Scabies
Order: Acari
Family: Sarcoptidae
Description
Scabies is a common skin condition caused by a microscopic mite, Sarcoptes scabiei, that burrows and reproduces in the upper layers of skin. These mites are extremely small, measuring approximately 0.1–0.5 mm long and 0.15-0.25 mm wide, and cannot be seen without a magnifying device. Adults of S. scabiei have round, flattened bodies and eight legs that are short and stubby. Larvae have three pairs of legs, while nymphs have four pairs of legs and more closely resemble adults. Since the mites are too small to be observed directly on skin, identification and diagnoses are possible by assessing the appearance of rashes produced by S. scabiei. The eggs of S. scabiei are oval and approximately 0.1-0.15 mm long.
Adult of Sarcoptes scabiei observed under a microscope. These mites are too small to be seen with the naked eye, and scabies is often diagnosed based on the presence of symptoms. Image credit: Global Health, Division of Parasitic Diseases and Malaria
Quick Facts
- Scabies is a skin condition caused by a small mite, Sarcoptes scabiei, that burrows and reproduces in the skin. Since these mites are too small to be seen with the naked eye, scabies is often diagnosed based on symptoms such as a rash with intense itching that worsens at night.
- Scabies can spread when pregnant females are transferred via close bodily skin contact for prolonged periods. People of all ages, races, and social classes can be affected. Globally, it is estimated that no fewer than 200 million people suffer from scabies infections at any point in time. In developed countries, outbreaks are most likely to occur in households, nursing homes, extended-care facilities, prisons, and schools.
- When an individual is diagnosed with scabies, it is recommended that everyone inhabiting the household undergo treatment with topical creams, or in extreme cases, oral medications. After successfully treating for crusted scabies, sanitizing the treated person’s living area can help prevent reinfestation with mites dwelling in the environment.
Life history and habits
Sarcoptes scabiei has a life cycle that consists of egg, larva, nymph, and adult stages. Adults of S. scabiei mate on the skin surface, and males die shortly after mating. Mated females remain fertile for the rest of their life, and burrow into the stratum corneum of the epidermis, where they lay two to three eggs per day. Females live for 4-8 weeks and continue burrowing at a rate of 0.5-5 mm per day. Eggs are deposited within the burrows and hatch after two to five days. The newly emerged larvae mature into adults after three weeks, at which point they migrate to the skin surface to mate.
To penetrate the skin, the mites secrete a clear saliva-like fluid that dissolves the stratum corneum of the epidermis and causes a sunken pocket or depression to form. The mite then begins digging or crawling actions that form a tunnel or burrow in the stratum corneum as the mite is propelled forward and becomes completely submerged in skin tissue. This entire process can occur in as little as five minutes after a mite contacts the skin.
Transmission primarily occurs through prolonged contact with an infected host. However, these mites can survive and remain infective in the absence of a host. The survival duration is dependent on environmental conditions such as temperature and relative humidity, with low temperatures and high relative humidity associated with longer survival times of up to 48 hours. Studies on a strain of S. scabiei on rabbit hosts suggest that the infectivity of mites decreases the longer they exist without a host.
Symptoms
Research suggests that individuals diagnosed with ordinary scabies (as opposed to crusted scabies) are likely to have fewer than 15 mites across all body regions. For this reason, diagnosing an infestation with 100% certainty is challenging due to the low probability of detecting mites in skin scrapings. Rather, scabies infestations are initially diagnosed based on the presence of symptoms on skin, and confirmed by analyzing skin scrapings for mites, eggs, fecal pellets, and burrows in the epidermis. On very young children, the head, face, neck, palms, and soles are the most common sites of scabies. On adults and older children, scabies is most frequently observed on the hands, wrists, and elbows, although the genitals, feet, buttocks, armpit region, breasts, and waistline are also favored sites of S. scabiei.
The most common symptoms of scabies include pimple-like rash with intense itching, especially at night. These rashes can occur in a line and may include blisters and scales that rupture when scratched and increase exposure to secondary bacterial infections. The burrows produced by S. scabiei can appear as thread-like gray-, white-, or skin-colored tracks on the skin surface that measure 5-10 mm (1/5-2/5 inch) long, though the burrows can be very subtle and inconspicuous. These symptoms can take 4-8 weeks to develop on individuals who have never had scabies before, while people who have had scabies tend to display symptoms within 1-4 days after exposure to the mites. It is worth noting that the itchiness often persists for several weeks after successful treatment, and that some common skin conditions may cause similar symptoms, such as allergic reactions, contact dermatitis, impetigo, eczema, and psoriasis.
Crusted scabies
Some individuals, such as those with medical conditions that weaken the immune system, the elderly, or those living in institutions, are susceptible to a more severe case of scabies, known as crusted scabies (formerly known as Norwegian scabies). Among the most susceptible are individuals who have HIV, leprosy, or down syndrome, who may have hundreds or even thousands of mites living in their skin. The mites cause affected areas of the skin to dry and become scaley, and the crusts that form are gray, tend to be thick, and crumble easily when touched. Individuals with crusted scabies may not experience intense itchiness, and unlike ordinary scabies, diagnosing crusted scabies with 100% certainty is relatively easy since skin scrapings are very likely to contain mites.
The webbing between fingers is a common area for infestations of S. scabiei, the mite that causes scabies. Image credit: Webmd.com, Skin Problems and Treatments Guide
Burrow caused by S. scabiei. The burrows appear as raised tracks in the skin that are usually gray, white, or skin colored. Image credit: Webmd.com, Skin Problems and Treatments Guide
Example of crusted scabies, which is a more severe form of this skin condition. Image credit: DermNet, Crusted Scabies
Prevention
Avoiding skin-to-skin contact with infected individuals is an effective way to avoid exposure to S. scabiei. All bedding and clothing that have been in contact with the infected person should be dry cleaned or washed with hot water and dried directly in sunlight or in a hot dryer cycle. After treating individuals with crusted scabies, thoroughly cleaning all dwellings is highly recommended since there may be a substantial number of mites present in the environment. Items that cannot be washed should be sealed in a plastic bag for one week.
Chemical control
Diagnosing scabies with 100% certainty is not necessarily required for treatment. If one person in a household is diagnosed with scabies, it is recommended that all individuals inhabiting the household undergo treatment regardless of whether they are displaying symptoms. Chemical treatments are available as topical creams containing various insecticides or, in more extreme cases, oral medications containing ivermectin. It is important to note that oral medications containing ivermectin should not be taken by pregnant women or children under 15 kg (33 pounds), and that acaricide treatments may be ineffective against populations of S. scabiei that have developed resistance. Other treatments such as tea tree oil, neem, aloe vera, cayenne pepper, and clove oil work to promote skin healing and reduce itchiness but may not be as effective at killing mites.
References
Arlian, L., and M. Morgan. 2017. A review of Sarcoptes scabiei: past, present, and future. Parasites & Vectors. 10:297-319. Available here.
CDC. 2020. Scabies Frequently Asked Questions (FAQs). Centers for Disease Control and Prevention. Available here.
CDC. 2018. Scabies. Centers for Disease Control and Prevention. Available here.
Heukelbach, J., and H. Feldmeier. 2006. Scabies. The Lancet. 367:1767-1774. Available here.
Hicks, M., and D. Elston. 2009. Scabies. Dermatologic Therapy. 22(4):279-292. Available here.
WHO. 2023. Scabies. World Health Organization. Available here.