Scabies

Scabies is a contagious parasitic infection of the skin endemic throughout the world with a global prevalence of 300 million but particularly problematic in areas of poor sanitation, overcrowding and social disruption (Strong and Johnstone, 2007). Despite its incidence, it is often missed or misdiagnosed. It can affect people of any age but is mostly seen in children, young adults, elderly people especially those in institutions and those who are immunocompromised. It is more common in overcrowded situations and in urban areas. It is a huge source of embarrassment and misery from severe itching and sleepless nights. Disease control is often hampered by inappropriate or delayed diagnosis and poor treatment compliance (Heukelbach and Feldmeier, 2006).

It is caused by the Sarcoptes scabeii mite and spreads from person to person by direct skin contact which includes sexual, though transfer via clothing or furnishings is possible. The pregnant female lays eggs in burrows in the stratum corneum; 50–72 hours later, the larvae appear and make new burrows. They mature, mate and repeat this 10- to 17-day cycle. Physical findings include burrows (Figure 12.12), erythematous papules, excoriations, nodules, vesico-pustular or bullous lesions and secondary bacterial infection. The classic sites of infection are between the fingers, the wrists, axillary areas, female breasts (particularly the skin of the nipples), peri-umbilical area, penis, scrotum and buttocks (Strong and Johnstone, 2007). Infants are usually affected on the face, scalp, palms and soles. There are more pustules in younger children. The condition is very itchy which is often worse at night. The host immune reaction to the presence of mites and their products in the epidermis (Heukelbach and Feldmeier, 2006) is the source of much of the itching and can appear about a month after initial infection and persist for up to 6 weeks after treatment.

Crusted (Norwegian) scabies is much more severe and is associated with extreme incapacity and immunosuppression such as in HIV infection. This form of scabies presents with a hyperkeratotic dermatosis which can resemble psoriasis and lymphadenopathy and eosinophilia may also be present. Itching may be surprisingly mild. These patients are highly infectious and may harbour millions of mites which may also be on the scalp (Strong and Johnstone, 2007). Complications are few but secondary bacterial infection with S. aureus or Group A Streptococcus can occur. Scabies is a risk factor for developing acute post-streptococcal glomerulonephritis (Heukelbach and Feldmeier, 2006). In crusted scabies, a generalised lymphadenopathy is common and secondary sepsis can lead to death.

Diagnosis
This is usually made on clinical grounds and good history-taking. Other family members may be infected and although they may be asymptomatic, more classically itching which starts at the same time amongst family members or those living in an institution, indicates scabies. Differential diagnosis includes atopic eczema, allergic contact dermatitis, insect bites, papular urticaria and impetigo.

Microscopic identification of the mite can be made by picking out a mite from a burrow with a needle. Alternatively, scrapings can be looked at in the same way.

Management
The management of scabies falls into two equally important halves:

1. Getting rid of the patient’s own scabies and ensuring that all family and those who have had prolonged contacts are treated, even if they are asymptomatic. This means checking who lives at home and who visits regularly and ensuring the patient knows how to apply the treatment correctly.
2. Making sure that the patient and contacts do not catch it again which means that all family members and sexual contacts must be treated too, whether they say they are itchy or not (BAD, 2004).
   
   

Of the topical scabicides in use (Maliathon, Permethrin and Sulphur), on recent review, Permethrin appears to be the most effective (Strong and Johnstone, 2007). This must be applied correctly in order to be effective.

Procedure:

1. All those who need treatment should apply it at the same time.
2. Avoid bathing before application as this increases absorption into the blood and removes the treatment from their site of action on the skin (BNF, 2008).
3. Treatment should be applied to the whole body, including the scalp, face, neck and ears.
4. Special care needs to be taken with genitalia, flexures, fingernails, webs of the fingers and toes.
5. Leave the treatment on for at least 12 hours before washing off.
6. When hands are washed during this period (or a child’s nappy changed), treatment should be reapplied.
7. Apply two treatments 1 week apart.
8. Ordinary washing of clothes and bedding is sufficient (BAD, 2004).
   
   

Ivermectin, an oral antihelminthic appears to be an effective (off license) treatment (Strong and Johnstone, 2007) and may be useful in the management of crusted scabies and epidemics. Plant derivatives, for example neem, turmeric and tea tree oil, are promising future treatments (Heukelbach and Feldmeier, 2006).

Following effective treatment, itching can take up to 6 weeks to subside unless reinfestation occurs. This allows time for lesions to heal and for eggs and mites to reach maturity (i.e. beyond the longest incubation interval), if treatment fails (Strong and Johnstone, 2007). Scabies nodules can take longer to subside and topical corticosteroids may be indicated.