Scabies is a skin infestation caused by the human itch mite, Sarcoptes scabiei var. hominis. While highly contagious and typically spread through direct, prolonged skin-to-skin contact, scabies can also be acquired through less common means that do not involve direct human-to-human contact. This article explores these alternative pathways.
Indirect Transmission via Objects
Scabies can be transmitted indirectly through contaminated personal items, also known as fomites. These items include shared clothing, bedding, towels, or upholstered furniture that an infested person has recently used. Transmission through fomites is generally less frequent than direct skin-to-skin contact, as mites typically do not survive long when separated from a human host.
However, it remains a possibility, particularly with prolonged contact with heavily contaminated items or in situations involving severe infestations. For example, sleeping in the same bed as an infested person or sharing their clothes can facilitate this type of spread. This indirect route is more likely when there is a high concentration of mites on the items, such as in cases of crusted scabies.
Scabies from Animal Contact
Animals, including dogs, cats, and livestock, can develop sarcoptic mange, which is caused by a different variant of the mite, such as Sarcoptes scabiei var. canis in dogs. While these animal mites can temporarily burrow into human skin, causing an itchy, self-limiting rash, they cannot reproduce or establish a long-term infestation on humans. The infestation in humans typically resolves once the animal is treated or the human is no longer exposed to the infested animal. This is distinct from human scabies, which requires the human-specific mite Sarcoptes scabiei var. hominis for sustained infection.
How Long Mites Survive Off a Host
The human scabies mite, Sarcoptes scabiei var. hominis, has limited survival away from a human host. These mites generally die within 2 to 3 days (48 to 72 hours) when separated from human skin. Environmental factors like temperature and humidity influence their survival time. Mites are unable to move at temperatures below 20°C (68°F), and their activity significantly reduces below 15-16°C (59-61°F). Higher humidity and lower temperatures can prolong mite survival, but their viability outside a host is generally limited.
Crusted Scabies and Increased Risk
Crusted scabies, also known as Norwegian scabies, is a severe and highly contagious form of scabies. This variant is characterized by an extremely large number of mites, ranging from thousands to millions, and the presence of thick skin crusts. Due to this high mite burden, individuals with crusted scabies are significantly more contagious than those with typical scabies. The risk of indirect transmission through fomites, such as clothing, bedding, and furniture, is substantially higher and more common with crusted scabies. Crusted scabies often affects individuals with weakened immune systems, the elderly, or those who are physically or mentally incapacitated.