Scabies is a parasitic infestation caused by the microscopic mite Sarcoptes scabiei var. hominis. This mite burrows into the top layer of human skin where it lives, feeds, and lays eggs. The primary symptom is intense itching, often worse at night, caused by the body’s allergic reaction to the mites and their waste. Understanding how this infestation spreads, especially concerning transmission from inanimate objects, is important for assessing risk and implementing prevention measures.
The Scabies Mite: Survival Off the Human Body
The scabies mite is an obligate human parasite, depending entirely on a human host for survival and reproduction. The female mite burrows into the skin, lays eggs, and the resulting larvae mature on the skin’s surface. The entire life cycle requires the warmth and sustenance provided by the human body.
When separated from a host, the mites cannot survive for long periods. Scabies mites generally die within two to three days away from human skin. This limited survival time explains why transmission from objects is uncommon for typical infestations.
Primary Modes of Transmission
Scabies is highly contagious, but the standard route of infection is through direct, prolonged skin-to-skin contact with an infested person. Sustained contact is necessary to allow the mites time to successfully transfer. Brief interactions, such as a quick handshake or hug, typically do not result in transmission.
The infestation commonly spreads through intimate personal contact, sharing a bed, or living in the same household. Transmission often occurs before symptoms appear, as development can take four to eight weeks after initial infestation. Because extended contact is required, crowded living conditions, such as nursing homes or childcare facilities, increase the risk of spread.
Addressing the Risk of Fomites
Transmission via objects, known as fomites (clothing, towels, or bedding), is possible but rare for typical scabies infestations. A classic infestation involves a low number of mites, usually 10 to 15, which, combined with the mite’s limited survival time off the host, makes object-based spread uncommon. Sharing items like clothes or towels used by an infected person is considered a possible, though less common, risk.
Crusted Scabies
The significant exception to this low risk is crusted scabies, previously known as Norwegian scabies. This severe form occurs primarily in people with weakened immune systems and involves thousands or even millions of mites. Due to this extremely high mite burden, people with crusted scabies are highly contagious, and transmission via fomites becomes a genuine concern.
Environmental Cleaning and Eradication
Decontaminating the environment is a necessary part of treatment to prevent re-infestation. All clothing, towels, and bedding used by the infested person during the three days prior to treatment must be washed. Mites and their eggs are killed by heat, so items should be machine washed using hot water and dried on the hot cycle. A temperature exceeding 122°F (50°C) for at least ten minutes is effective.
For items that cannot be washed or dry-cleaned, such as shoes or stuffed toys, isolation is the recommended method. These items should be sealed in a plastic bag and removed from body contact for a minimum of 72 hours. This isolation period ensures any mites or eggs present die naturally. Thorough vacuuming of carpets and upholstered furniture is also advised.