Scabies is a skin infestation caused by the microscopic mite Sarcoptes scabiei var. hominis. The female mite burrows into the outermost layer of the skin, where it lives, feeds, and lays eggs. This activity triggers a delayed hypersensitivity immune reaction in the host, which causes the characteristic symptoms.
Visual Clues on the Skin
The most specific physical sign of scabies is the presence of a burrow, which appears as a small, slightly raised, grayish, or skin-colored line measuring a few millimeters to one centimeter in length. These tracks are created by the female mite tunneling through the epidermis to deposit eggs and fecal matter. Burrows can be difficult to find because a person with classic scabies typically hosts only 10 to 15 mites.
The skin reaction often presents as a rash of small, red bumps (erythematous papules), resulting from the body’s allergic response to the mite, its eggs, and its waste. This rash may resemble hives, small blisters, or pimples. In adults and older children, the rash is usually concentrated in warm, protected sites. These areas include the webs of the fingers, inner wrists, elbows, armpits, belt line, and the genital area.
In vulnerable populations, such as infants and young children, the rash frequently affects the head, neck, palms, and soles of the feet. A severe, highly contagious variant called crusted scabies (formerly Norwegian scabies) occurs in individuals with weakened immune systems. This type involves a massive hyperinfestation of mites, often numbering in the millions, and presents as thick, gray, crumbling crusts on the skin.
The Distinct Sensory Experience
The cardinal symptom of scabies is intense, persistent itching (pruritus), often described as one of the most severe forms of itch. This sensation results primarily from a Type IV delayed hypersensitivity reaction to mite products deposited under the skin. This allergic response involves the activation of immune cells and the release of inflammatory mediators that stimulate nerve endings.
The itching typically worsens significantly at night. This nocturnal intensification is thought to be related to increased mite activity during warmer periods and changes in the host’s nervous system sensitivity due to the body’s natural circadian rhythm. This severe itching often leads to sleep disturbance.
Constant scratching (excoriation) can cause secondary skin damage, including open sores and crusting. These breaks in the skin barrier increase the risk of secondary bacterial infection, most commonly impetigo, caused by Staphylococcus aureus. Even after successful treatment, the itching can persist for up to six weeks while the residual allergic inflammation resolves.
Understanding How Scabies Spreads
Scabies is highly contagious and spreads most commonly through direct, prolonged skin-to-skin contact with an infested person. Transmission generally requires contact lasting 10 to 15 minutes or more, common among sexual partners or household members. A quick handshake or brief hug usually presents a low risk of transmission for classic scabies.
The risk of spread is higher in crowded living conditions and institutional settings, including nursing homes, extended care facilities, and childcare centers. Close physical proximity in these environments facilitates the transfer of mites. Transmission via contaminated inanimate objects (fomites), such as shared clothing, towels, or bedding, is considered less common in typical scabies cases.
However, the risk of indirect spread through fomites increases when the source person has crusted scabies. Because these individuals harbor millions of mites, they shed large numbers of parasites that can survive off the human host for up to 72 hours. In this scenario, even brief contact with contaminated furniture or bedding can be sufficient to cause infestation.
Seeking Medical Confirmation and Management
If a person suspects scabies based on visual and sensory symptoms, seeking professional medical confirmation is necessary, as self-diagnosis can be unreliable. A healthcare provider can often make a presumptive diagnosis based on a physical examination and the patient’s history. Definitive diagnosis involves a skin scraping, where a small sample of skin is examined under a microscope to identify the mites, eggs, or fecal matter.
Management relies on prescription medications, as over-the-counter products are not effective. The primary treatment involves topical scabicides, typically a medicated cream applied to the entire body from the neck down and left on for a specified period. Oral medications may also be prescribed in cases of crusted scabies or when topical treatments fail.
To ensure successful eradication and prevent reinfestation, it is important to treat all close contacts simultaneously, even if they have not yet developed symptoms. This proactive approach addresses the incubation period, which can last up to six weeks following exposure, during which a person can be contagious without showing signs. Clothing, bedding, and towels used by the infested person must also be thoroughly cleaned or isolated to eliminate surviving mites.