Cause of Scabies
Scabies is caused by infestation with the parasitic mite Sarcoptes scabiei var. hominis, which burrows into the stratum corneum (top layer) of human skin. 1, 2
The Causative Organism
The disease results from the female mite invading the stratum corneum where she creates tunnels to reside, move, feed, deposit fecal pellets, and lay eggs 2. These mites are microscopic parasites that live exclusively in human skin and do not enter the bloodstream or internal organs 3.
Transmission Mechanisms
The mite spreads through:
- Direct skin-to-skin contact (primary route) 1
- Sexual contact 1
- Contact with infested fomites (less common) - including bedding, clothing, and other personal items 4, 1
Close personal or household contact is the key risk factor, which explains why outbreaks consistently occur in nursing homes, residential facilities, hospitals, and other institutional settings 4.
Important Clinical Context
The host-parasite interaction is complex and involves immune-mediated mechanisms 2. The intense itching that characterizes scabies is actually caused by the host immune response to the mite, not the mite itself 5. This explains why symptoms may persist for up to 2 weeks after successful treatment, even when mites are eradicated 4.
Risk Factors for Transmission
- Poverty and overcrowding 6
- Institutional living (nursing homes, prisons, hospitals) 4
- Immunodeficiency states (particularly for crusted scabies) 4
- Poor hygiene conditions (though scabies can affect anyone regardless of personal hygiene) 3
Special Variant: Crusted Scabies
Crusted (Norwegian) scabies represents an aggressive form that occurs in immunodeficient, debilitated, or malnourished persons, including those receiving systemic glucocorticoids, organ transplant recipients, and persons with HIV infection 4. This variant is transmitted more easily than typical scabies due to the massive mite burden 4.