Erythroderma infections

Scabies is a parasitic infestation of the skin by the mite Sarcoptes scabiei. It is a 0.3mm-0.5mm mite that occupies the epidermis. It burrows through the outermost layer of the epidermis (the stratum corneum) and tunnels through it.

🔢 Pathophysiology and classification

It lays about 2-4 eggs daily which hatch in 2-4 days. It takes about 2 weeks before the eggs form adults. The female parasite lays approximately 25 eggs before dying.

On average, an individual may have 10-12 mites. It is transmitted through skin-to-skin contact but can also occur through clothing and bedding. Sexual transmission may also occur in young adults.

The mite burrows through the stratum corneum which leaves an erythematous trail. The immune system attacks the mite with Th2 cells and it is this response that causes the symptoms of pruritus, erythema, papules and nodules.

There are 2 variants of scabies:

1. Classical scabies - this is the infection as described above. There are only a few mites (5-15) on the individual at any given point.
2. Crusted scabies/Norwegian scabies - this is a more contagious form of the infestation that occurs in individuals who have an insufficient immune response. For example, in HIV patients, or patients with leukaemia, immunosuppressive medication. It leads to an infection of thousands to millions of parasites infesting the skin.

⚠️ Risk factors

• Living in crowded areas
• Winter months
• Poverty
• Immunosuppression
• Inability to scratch - such as physical incapacity, or because of skin anaesthesia.
• Elderly

😷 Presentation

Features start to arise after 3-6 weeks (the first wave of reproduction). However, in a re-infested individual, features develop within 1-3 days of infestation due to prior sensitization.

• Intense pruritus - worse at night.
• Burning sensation
• Erythematous papules - in a characteristic distribution on the skin regions that have thinner keratinous layers, such as the periumbilical area, waist, genitalia, breasts, buttocks, axillary folds, fingers (including interdigital spaces), wrists, and extensor aspects of the limbs.
• Burrows -  this is a pathogonomic sign. It appears as a thin, brown-grey line of 0.2–1 cm in length. The burrows are produced by the moving mite and are difficult to observe if the skin has been scratched, has become secondarily infected, or if eczema is present.

💡 The absence of itching does not exclude scabies (for example, in young babies and in people with neurological conditions with decreased/loss of sensation and in crusted scabies)

• Fissured plaques and crusts - for crusted scabies. They are mainly on bony prominences and are yellow-brown in colour.

🔍 Investigations

• 🥇 History and examination - this may be sufficient but often we can confirm the test easily with an ink burrow test or microscopy of skin scrapings.
• 🏆 Ink burrow test - blue or black ink is applied on the surface of a papule. It is then wiped off. If scabies is present and a burrow has been made then the ink will track down the burrow and show the pattern of the burrow.
• 🏆 Microscopy of skin scrapings - papules are scraped off and then viewed under a microscope. It should show the mites, eggs, mite faeces. It is the most specific test but not has a sensitivity of <50% has the mite is often not present in the scrapings.

🧰 Management

• Permethrin 5% cream - the whole body should be treated and more attention on the areas that are more susceptible (as mentioned above). The treatment should be washed off after 8-12 hours. The whole household and all the sexual partners should be treated even if asymptomatic. Itching may persist for up to 4 weeks. If it persists longer than 2-4 weeks, they should be advised to return.
• Wash bedding, clothing and towels - they should be washed at temperatures of at least 60ºC and dried with a hot dryer or dry-cleaning for at least 72 hours.
• Crotamiton 10% cream or hydrocortisone 1% - to treat post-scabietic itch.
• Admit to hospital if crusted scabies is suspected. It may require treatment with oral ivermectin. This can also be used if they have persistent nodular scabies.