Human infection with the parasite Sarcoptes scabiei var hominis causes scabies. Humans are the only known host, and the parasite is transferred from one person to another by close physical contact.
Clinical Findings: Scabies mites can affect any human. Men and women are equally affected, and there is no race predilection. The rash of scabies is highly pruritic. Patients often scratch in front of the examining physician and cannot stop themselves from doing so. Patients often state that it is the worst itching sensation they have ever experienced. The itching sensation is worse in the evening, especially when one is trying to sleep. Cutaneous findings are variable. Burrows are the hallmark of scabies and are pathognomonic for the disease.
Burrows consist of a fine, 0.5- to 1.0-mm-wide, 0.5- to 1.5-cm-long area of undulating or serpentine regions with a tiny black speck at one end. This tiny black speck is the scabies mite that is burrowing along the skin. If one were to scrape the area of the burrow where the mite is located and examine the scraping under the microscope, a mite would surely be seen. The mite may also be seen in association with eggs and scybala (mite feces). Any of these findings confirms the diagnosis. Burrows are most commonly appreciated along the sides of the fingers and the wrists.
The palms are commonly affected with tiny (1 mm) patches within the skin lines. They are intensely pruritic and are associated with excoriations. Scabies mites avoid the areas of the body that contain numerous sebaceous glands and for this reason are almost never seen on the face of anyone past the age of puberty. They may be found on the face of infants and children, who have not yet formed mature sebaceous glands. Scabies also has a propensity to affect the genitalia. The scrotum is almost always affected in cases that are more than a few weeks old. Very few rashes cause papules or nodules on the scrotum, and the presence of itchy nodules on the scrotum should be considered a sign of scabies until proven otherwise.
Crusted or Norwegian scabies is a rare form of scabies seen in immunosuppressed individuals. The crusted lesions represent the actions of hundreds to thousands of scabies mites. Patients are often covered from head to toe and are extremely pruritic. A scraping shows the presence of numerous mites. Individuals with crusted scabies should be treated with a multimodal approach. Scabies can cause outbreaks in long-term care facilities. These outbreaks can affect many individuals within the facility and are difficult to eradicate.
Histology: Skin biopsies are rarely performed. If a biopsy were to be done, one would see a mixed inflammatory infiltrate in the dermis with many eosinophils. This is a nonspecific finding and can be the result of any bug bite reaction. If the actual mite is biopsied, scabies parts will be present within the epidermis.
Pathogenesis: S. scabiei is spread from one human to another by close physical contact. The mite burrows into the epidermis but is unable to penetrate the basement membrane zone. Its presence sets off a massive inflammatory response. The female mites lay eggs as they burrow through the skin. Each egg hatches within 2 to 3 days and releases a larva. The larvae quickly grow and form nymphs and then mature adult mites. This process occurs within 1 week’s time. The mites have a life span of 2 months. The female mite can lay 3 eggs per day.
Treatment: Permethrin is currently the drug of choice to treat scabies. It should be applied overnight and repeated in 1 week, because it is pediculicidal but not ovacidal. The second application makes sure that any recently hatched mites are killed before they can reach reproductive age. Colloidal sulfur may be used on pregnant women. It is efficacious and safe but has a terrible odor. Outbreaks in long-term care facilities are often treated with oral ivermectin, which has shown good efficacy. Lindane has fallen out of favor because of its potential neurotoxicity. The use of malathion is advocated if permethrin fails.