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Sarcoptes scabiei (Scabies or Itch mite)
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> Arachnida > Acari (mites and
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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 © |
