Scabies

Overview

By Tina Shahian, PhD

Scabies is a fully treatable skin rash caused by the microscopic human mite Sarcoptes scabiei. It is transmitted by direct skin-to-skin contact or by contact with infested clothing and bedding. The mites burrow into the upper skin layer (epidermis) to live and lay eggs, causing raised itchy bumps along the way.

Sarcoptes scabiei mite (source: Alan R Walker)The elderly and those with a weakened immune system may develop a severe form of scabies called crusted scabies, also called Norwegian scabies. Crusted scabies is highly contagious because of thick brittle crusts that form on the skin, each carrying hundreds to thousands of mites and eggs.

Mites are species-specific, which means mites that infect other animals cannot infect humans, and vice versa. Other species of mites may cause a minor skin reaction in humans, but they cannot reproduce.

Human scabies is very common worldwide and can affect anyone regardless of age, gender, race and ethnicity. It spreads easily in places where there is close human-to-human contact, such as hospitals, childcare centers, prisons and nursing homes. Scabies is fully treatable using medicated creams/lotions. The most common complication associated with scabies is infection of open sores due to excessive itching.

Causes and Risk Factors

Human scabies is caused by the mite Sarcoptes scabiei. Pregnant females burrow into the upper epidermis, where they lay their eggs and remain for future reproductive cycles. New mites develop and mate under the skin, then travel to the surface where they can infect a new host. In a typical infection, there are fewer than 20 mites per person and they are typically found on the hands, wrists, knees, penis, breasts and shoulder blades. The itchy skin rash that develops is caused by an allergic reaction to mite proteins, eggs and feces.

Transmission primarily takes places during prolonged, direct skin-to-skin contact (e.g. sexual contact) with an infected person. Hospitals, nursing homes and other extended care facilities are especially prone to a mite infestation. A less common means of transmission is through contact with infested clothing or linens. Crusted scabies is the most contagious form of scabies because of thick crusts on the skin that can carry thousands of mites and mite eggs. These crusts can also shelter and nourish mites for up to a week when they shed. Risk factors for acquiring scabies are:

  • Sexual contact with an infected partner
  • Infection within the household or immediate family
  • Extended care facilities (e.g. nursing homes, hospitals and childcare centers).

Risk factors for crusted scabies are:

  • Weakened immune system
  • Advanced age.

Symptoms

The most common symptoms of scabies are severe inching and body rash, which are caused by an allergic reaction to the mites. Those infected for the first time may not develop symptoms for up to two months; however, they are still contagious and able to transmit scabies. Repeat infection can produce symptoms within days. The symptoms of scabies are:

  • Severe itch that worsens at night
  • Rash made of small bumps that look like pimples or hives
  • Greyish thick crusts that easily crumble (crusted scabies).

Scabies can affect any area of the body, but mites favor the following sites:

  • Between fingers and around nails
  • Wrists and elbows
  • Buttocks and penis 
  • Shoulder blades and armpits
  • Skin around nipples
  • Skin covered with jewelry or belt.

The most common areas of infestation in young children and infants are:

  • Scalp, neck and face
  • Palms of hands
  • Soles of feet.

Diagnosis and Treatment

Doctors diagnose scabies by examining the skin for evidence of mites and/or mite burrows. The burrows are hard to find and appear as tiny greyish or skin-colored raised lines on the skin. Scrapings from the affected skin can also be studied under a microscope to find mites or eggs.

The goal of treating scabies is to alleviate the symptoms and eliminate the parasite. Since scabies is very contagious, doctors usually recommend treatment for anyone living with the infected person, as well as recent sexual partners. Scabies is treated with prescription medications (called scabicides) that kill mites and mite eggs. In most cases adults and children can resume normal activity the day following treatment, although the itch can persist for a few more weeks. If itching and rash reappear after four weeks, retreatment is needed. Treatment for scabies includes:

  • Scabicides: Creams and lotions (permethrin, lindane, crotamiton) and oral ivermectin are available for treating scabies. Choice of treatment depends on the patient’s age, pregnancy, overall health and response to previous scabies drugs. Ivermectin is used to treat crusted scabies or patients with weak immune systems. Treatment lotions/creams are applied all over the body – including head, neck and scalp for children - and left for several hours.
  • Decontamination: All clothing and linens belonging to the infected person must be washed in hot water. Items that cannot be laundered can be sealed in a bag for 2–3 days to kill mites and eggs.
  • Antihistamine: Over-the-counter allergy medications may be used to manage the rash and itching.
  • Antibiotics: Excessive itching of the rash area(s) can lead to open sores and infection. Antibiotics are prescribed to treat the infection.

Prevention

Repeat infestation or a scabies outbreak can be prevented by avoiding contact with an infected person and laundering all contaminated linens and clothing in hot water. Items that cannot be washed can be sealed in a bag for 2–3 days to starve the mites.

Sources

  • “Scabies”. Centers for Disease Control and Prevention (CDC). Retrieved Jul 24, 2015. http://www.cdc.gov/parasites/scabies/index.html.
  • "Scabies". Mayo Clinic. Mayo Foundation for Medical Education and Research. Retrieved Jul 24, 2015. http://www.mayoclinic.org/diseases-conditions/scabies/basics/definition/con-20023488.
  • “Scabies” American Academy of Dermatology. Retrieved Jul 24, 2015. https://www.aad.org/dermatology-a-to-z/diseases-and-treatments/q---t/scabies.