverished and immune compromised individuals, as well as those in crowded living conditions. Treatment of scabies infestation, and in particular recurrent infestations, poses a significant financial burden on health systems.¹³ Thus, improving existing diagnostic and treatment protocols may help reduce disease recurrence and the associated costs.

Topical 5% permethrin cream is the treatment of choice for classic scabies, while a combination of oral ivermectin and a topical agent is recommended for crusted scabies.^{13 14} Permethrin remains the gold standard of treatment of scabies, and has been associated with greater efficacy and a lower side effect profile than oral ivermectin.¹³ However, a recent Cochrane Review found no significant difference between several permutations of permethrin and ivermectin therapies, so further research is needed to elucidate what regimen is truly most therapeutic.¹⁵

Existing literature has identified the importance of controlling subungual and ungual scabies, as these presentations are particularly difficult to treat and may serve as reservoirs for recurrent infestations.⁵ This case reinforces those findings, as the duration of infestation for this patient would likely have been significantly shortened had the subungual and ungual infestations been identified sooner. However, this clinical profile—an atypical presentation of crusted scabies with an unidentified subungual reservoir—is rare and unfortunately reflects a poorly described patient population.¹⁶

While it is not immediately clear why initial treatments failed in this patient, it is likely that her immune compromised status put her at increased risk of developing a severe presentation. This difference in risk is not accounted for in treatment guidelines for the first presentation of classic scabies. It is possible that asymptomatic subungual or ungual infestations, which are unlikely to respond to topical treatments, can lead to crusted scabies, which will present more severely in high-risk patients. Thus, we suggest that establishing a more rigorous initial treatment protocol (including both topical and oral agents) for high-risk patients may help reduce the incidence of recurrent scabies and crusted scabies. Doing so may improve the symptom burden among affected individuals, and may reduce subtherapeutic exposure to first-line treatment agents and thereby limit the development of truly refractory scabies.

Certain patient populations are known to face a greater risk of severe scabies infestation on exposure to mites. Furthermore, atypical presentations, such as the predominance of facial and scalp lesions seen in this patient, more commonly occur with crusted scabies as opposed to less severe infestations.^{16 17} This must also be taken into account, as atypical presentations may lead to a lower clinical index of suspicion for ungual and subungual reservoirs, despite the paradoxical reality that both atypical presentations and subungual reservoirs are more likely to occur with crusted scabies. Increasing the scope of initial treatment among high-risk populations may allow for shorter, more effective treatment with fewer complications.

Learning points.

• Subungual and ungual scabies infestations may be subclinical or difficult to detect, and may serve as reservoirs for severe and continued disease in some patients.

• Rigorous initial assessment and treatment of detected or suspected subungual and ungual scabies may prevent the presentation of severe crusted scabies, particularly among high-risk patients (such as immunocompromised).

• High clinical suspicion should be used to detect subungual and ungual scabies reservoirs, as they are unlikely to respond to standard topical treatments.