Skin Conditions, General | October 27, 2015 | Author: The Super Pharmacist
Scabies is a skin infestation with the Sarcoptes scabiei mite. Scabies is in infestation rather than an infection, since the mite is a parasite rather than a microorganism. The Sarcoptes scabiei mite is whitish brown, has eight legs, and vaguely resembles a turtle, though it is much smaller (0.3 X 0.4 mm).1 It is so small, in fact, that a single mite is just barely perceptible with the naked eye. Even if Sarcoptes scabiei could be readily observed with the naked eye, it is almost always found beneath the skin in tracks that the mite has burrowed, rather than on the surface.
Interestingly, only the female mite causes the clinical disease known as scabies.2 Once the female mite has been fertilised by a male, it quickly burrows into the epidermis, specifically the stratum granulosum (i.e. third layer of five epidermal layers).3 The female mite releases digestive enzymes to burrow about 2 mm through the skin each day. Along the way, she lays two or three eggs at a time, up to 25 total, in the skin burrows. After one to two months of burrowing and laying eggs, the mite dies at the end of the tunnel. During this time, however, the eggs that were laid hatch within four days, molt several times, return to the surface of the skin to find new mates and repeat the cycle.3
The most obvious and prominent feature of scabies is severe, unrelenting itchiness. The itchiness is caused by an immune hypersensitivity reaction directed against the dead mite, its feces and eggs.4 This immune reaction causes severe itchiness, skin inflammation, and papules (i.e. non-fluid filled, raised bump). Because of constant scratching, the skin is often excoriated, red, cracked, and bleeding. It is sometimes possible to see evidence of the mite’s burrow, but most often this simply looks like just another scratch on the skin. Scabies tends to affect only certain, discrete regions of the body. Scabies infestation can be found in the armpits and nipples, in a band across the naval, on the wrists, elbows, ankles, and knees, in the webbing of the fingers, and around the genitalia and anus.3 Since symptoms are caused by a delayed immune reaction, patients experiencing their first infestation may not have symptoms for up to four weeks.2 This is the time required for the immune system to react to the mite with a delayed hypersensitivity reaction. Subsequent infestations, however, cause symptoms within 24 to 48 hours because the immune system is already primed to respond to the mite antigens.2 Fortunately, it appears that a majority (60%) of people will develop an immunity to scabies infestation after the first encounter.5
Scabies is incredibly contagious through person-to-person contact.5,6 The mite must be on the surface of the infested person and must directly make contact with the second person's skin. Given the distribution of scabies infestation, (i.e. genitals, anus, nipples) sexual contact is a primary means of spreading the mite. Indirect contact is not sufficient to spread the infestation.
The Sarcoptes scabiei mite is an obligate parasite, which means it must live on a host in order to survive. When it is off the host (i.e. human) the mite is highly susceptible to dehydration. Thus, Sarcoptes scabiei mites tend to die once they are shed from the person.6 As such, bed linens, blankets, and clothing are not major mechanism by which scabies can be transmitted between people.2 Nevertheless, it is possible for scabies to be transmitted through heavily infestedfabrics especially, if the surface is moist and/or the relative humidity is high (e.g. >75%).7
The most effective means of avoiding scabies infestation is to avoid direct contact with someone who is infested.8 Unfortunately, infestation may not cause symptoms in the primary patient for several weeks.2 Therefore, it may be impossible to avoid contact since the primary patient may not know that they are infested. In someone with confirmed scabies, direct contact should be avoided until the mites are completely eradicated. Importantly, infested individuals can return to work, school, or other daily activities the day after they have been treated.9 Transmission of scabies through blankets and other fabrics is unlikely, but possible. Therefore, it is reasonable to thoroughly launder or discard linens that have been used by someone who has or recently had scabies. Since Sarcoptes scabiei mites do not typically live off the host for more than three days, one could place the used linens in a sealed plastic bag for at least three days until the mites have died. After that time, the fabric should be washed in hot water or subjected to dry cleaning chemicals. It is important to remember that the eggs may last up to 10 days of the host.2 Thus, the safer approach may be to keep the fabrics in a sealed plastic bag for at least 10 days before laundering, if not discarded.
Treatment for scabies has three main goals: eliminate the mites, stop the severe itching, and prevent secondary bacterial infections. Various topical and oral mite-killing treatments are available. Unfortunately, the more effective treatments are limited by potentially serious side effects. Therefore, the choice of treatment to eliminate scabies must consider both the efficacy and toxicity of the drugs used.
Given its reasonable efficacy and relatively minor toxicity, topical permethrin 5% cream is a first-line treatment for scabies.10 While scabies only affects discrete areas of the body, permethrin 5% cream must be applied to every area of the body from the neck down. Furthermore, the cream must be left on for up to 14 hours before it is washed off. This process is usually repeated again in one week.10 Another agent used to eliminate mites is an oral treatment, ivermectin, at a dose of 200 micrograms per kilogram.10,11 While the oral medication is more convenient than the topical cream, it may be less effective than permethrin.12 While some physicians may prescribe ivermectin first, it is generally considered a second line agent if permethrin fails.10,12 Other, third-line treatments include lindane, benzyl benzoate, crotamiton, malathion, and sulfur in petrolatum, where available.1,13
The itch of scabies can be curbed by anti-histamines such as diphenhydramine, prochlorperazine, and cyproheptadine.4 Topical corticosteroids may be used to stop itching, but only after mites have been eradicated as they reduce the immune reaction.3
Since the itchiness of scabies is so severe and prolonged, especially at night, patients will scratch the affected regions of skin nearly constantly during infestation. Even treatments that attempt to stop the itch are often be only partially effective. This prolonged and aggressive scratching causes a breakdown in the skin that allows bacteria to penetrate into deeper layers of tissue or enter the bloodstream. This places patients at risk for invasive Streptococcus and Staphylococcus infections, rheumatic fever, and glomerulonephritis.8 Consequently, the first sign of skin or systemic bacterial infection is treated aggressively with antibiotics.