Sarcoptes scabiei (Scabies or Itch mite)

Life > Eukaryotes > Opisthokonta > Metazoa (animals) > Bilateria > Ecdysozoa > Panarthropoda > Tritocerebra > Arthropoda > Arachnomorpha > Cheliceriformes > Chelicerata > Euchelicerata > Arachnida > Acari (mites and ticks)

The Scabies mite Sarcoptes scabiei is a skin parasite of mammals, including people. Despite being placed in one species, these mites are host specific so infection of one species of mammal by mites from another species (e.g. the mite moving from dog to human) is usually unsuccessful.

Life Cycle

1. Adult female burrows into skin. The newly mated female selects a site on the skin for burrowing. This is usually in the vicinity of the hands, wrists, elbows, feet or groin. The female uses suckers on her legs (called ambulacral suckers) to attach herself to the skin surface and then cuts a channel into the skin using her jaws (termed chelicerae) as well as cutting edges on the tibiae of her first pair of legs.  She burrows only in the top layer of skin (the cornified epithelium) and over her full life the burrow can reach a length of 2-3 cm and can often be observed through the skin as a thin zigzag line [check]. 2. Female lays her eggs. The female starts laying eggs in the burrow a few hours after she has began burrowing and she continues to lay eggs at a rate of 2-3 per day for up to 2 months. 4. Larvae mature. After emerging from the egg, the larva moves out of the burrow onto the skin and wanders around till it finds a suitable hair follicle within which to shelter and find food. It lives here, passing through two moults before becoming adults. 
3. Eggs mature. Larvae emerge from the eggs 3-4 days after they have been laid. 5. Adult male and female mate. Mating probably takes place on the surface of the skin.

The entire life cycle can be completed in 10-14 days.

Scabies infections 

Scabies mites are transferred from one person to another through direct contact. This usually occurs through sexual contact, children playing, or in settings where people have to live in close proximity to one another. Very rarely are they transferred via clothing and bedding.

Infection by the mites causes problems because they produce a substance that causes an allergic reaction in the host person and makes them feel very itchy (especially at night when they are warm in bed). The itchiness leads to the infected person scratching themselves which causes wounding and often subsequent infection by bacteria. People who have low immunity are more likely to develop secondary infections. It has been found that people with AIDS are particularly prone to allergic responses from scabies infections and to severe secondary infection by bacteria.

Scabies is evidently easily treated using topical creams containing a pesticide (see links below). However, the main problem is that infection by these mites is often misdiagnosed by doctors and other health professionals. For instance, there have been cases of people complaining to their doctor of itchiness who have then been referred on to a psychiatrist on the grounds that the itchiness is not real but related to psychological disturbances.

Related causes of itchiness

If you get an itchy feeling in bed at night it might not be caused by scabies mites but by another little mite called the Fowl Itch Mite Dermanyssus gallinae. These mites live on birds and are particularly associated with birds' nests. People who complain of 'bird lice' from birds nesting in their ceiling are almost certainly referring to the Fowl Itch Mite as true bird lice do not bite humans.

References

  • Mellanby, K. 1943. Scabies. Oxford University Press, Oxford, 81 pp. [reprinted 1973].

Links

Text by Hamish Robertson


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