Scabies

Scabies is an intensely pruritic (itchy) skin infestation caused by the host-specific mite, Sarcoptes scabiei var hominis. These mites burrow into the top layer of human skin to lay their eggs. The burrows sometimes appear as short, wavy, reddish, or darkened lines on the skin's surface, especially around the wrists and between the fingers. A person with scabies can also develop a bumpy red rash.

This mite is distributed worldwide, and can affect all socioeconomic groups. 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 lasts 30 days and is spent within the human epidermis. After copulation, the male mite dies and the female mite burrows into the superficial skin layers and lays a total of 60-90 eggs. The ova require 10 days to progress through larval and nymph stages to become mature adult mites. Less than 10% of the eggs laid result in mature mites. They feed on dissolved tissue but do not ingest blood. Scybala (feces) are left behind as they travel through the epidermis, creating linear lesions clinically recognized as burrows. Most burrows occur in the webs of fingers, flexing surfaces of the wrists, around elbows and armpits, the areolae of the breasts in females and on genitals of males, along the belt line, and on the lower buttocks.

The incubation period depends on whether the infestation is an initial exposure or a relapse/reinfestation. Upon initial infestation, a delayed type IV hypersensitivity reaction (allergy) to the mites, eggs, or scybala develops over the ensuing 4-6 weeks. Previously sensitized individuals can develop symptoms within hours of reexposure. The hypersensitivity reaction is responsible for the intense pruritus (itching) that is the main symptom of the disease.

Scabies is transmitted through prolonged skin-to-skin contact with an infected person, which is why it is sometimes classed as a sexually transmitted disease. Spread by clothing, bedding, or towels is a less significant risk, though possible.

Approximately 300 million cases of scabies are reported worldwide each year. In developed countries, scabies epidemics occur primarily in institutional settings such as prisons and long-term care facilities such as nursing homes and hospitals. Prevalence rates in developing countries are higher than those in developed nations. Natural disasters, war, and poverty lead to overcrowding and increased rates of transmission. 

SIGNS AND SYMPTOMS 

Symptoms include primary and secondary lesions. Primary lesions are the first manifestation of the infestation, and these typically include small papules, vesicles, and burrows. Secondary lesions are the result of rubbing and scratching, and they may be the only clinical manifestation of the disease. The characteristics of secondary lesions include excoriations, widespread eczema, honey-coloured crusting, postinflammatory hyperpigmentation, erythroderma (abnormal redness of the entire skin surface), prurigo nodules, and frank pyoderma.

The hallmark of scabies is itching, which is insidious and relentless. For the first weeks, the itch is subtle. It then gradually becomes more intense until, after a month or two, sleep becomes almost impossible.  

TREATMENT 

Scabies infestations require medical care. Treatment includes administration of a scabicidal agent such as Permethrin (Lyclear, Elimite), Lindane (Kwell), Precipitated sulfur in petrolatum, Crotamiton (Eurax), Ivermectin (Stromectol); an antipruritic agent such as a sedating antihistamine, and an appropriate antimicrobial agent if secondarily infected. All family members and close contacts must be evaluated and treated, even if they do not have symptoms. Pets do not require treatment. All carpets and upholstered furniture should be vacuumed and vacuum bags immediately discarded. If a member of the member is infested with scabies, you should launder clothing, bed linens, and towels used within the last week in hot water the day after treatment is initiated and again in 1 week. Items that cannot be washed may be professionally dry cleaned or sealed in plastic bags for 1 week.

Infested persons should avoid skin-to-skin contact with others. 

PREVENTION 

The only way to avoid becoming infested with scabies is to avoid contact with infested persons. Also if a member of your family has become infested it is important that you disinfect the entire household.

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