Scabies in Humans

 
Scabies is a chronic skin disease caused by the mite Sarcoptes scabiei.
 
The mite lives in the superficial layers of the skin  where it eats keratinized skin cells. As it eats, it makes tunnels. The mites are unable to live more than three to four days away from the host.
 
Symptoms:
 
Typical Scabies in humans presents as itchy papules, symmetrically distributed, mostly on the body and limbs. Vesicles and nodules are also seen.  The nodules are usually inflamed and very irritating. In infants lesions commonly occur on the hands and feet. This condition can be chronic. Infectivity is low as there are only about 10 mites per gram of skin.   
Occasionally secondary bacterial infection may obscure underlying scabies.  
 
Scabies can also be transient. The reaction is mostly allergic, mite numbers are low, (10 mites per gram of skin) and the mites do not reproduce. It disappears when the person is separated from the source of the scabies.
Some people may develop Crusted Scabies. The superficial skin layer builds up and thickens. It is more likely to happen in immunocompromised individuals, such as people with HIV, leprosy, Down's syndrome, certain cancers and also some ethnic groups, such as Aboriginals in Australia.
In Crusted Scabies, there are to 2000 mites per gram of skin. It is very infectious. Where patients have been treated in hospital wards, mites were found on walls and curtains.  The crusts are thick layers of keratinized skin mixed with mites, mite eggs, shed mite skins, mite faeces and bacteria.
Crusted scabies can spread all over the body, or be localized to certain areas. It is not necessarily symmetrical.
If the crusts crack, raw skin is exposed and the patient is prone to infection and septicaemia.
 
Diagnosis:
 
Diagnosis of scabies by identification  of the mite can be difficult.If affected skin is coloured with a felt tip pen, and the ink then wiped off the skin surface, the tunnels in the skin will often retain the ink and be visible. The mites, if found, can be seen and identified under a microcope. Your doctor will apply some saline or mineral oil to the skin and gently scrape the superficial layers with a  scalpel blade.
Diagnosis of crusted scabies by lab confirmation is easier, because of high mite numbers.   
 
 Treatment:
 
Typical scabies is readily cured with correct treatment.
 A 5% permethrin cream (Elimite or Nix) is applied from head to foot soles and left on for 10 to 14 hours (usually overnight). Do not apply to face, eyes, or mucous  membranes. The following morning the ointment is washed off by showering. The treatment is repeated after 1 week.
 
Ivermectin tablets (Stromectol) are very effective, but need a prescription.
 
Other scabies medicines are:
1%  lindane (Kwell, Scabene)
Malathion 0.5% (Ovide lotion)
Crotamiton (Eurax)
 
Although Scabies is effectively cured within a day or two of treatment, itching may persist for up to 4 weeks. Antihistamines such as Benadryl may be used to control  the itch.
 
In some cases Scabies is complicated by secondary infection and will require a prescription of antibiotic, eg Bactrim.