Scabies is a skin infestation caused by the microscopic mite, Sarcoptes scabiei var. hominis, which burrows into the top layer of human skin to live and lay eggs. The infestation results in intense itching and a rash, which is an allergic reaction to the mites, their eggs, and their feces. While the primary concern is the direct transfer of mites through prolonged skin-to-skin contact, worry often extends to the environment, particularly items like footwear. Understanding the biological limits of the mite outside of a human host is key to assessing the realistic risk posed by contaminated personal belongings.
Biological Limits of the Scabies Mite
The scabies mite is an obligate parasite, meaning it must live on a human host to feed, reproduce, and complete its life cycle. Once dislodged from the skin, the mite struggles to survive due to its inability to maintain water balance. It requires the warmth and consistent humidity of human skin to prevent fatal dehydration.
The mite’s survival time off the human body is usually limited to 48 to 72 hours under typical indoor conditions. Survival is heavily influenced by ambient temperature and relative humidity. Cooler temperatures and higher humidity can extend the mite’s lifespan, with some studies showing survival for up to a week at 59°F (15°C).
Environments like shoes, which are often dry, cool, and lack the necessary warmth and consistent moisture, are generally hostile to the mite’s survival. Mites cannot feed or reproduce once off the host, and their infectivity drops significantly.
Assessing Transmission Risk from Objects
The majority of scabies cases are spread through direct, prolonged skin-to-skin contact, which is necessary for the mite to successfully transfer and burrow into a new host. Transmission from an object, known as fomite transmission, is considered less common, especially in cases of typical scabies where only a small number of mites are present on the body.
The risk of contracting scabies from brief contact with an inanimate object like a shoe is extremely low because the mite’s ability to remain infective declines rapidly off the host. For a shoe to pose a risk, it would need to contain a live, infective mite that successfully transfers to a new host’s skin and burrows before succumbing to environmental conditions.
A notable exception is crusted scabies, sometimes referred to as Norwegian scabies. People with this severe condition have massive numbers of mites and shed large amounts of skin crusts containing them. In these severe cases, the risk of transmission via fomites, including clothing and possibly footwear, is significantly higher and requires stringent environmental cleaning protocols.
Decontamination Strategies for Footwear and Personal Items
For peace of mind, or after exposure to a person with scabies, simple decontamination steps can eliminate any lingering mites on non-washable items like leather shoes or boots. The goal of decontamination is to exceed the mite’s maximum survival time off the host.
One effective method is to seal the footwear in a plastic bag and store it at room temperature for at least 72 hours. A full seven days is often recommended for maximum assurance. This isolation period ensures that any mites on the item die from dehydration and lack of a food source.
Alternative strategies involve using extreme temperatures, which are rapidly lethal to the mites. Items that can withstand high heat, such as cloth sneakers, can be placed in a clothes dryer on the high heat setting for a minimum of 20 minutes.
Washable Items
For washable items like socks or insoles, washing in hot water followed by a high-heat drying cycle is sufficient. Temperatures exceeding 122°F (50°C) for 10 minutes will kill all mites and eggs.
Cold Decontamination
Conversely, sealing items in a plastic bag and placing them in a freezer for 12 hours or more is also an effective way to kill the mites. This method uses extreme cold to rapidly halt the mite’s biological functions.