What is the cause of scabies?
Scabies is caused by a mite (like a tiny insect) called Sarcoptes scabiei. The mite is a parasite, meaning it lives off the host (a human) with no benefit to the host.
Scabies mites are so tiny, you can't see them with the naked eye. They have a cream-coloured body, bristles and spines on their back, and four pairs of legs. The female mite is bigger (about 0.4 mm x 0.3 mm) compared with the male (0.2 mm x 0.15 mm). With the naked eye you might be able to see them as a speck. The female mites tunnel into the skin and lay eggs. About 40-50 eggs are laid in the lifetime of a mite. The eggs hatch into larvae after 3-4 days; these then grow into adults within 10-15 days.
Most of the symptoms of scabies are due to your immune system's response to the mites, or to their saliva, their eggs or their poo (faeces). In other words, the rash and the itching are mostly caused by your body's allergic-like reaction to the mites, rather than the mites themselves.
The average number of mites on an infested person is 12.
How do you get scabies?
You need close skin-to-skin contact with an infected person to catch scabies. This is because the scabies mite cannot jump or fly.
Most cases of scabies are probably caught from prolonged hand-holding with an infected person. The hand is the most common site to be first affected.
Close skin-to-skin contact when having sex is another common way of catching scabies.
The skin-to-skin contact needs to be for a reasonable time to catch the mite. You usually need to be in skin contact for 15-20 minutes to catch scabies. Therefore, you are unlikely to catch scabies from an infected person by casual short contact such as a handshake or a hug. The more mites there are, the faster they can spread to another person.
The mites live in skin and can survive away from the host human for about 24-36 hours. You are unlikely to catch scabies from bedding and towels unless you use them immediately after being used by someone with scabies. However, just in case, it is best to treat bedding and towels by hot washing (described later).
Scabies is fairly common and can usually be recognised and treated by GPs. In the UK, about 1 in 1,000 people develop scabies each month. Scabies is more common in town (urban) areas, in women and children and in the winter because people often share beds in the cold. It is common worldwide, particularly where there is poverty and overcrowding such as poor areas of Asia and Africa. There may be outbreaks in overcrowded places such as refugee camps.
Sometimes outbreaks of scabies occur in places such as nurseries and residential homes, where people are in regular close contact.
What is crusted scabies ('Norwegian' scabies)?
This type of scabies is unusual, and most people who develop scabies do not get 'Norwegian' scabies.
It is a more severe form of scabies with a hyperinfestation of lots of scabies mites. This means that there are thousands or millions of the mites, causing excessive scaling and flaking of the skin. Because there are so many mites, and because the skin is flaking off, scabies can be spread to others extremely easily. The rash is crusting (and looks a bit like psoriasis). It can be very extensive and can, if severe, lead to serious secondary bacterial skin infections.
Crusted scabies mainly occurs in people who have a poor immune system (are immunocompromised). The types of people who might get crusted scabies include:
- People with HIV/AIDS.
- People having chemotherapy.
- Frail people who are elderly or ill due to other conditions.
- Those who are poorly fed (malnourished).
- Those with learning difficulties (because they cannot comprehend or respond to the itch).
- People who have nerve-related problems (because they cannot feel the itch).
Crusted scabies is more easily transmitted through contact with towels, bedding and upholstery. In crusted scabies, the mites can survive for a few days away from their host. Even minimally exposed people, such as cleaners and laundry personnel, are at risk from crusted scabies and need insecticide treatment if they are working in an institution with an outbreak. An outbreak in an institution with many vulnerable people, such as a nursing home, can be extremely challenging. It is likely that all residents and all staff would require treatment.
A healthy person with a normal immune system would develop 'normal' scabies if infested with the scabies mite following contact with someone who had crusted scabies.
Crusted scabies may be impossible to eradicate in people with HIV infection, and recurrences are common.
Further reading and references
Scabies; DermNet NZ
Strong M, Johnstone PW; Interventions for treating scabies. Cochrane Database Syst Rev. 2007 Jul 18(3):CD000320.
Gunning K, Pippitt K, Kiraly B, et al; Pediculosis and scabies: treatment update. Am Fam Physician. 2012 Sep 1586(6):535-41.
FitzGerald D, Grainger RJ, Reid A; Interventions for preventing the spread of infestation in close contacts of people with scabies. Cochrane Database Syst Rev. 2014 Feb 242:CD009943. doi: 10.1002/14651858.CD009943.pub2.
Engelman D, Kiang K, Chosidow O, et al; Toward the global control of human scabies: introducing the International Alliance for the Control of Scabies. PLoS Negl Trop Dis. 2013 Aug 87(8):e2167. doi: 10.1371/journal.pntd.0002167. eCollection 2013.
Scabies; NICE CKS, May 2016 (UK access only)
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