Scabies is a highly contagious skin condition caused by an infestation of the microscopic mite Sarcoptes scabiei var. hominis. This infestation is a common public health problem worldwide, affecting people of all ages and socioeconomic backgrounds. While many people associate it with poor hygiene, the primary risk factor is close personal contact.
Understanding the Scabies Mite
The female mite measures only about 0.3 to 0.45 millimeters in length. After mating on the skin’s surface, the impregnated female mite burrows into the top layer of the skin, known as the stratum corneum, creating a shallow tunnel. She then lays two to three eggs each day inside this burrow for the remainder of her one to two-month lifespan. The eggs hatch into larvae in about three to four days, and these larvae migrate to the skin’s surface. They develop through nymphal stages before maturing into adults, completing the entire life cycle on the human host. The intensely itchy symptoms associated with scabies are not caused by the burrowing itself but by the body’s allergic reaction to the mites, their eggs, and their fecal matter, often called scybala.
Transmission Routes: Prolonged Skin-to-Skin Contact
Scabies is primarily transmitted through direct, prolonged skin-to-skin contact with an infested person. Transmission requires a sufficient duration of contact for the mite to move, which is typically cited as 10 to 20 minutes. Because sexual contact inherently involves this extended, close physical contact, it is a very common route of transmission, leading to a high rate of spread among sexual partners. However, scabies is not exclusively a sexually transmitted infection (STI), as the transmission mechanism is prolonged physical contact, not a specific exchange of bodily fluids.
Sharing a household or providing intimate care, such as bathing or dressing, also presents a high risk of transmission. Brief contact, such as a handshake or a quick hug, is considered low-risk for the classic form of scabies. Transmission through indirect contact, like sharing clothing, bedding, or towels, is less common because the mites usually only survive for 48 to 72 hours away from a human host. An exception to this is crusted scabies, a severe form of the infestation where the host harbors thousands of mites, making transmission via contaminated objects much more likely.
Identifying Symptoms and Infestation
The most common sign of scabies infestation is severe, unrelenting itching that characteristically worsens at night. For people who have never had scabies before, symptoms can take anywhere from two to six weeks to appear following exposure. The physical appearance of the infestation often includes a pimple-like rash with small red bumps or blisters. A unique, though sometimes difficult to find, sign is the burrow itself, which appears as a thin, wavy, raised line that is grayish or skin-colored and may be up to a centimeter long. In adults, the rash and burrows are most commonly found in skin folds, such as the webbing between the fingers, the insides of the wrists and elbows, the armpits, the waistline, and around the genitals.
Treatment and Preventing Reinfestation
Scabies treatment requires prescription medication, as no over-the-counter products are approved to eradicate the mites. The standard first-line treatment is a topical cream, often Permethrin 5%, which is applied to the entire body from the neck down and left on for eight to fourteen hours before being washed off. A second application is typically required one week later to kill any mites that have hatched from eggs surviving the first treatment. Oral Ivermectin is another effective medication, often used in cases of crusted scabies or when topical treatment is impractical, and it is usually taken in two doses spaced a week apart. A crucial part of preventing reinfestation is the simultaneous treatment of all sexual and close personal contacts, even if they do not yet show any symptoms. Additionally, all clothing, bedding, and towels used by the infested person during the three days before treatment must be decontaminated. This is accomplished by:
- Washing these items in hot water and drying them on a hot cycle.
- Sealing non-washable items in a plastic bag for a minimum of 72 hours.
Itching may continue for two to four weeks after successful treatment as the body resolves the allergic reaction.