modified from: Kettle, D.S. (1995). Medical and Veterinary Entomology. CAB International. Wallingford
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Sarcoptes scabiei is a parasitic mite that lives within the subcutaneous tissues of skin on humans, causing the condition known as scabies; similar mites cause what is called "mange" in wild and domestic animals. This mite is distributed worldwide, and can affect all socioeconomic groups. Scabies mites are generally host specific and S. scabiei is dependant on humans for its life cycle. These oval, straw coloured mites are very small, measuring 0.2-0.4mm in length. Their bodies are covered with fine lines and several long hairs. The female mite has scattered on the dorsal surface some short blunt spines, which aid her in maintaining her position within the tunnel. The mites have no eyes, and they have short and thick legs, with the first two pair of legs stalked. The immature stages of the scabies mite are comprised of a six legged larval stage, followed by 2 nymphal stages that have eight legs, and each stage resembles the adult mite.
The entire life cycle of the mite occurs over 10-17 days. Newly mated females take approx. an hour to burrow into the outer layer of human skin and excavates a tunnel. The mite lays her eggs singly, depositing behind her 2-3 eggs each day. Females burrow without direction, using their mouthparts to tunnel 0.5-5mm a day, eating the skin and tissue fluids that ooze from their excavations. Each tunnel contains only one female, her eggs and faeces. After 48 hours the eggs hatch and the larval stages dig their way to the surface of the skin, where they immediately burrow. This burrow may only be a short distance into the skin, or they make use of hair follicles, to moult to the next stage. Larval and nymphal stages remain in these moulting pockets feeding on fluids secreted from the follicles before moulting to the adult stage. Newly moulted male and female mites construct short burrows <1mm before mating.
After fertilisation, female mites wander on the skin to seek a suitable site for a permanent burrow, the transfer of a female to another host at this stage will initiate a new infection. A fertilised female mite can only initiate successful scabies infections. Female mites rarely leave their burrows, and if removed by scratching and remain undamaged, they will attempt to burrow again. During an infection the number of mites increases rapidly, then drops off, leaving infected persons with a relatively stable mite population of 15-20 females. The mortality rate of mites is high, 90% of mites that hatch will die, and mites removed from their host can only live a short time.
Scabies infestations can present different clinical pictures and may be difficult to diagnose. The initial infestation may remain undetected for a month or more, before sensitisation develops and a immunological response in the host is triggered. The allergic reaction is from components of the mite's faeces, skin moults, saliva or moulting fluids diffusing into the tissues of the host from the burrows. The patient can experience severe itching all over the body, and especially at night. Large areas of the body can be covered by a rash that can last for weeks but which will not (or only rarely) coincide with the areas of mite infestation. Eruption of the skin into small itchy lesions may occur in conjunction with the rash. Scabies mites tend to burrow into the skin where there is a natural crease and the host's reaction will be minimal. The hands, and webbing between the fingers, the wrists, and elbows are common areas. From the surface of the skin, the tunnels appear as greyish pencil marks, in darker skin the tunnels appear paler. The severe itching and scratching can lead to secondary infections and, in cases of heavy infestations anaemia can develop. There has been no transmission of disease pathogens associated with this mite.
Untreated scabies infestations, especially in infants, immobilised geriatric patients, AIDS and other immunologically compromised patients can support huge numbers of female mites. The patient's skin may become crusted on the surface, with the underlying layers soft and honeycombed with tunnels, these infections are referred to as "Norwegian" or "crusted"scabies. Patients with this advanced state of infection can act as a source for local epidemics in health care facilities. In some cases, scabies infections in nursing staff or family that have had contact with the patient will lead to diagnosis of the primary patient. Reinfected patients will develop an immediate itch when another scabies infection is initiated.
Skin scrapings are examined with a compound light microscope for the presence of mites, eggs or faeces. A glass slide mount is prepared, using dilute potassium hydroxide or lactic acid to mix with the skin scraping. This aids in clearing any thick layers of skin cells in the sample to reveal any evidence of the scabies mite, but clearing may take some time (hours to days).
Treatment & Control
Once diagnosed, most scabies infections are easy to control, providing the directions of the scabicide treatments are followed. Any pharmacy will supply a chemical preparation, and a prescription is not necessary. Care should be taken in re-applying scabicides unnecessarily, to avoid skin irritation and added costs. In most cases, itching may persist for a week or more after the treatment, but this is not necessarily a sign of treatment failure. Re-examination of the patient at four weeks after the treatment is appropriate. At the commencement of the treatment, bed linen and underwear of the patient should be washed in hot water and hot tumble dried, but there is no need to treat furniture or rooms with an insecticide. A common problem of treatment failure is insufficient coverage of the body with the scabicide, and resistance is rare. For patients diagnosed with crusted scabies, the patient should be isolated, and barrier nursing implemented throughout the treatment. All individuals that have had significant contact with the primary patient should also be treated. Scabies is highly contagious in overcrowded situations and close contact with infected individuals should be avoided. Touching, shaking hands, or sharing beds and contaminated objects of an infected person are common modes of transmission.
Confirmation & Enquiries
Information and identification of Scabies mites, and all other medically important arthropods, is provided through the Medical Entomology Department at ICPMR, Westmead Hospital.
See 'Contacts' for further information.
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