What is scabies?

What is Scabies?

Scabies is a highly contagious parasitic skin infestation caused by the mite Sarcoptes scabiei var. hominis, characterized by intense nocturnal itching, burrows, and a pruritic rash in typical body locations. 1, 2

Causative Agent and Transmission

• Scabies is caused by an obligate human parasite mite, Sarcoptes scabiei var. hominis, which infests the skin 3
• The mite infests humans and can also infest animals including swine, dogs, cats, foxes, cattle, and coyotes, though human infestation from animal sources is usually self-limiting 4
• Transmission occurs primarily through prolonged, direct skin-to-skin contact with an infested person 3, 5
• The disease spreads rapidly through close personal contact including relatives, sexual partners, schoolchildren, chronically ill patients, and crowded communities 6
• Scabies mites survive on clothing for up to 4 days without skin contact but only 1-2 days at room temperature (21°C) 1

Clinical Presentation

• Classic symptoms include: intense nocturnal pruritus (nighttime itching), burrows (serpiginous tracks in the skin), and an erythematous papular rash arranged in linear order 1, 6, 7
• Typical body locations affected: finger webs, wrists, axillary folds, abdomen, buttocks, inframammary folds, waist, navel, groins, extensor surfaces of elbows and knees, and in men, the genitalia 6, 3
• Both symptomatic patients and asymptomatic carriers can spread the disease rapidly 5
• In debilitated or elderly patients, presentation may be atypical—burrows and pruritus may be absent, with only hyperkeratosis, papules, or vesicles present 1

Clinical Variants

• Crusted (Norwegian) scabies: occurs in debilitated older adults who are often heavily infested with high mite burden and high infectivity 1
• Nodular scabies and bullous scabies: other recognized clinical variants 3

Epidemiology and Risk Factors

• Worldwide, scabies affects 200 to 300 million individuals annually 8, 3
• The average prevalence is estimated at 5-10% in children of developing countries 3
• Children under age 2 and elderly individuals are at greatest risk 3
• Poverty, overcrowding, and migration (including refugees and asylum seekers) are social factors contributing to higher risk of transmission and treatment failure 8, 5
• The COVID-19 pandemic and migration have contributed to increased incidence 8

Complications

• Scratching leads to excoriations and secondary bacterial infections 7, 3
• Bacterial complications are responsible for mortality associated with scabies 8
• Treatment with scabicides may temporarily exacerbate pruritus, edema, and erythema 9

Common Diagnostic Pitfalls

• Clinical diagnosis alone may be necessary in outbreak settings, though incorrect diagnosis can lead to pseudo-outbreaks with widespread "psychogenic" scabies (itching without actual infestation) 1
• Misdiagnosing outbreaks can lead to unnecessary mass treatment 1