Misdiagnosed Crusted Scabies in an AIDS Patient Leads to Hyperinfestation

Comment

Classic (noncrusted) scabies is common worldwide, with an estimated 300 million cases per year. It is caused by the mite Sarcoptes scabiei var hominis, and transmission occurs by direct skin-to-skin contact or less commonly by fomites (eg, linens, bedsheets) and therefore is common in overcrowded environments.² Crusted scabies is a severe, highly contagious form of the disease in which the host’s immune system is overwhelmed and unable to defend against mites on the skin, resulting in hyperinfestation of the host. The mites use secretions to dissolve the epidermis and burrow through the skin, leaving feces in their tracks.³ Interestingly, the native aboriginal populations of Australia have a high incidence of crusted scabies even though they show no signs of immunosuppression. The reason remains unclear but may be due to a skewed T-cell response.⁴ Various mechanisms have been described for the symptoms of scabies, and it is believed that there is a hypersensitivity reaction to the mites and the feces. Increased IL-17 production by skin T cells may be responsible.⁵

Clinical Features

Crusted scabies is characterized by severe hyperkeratosis and plaques with desquamation and erythroderma that is worse in the acral regions and large joints, such as the elbows and the knees, as seen in our patient. Because of the deep burrows, patients are predisposed to secondary superinfections by bacteria. In our case, the patient had methicillin-resistant S aureus bacteremia, which persisted for some time despite treatment with intravenous antibiotics.

Diagnosis

Because scabies can imitate different conditions, it can be difficult to diagnose. Misdiagnosis of psoriasis in our patient led to ineffective treatment and subsequent worsening of his condition. Burrows are pathognomonic for scabies, though in severe cases, the burrows may be concealed by extreme hyperkeratosis. Diagnosis is confirmed by mineral oil preparation from the plaques showing numerous scabies mites and feces.

Treatment

It is important to control the spread of scabies, as it is highly contagious, and if the living environment is not properly cleaned, the patient can be reinfected. All clothing, bedsheets, and linens in the household must be washed in hot water and dried in a hot dryer, and nonwashable items should be placed in a closed plastic bag for 72 hours. All contacts also should be treated with 1 application of permethrin cream to the entire body including the head and neck, left on overnight, and washed off with warm water.¹ The washing also helps remove some of the skin crusts. Patients should be educated that pruritus and burning may initially worsen with permethrin treatment due to the body’s reaction to the parasite.^1,2 In addition, keratolytic agents such as topical urea or salicylic acid can be used as an adjuvant therapy to improve the efficacy of permethrin.

Permethrin is effective against both mites and eggs and works by inhibiting sodium channels, resulting in nerve signal conduction block and subsequent paralysis. Ivermectin is thought to act on glutamate-gated chloride channels, which are present in invertebrates but absent in vertebrates, causing hyperpolarization and paralysis of the adult mite.^1,6

Conclusion

Crusted scabies is a highly contagious and intensely pruritic condition. Scabies can mimic other conditions, such as psoriasis or severe dermatitis, so it is important to keep this diagnosis in mind, especially in immunocompromised patients or populations in overcrowded areas (eg, those who are incarcerated or in nursing homes). Treatment consists of isolating the patient, starting topical permethrin and oral ivermectin (in severe cases), washing all linens, and prophylactically treating contacts. A delay in diagnosis can lead to severe debilitating disease, as seen in the extreme case of our patient. However, our patient made a full recovery with appropriate treatment and care.