Sarcoptes scabiei
Encyclopedia
Sarcoptes scabiei or the itch mite is a parasitic arthropod
that burrows into skin
and causes scabies
. Animals affected include not only human
but also wild and domesticated dog
s and cat
s in which it is one cause of mange
. Also affected in the wild are ungulate
s, boar
s, bovid
s, wombat
s, koala
s, and great apes.
The Italian biologist Diacinto Cestoni showed in the 18th century that scabies is caused
by the mite
Sarcoptes scabiei, variety hominis. The disease produces intense, itchy skin rash
es when the impregnated female tunnels into the stratum corneum
of the skin and deposits eggs
in the burrow. The larva
e, which hatch in 3–10 days, move about on the skin, moult
into a nymphal
stage, and then mature into adult mites. The adult mites live 3–4 weeks in the host's skin.
Sarcoptes
is a genus of skin parasites, and part of the larger family of mites collectively known as “scab mites”; they are also related to the scab mite Psoroptes, also a mite that infests the skin of domestic animals. Sarcoptic mange affects domestic animals and similar infestations in domestic fowls causes the disease known as “scaly leg”. The effects of Sarcoptes scabiei are the most well-known, causing “scabies”, or “the itch”. The adult female mite, having been fertilised, burrows into the skin (usually at the hands or wrists; however, other parts of the body may also be affected), and lays its eggs.
The burrowing is carried out using the mouthparts and special cutting surfaces on the front legs. While these are being used, the Sarcoptes scabiei anchors itself with suckers on its feet. Eggs are laid in small numbers as the mite burrows, and, as these hatch, six-legged larvae climb out on to the skin and search for hair follicles, where they feed and moult (discard old cuticles to grow). It is in the hair follicles that the larvae show the first nymphic stages, with eight legs.
In the nymphic stages, the creature feeds and moults and, if male, gives rise to the adult. In the case of females, another moult occurs before adulthood. The female has more moults than a male and therefore takes longer – seventeen days to the other nine to eleven days for a male - to reach adulthood. The female is about twice the size of the male.
Although the life-cycle is only about two weeks, individual patients are seldom found to have more than about a dozen mites on them. Even so, this number can cause agonising itching, especially at night, and severe damage to the skin often comes as a result of scratching, in particular by the introduction of infective bacteria, which may lead to impetigo
or eczema
.
The eggs are laid by the female at a rate of about two to three eggs a day for about two months. It is considered that about two percent of the British population is infected with these mites, which take about twenty-five minutes to an hour to burrow into the skin.
The best conditions in which to harbour Sarcoptes scabiei is in areas where there is frequent skin to skin contact, therefore the hands and wrists, as the mites are transmitted by skin contact with carriers, and they very easily spread. Infestations of Sarcoptes scabiei are commonly found in pigs. They significantly depress growth and feeding rate, but usually die out in around five days in typical farm conditions. However, once in a herd, the mites are very difficult to eliminate without great measures taken.
Upon infesting a human host, the adult female burrows into the skin, where she deposits 2-3 eggs per day. These oval eggs are 0.1–0.15 mm (0.00393700787401575–0.00590551181102362 ) long and hatch as larvae in 3–4 days. Upon hatching, the 6-legged larvae migrate to the skin surface and then burrow into molting pouches (these are shorter and smaller than the adult burrows). After 3–4 days, the larvae molt, turning into 8-legged nymphs. This form molts a second time into slightly larger nymphs, before a final molt into adult mites. Adult mites then mate when the male penetrates the molting pouch of the female. Mating occurs only once, as that one event leaves the female fertile for the rest of her life (1–2 months). The impregnated female then leaves the molting pouch in search of a suitable location for a permanent burrow. Once a site is found, the female creates her characteristic S-shaped burrow, laying eggs in the process. The female will continue lengthening her burrow and laying eggs for the duration of her life.
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
that burrows into skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
and causes scabies
Scabies
Scabies , known colloquially as the seven-year itch, is a contagious skin infection that occurs among humans and other animals. It is caused by a tiny and usually not directly visible parasite, the mite Sarcoptes scabiei, which burrows under the host's skin, causing intense allergic itching...
. Animals affected include not only human
Human
Humans are the only living species in the Homo genus...
but also wild and domesticated dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
s and cat
Cat
The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...
s in which it is one cause of mange
Mange
Mange is the common name for a class of persistent contagious skin diseases caused by parasitic mites. Since mites also infect plants, birds, and reptiles, the term "mange," suggesting poor condition of the hairy coat due to the infection, is sometimes reserved only for pathological...
. Also affected in the wild are ungulate
Ungulate
Ungulates are several groups of mammals, most of which use the tips of their toes, usually hoofed, to sustain their whole body weight while moving. They make up several orders of mammals, of which six to eight survive...
s, boar
Boar
Wild boar, also wild pig, is a species of the pig genus Sus, part of the biological family Suidae. The species includes many subspecies. It is the wild ancestor of the domestic pig, an animal with which it freely hybridises...
s, bovid
Bovid
A bovid is any of almost 140 species of cloven-hoofed ruminant mammal at least the males of which bear characteristic unbranching horns covered in a permanent sheath of keratin....
s, wombat
Wombat
Wombats are Australian marsupials; they are short-legged, muscular quadrupeds, approximately in length with a short, stubby tail. They are adaptable in their habitat tolerances, and are found in forested, mountainous, and heathland areas of south-eastern Australia, including Tasmania, as well as...
s, koala
Koala
The koala is an arboreal herbivorous marsupial native to Australia, and the only extant representative of the family Phascolarctidae....
s, and great apes.
The Italian biologist Diacinto Cestoni showed in the 18th century that scabies is caused
Etiology
Etiology is the study of causation, or origination. The word is derived from the Greek , aitiologia, "giving a reason for" ....
by the mite
Mite
Mites, along with ticks, are small arthropods belonging to the subclass Acari and the class Arachnida. The scientific discipline devoted to the study of ticks and mites is called acarology.-Diversity and systematics:...
Sarcoptes scabiei, variety hominis. The disease produces intense, itchy skin rash
Rash
A rash is a change of the skin which affects its color, appearance or texture. A rash may be localized in one part of the body, or affect all the skin. Rashes may cause the skin to change color, itch, become warm, bumpy, chapped, dry, cracked or blistered, swell and may be painful. The causes, and...
es when the impregnated female tunnels into the stratum corneum
Stratum corneum
The stratum corneum is the outermost layer of the epidermis, consisting of dead cells that lack nuclei and organelles. The purpose of the stratum corneum is to form a barrier to protect underlying tissue from infection, dehydration, chemicals and mechanical stress...
of the skin and deposits eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
in the burrow. The larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e, which hatch in 3–10 days, move about on the skin, moult
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
into a nymphal
Nymph (biology)
In biology, a nymph is the immature form of some invertebrates, particularly insects, which undergoes gradual metamorphosis before reaching its adult stage. Unlike a typical larva, a nymph's overall form already resembles that of the adult. In addition, while a nymph moults it never enters a...
stage, and then mature into adult mites. The adult mites live 3–4 weeks in the host's skin.
Clinical significance
The action of the mites moving within the skin and on the skin itself produces an intense itch that may resemble an allergic reaction in appearance. The presence of the eggs produces a massive allergic response that, in turn, produces more itching.Sarcoptes
Sarcoptes
Sarcoptes is a genus of mite.In some contexts, the types are all considered subordinate to Sarcoptes scabiei.* Sarcoptes scabiei var. bovis* Sarcoptes scabiei var. canis* Sarcoptes scabiei var. caprae* Sarcoptes scabiei var. equi...
is a genus of skin parasites, and part of the larger family of mites collectively known as “scab mites”; they are also related to the scab mite Psoroptes, also a mite that infests the skin of domestic animals. Sarcoptic mange affects domestic animals and similar infestations in domestic fowls causes the disease known as “scaly leg”. The effects of Sarcoptes scabiei are the most well-known, causing “scabies”, or “the itch”. The adult female mite, having been fertilised, burrows into the skin (usually at the hands or wrists; however, other parts of the body may also be affected), and lays its eggs.
The burrowing is carried out using the mouthparts and special cutting surfaces on the front legs. While these are being used, the Sarcoptes scabiei anchors itself with suckers on its feet. Eggs are laid in small numbers as the mite burrows, and, as these hatch, six-legged larvae climb out on to the skin and search for hair follicles, where they feed and moult (discard old cuticles to grow). It is in the hair follicles that the larvae show the first nymphic stages, with eight legs.
In the nymphic stages, the creature feeds and moults and, if male, gives rise to the adult. In the case of females, another moult occurs before adulthood. The female has more moults than a male and therefore takes longer – seventeen days to the other nine to eleven days for a male - to reach adulthood. The female is about twice the size of the male.
Although the life-cycle is only about two weeks, individual patients are seldom found to have more than about a dozen mites on them. Even so, this number can cause agonising itching, especially at night, and severe damage to the skin often comes as a result of scratching, in particular by the introduction of infective bacteria, which may lead to impetigo
Impetigo
Impetigo is a highly contagious bacterial skin infection most common among pre-school children. People who play close contact sports such as rugby, American football and wrestling are also susceptible, regardless of age. Impetigo is not as common in adults. The name derives from the Latin impetere...
or eczema
Eczema
Eczema is a form of dermatitis, or inflammation of the epidermis . In England, an estimated 5.7 million or about one in every nine people have been diagnosed with the disease by a clinician at some point in their lives.The term eczema is broadly applied to a range of persistent skin conditions...
.
The eggs are laid by the female at a rate of about two to three eggs a day for about two months. It is considered that about two percent of the British population is infected with these mites, which take about twenty-five minutes to an hour to burrow into the skin.
The best conditions in which to harbour Sarcoptes scabiei is in areas where there is frequent skin to skin contact, therefore the hands and wrists, as the mites are transmitted by skin contact with carriers, and they very easily spread. Infestations of Sarcoptes scabiei are commonly found in pigs. They significantly depress growth and feeding rate, but usually die out in around five days in typical farm conditions. However, once in a herd, the mites are very difficult to eliminate without great measures taken.
Morphology
Adult scabies mites are spherical, eyeless mites with four pairs of legs. They are recognizable by their oval, ventrally flattened and dorsally convex tortoise-like body and multiple cuticular spines. Females are 0.3–0.45 mm (0.0118110236220472–0.0177165354330709 ) long and 0.25–0.35 mm (0.00984251968503937–0.0137795275590551 ) wide, and males are just over half that size.Life-cycle
The scabies mite Sarcoptes scabiei var. hominis goes through four stages in its lifecycle: egg, larva, nymph, and adult.Upon infesting a human host, the adult female burrows into the skin, where she deposits 2-3 eggs per day. These oval eggs are 0.1–0.15 mm (0.00393700787401575–0.00590551181102362 ) long and hatch as larvae in 3–4 days. Upon hatching, the 6-legged larvae migrate to the skin surface and then burrow into molting pouches (these are shorter and smaller than the adult burrows). After 3–4 days, the larvae molt, turning into 8-legged nymphs. This form molts a second time into slightly larger nymphs, before a final molt into adult mites. Adult mites then mate when the male penetrates the molting pouch of the female. Mating occurs only once, as that one event leaves the female fertile for the rest of her life (1–2 months). The impregnated female then leaves the molting pouch in search of a suitable location for a permanent burrow. Once a site is found, the female creates her characteristic S-shaped burrow, laying eggs in the process. The female will continue lengthening her burrow and laying eggs for the duration of her life.