Scabies/Itch Mite Infestation (Sarcoptes scabiei)

Dating back to over 2500 years ago, scabies was known from the time of the Greeks and Romans through the Middle Ages as the “itch.” Although the disease was acknowledged by numerous cultures over several eras, the cause of the infliction was never clearly determined until 1687 by Giovanni Cosimo Bonomo. Several genera infect the skin of mammals, with S. scabiei being found in humans, although some mange mites of animals occasionally cause a pruritic rash in humans. S. scabiei is microscopic and lives in cutaneous burrows, where the fertilized female deposits eggs. Scabies is transmitted by close contact with infested individuals, including touching, shaking hands, sexual contact, and contact in day care centers with children and the elderly. Scabies is common among those in overcrowded conditions, and has been associated with immigrants, poor hygiene, poor nutritional status, homelessness, dementia and sexual contact.The usual skin sites that are susceptible to infection are the interdigital spaces, backs of the hands, elbows, axillae, groin, breasts, umbilicus, penis, shoulder blades, small of the back, and buttocks. The outstanding clinical symptom is intense itching. Scratching commonly causes weeping, bleeding, and sometimes leads to secondary infection. A form of the infestation, called crusted scabies, can occur in immunosuppressed or anergic individuals; many mites are present in keratotic excrescences on the body and extremities, but pruritus is usually absent. This infestation is highly contagious and has been reported to be the cause of hospital epidemics. Scabies occurs worldwide, with at least 300 million cases each year. Oral ivermectin, at a dose of 300 µg/kg, is given as a single dose and repeated after 7 days, and is effective for the treatment and prophylaxis of scabies in an infected institutional environment.

The most common symptoms are produced by the host immune reaction to burrowed mites and their products. Pruritus and rash may take up to 2~6+ weeks to develop after initial exposure to the scabies mite. For this reason, the early clinical presentation of scabies should be detected as soon as possible to prevent the spread of infestation. Skin scraping with dermoscopy is implicated as the diagnostic method of choice for scabies at the present time. Dermoscopy is especially useful in diagnosis of incognito scabies. In addition, the presence of visible burrows could be a reliable positive marker of scabies in the absence of dermoscopy or microscopy data. Specific techniques for recovery of the mites are listed below.  

Sarcoptes scabiei,  Left and Middle, The common itch mite; Right, Note the lesions on the hands.

Crusted scabies should be considered in the differential diagnosis of a generalized cutaneous eruption in a human T-cell leukemia virus type 1 (HTLV-1)-positive patient. Patients with crusted scabies from an HTLV-1-endemic population should be tested for a possible HTLV-1 infection. Infection with HTLV-1 is an important cofactor related to crusted scabies in Peru. Testing for HTLV-1 in all crusted scabies cases is highly recommended, especially when no other risk factors are apparent. These patients may be at increased risk of progressing to adult T-cell leukemia/lymphoma. Definitive parasitic diagnosis can be difficult; difficulties in management have led to renewed interest in both scabies and pediculosis. The diagnosis of scabies should always be considered in patients with advanced malignancies and associated pruritus.

The diagnosis of crusted scabies can easily be missed. Serpiginous tracks were noted on the surface of Sabouraud’s dextrose agar used for fungal culture of the skin scrapings from an elderly long-term-care facility resident. This unusual laboratory manifestation alerted clinical microbiologists to the possible diagnosis of scabies. Although many microbiology laboratories are aware of these unusual findings, personnel can forget to consider scabies in such situations.

Skin-scraping technique. The diagnosis can be confirmed by demonstration of the mites, eggs, or scybala (fecal pellets). Because the mites are located under the surface of the skin, scrapings must be made from the infected area.

1. Place a drop of mineral oil on a sterile scalpel blade. (Mineral oil is preferred to potassium hydroxide solution or water. Mites adhere to the oil, skin scales mix with the oil, the refractility differences are greater between the mite and the oil, and the oil does not dissolve fecal pellets.) 2. Allow some of the oil to flow onto the papule. 3. Scrape vigorously six or seven times to remove the top of the papule. (There should be tiny flecks of blood in the oil.) 4. Transfer the oil and scraped material to a glass slide (an applicator stick can be used). 5. Add 1 or 2 extra drops of mineral oil to the slide and stir the mixture. Any large clumps can be crushed to expose hidden mites. 6. Place a coverslip on the slide, and examine (first on low power). The adult mites range from approximately 215 to 390 µm in length, depending on sex. The eggs are 170 µm long by 92 µm wide, and the fecal pellets are about 30 by 15 µm. The fecal pellets are yellow-brown.

Plastic box or petri dish method. If mineral oil preparations of skin scrapings fail to demonstrate the mites, the encrusted skin scrapings, etc., can be placed in a small plastic box or small petri dish. The container should be left undisturbed at room temperature for 12 to 24 h. Away from the living host, the mites drop to the bottom of the box or dish and can be seen with a magnifying glass or dissecting microscope.