Scabies is a contagious disease caused by the mite,
Sarcoptes scabiei. Scabies is seen worldwide. It
is usually acquired by direct contact with an infested individual or acquired
from the bedding or clothing of infested individuals. Scabies is common in nursing
homes, where people are in close contact. The mites form tunnels within the top
layer of skin (stratum corneum of the epidermis). They do not go deeper than the top layer of
skin. The female mites lay eggs within
the tunnels they leave behind.
The skin findings include red bumps, nodules, and burrows. Itchy red papules can be on the web spaces of the hands, and the wrists, buttocks, trunk, arm-pits, abdomen and genitals. Adults do not get infestation on the head, but infants who are infected do get lesions on the head. Scabies causes very significant itching, which is often worse during night-time hours. Scabies can be much more severe in people with immunocompromise, such as those with HIV infection.
The linear tract produced by the mites are called “burrows”. The mite can only survive several days without the human host.
Skin scrapings can be collected and viewed under a
microscope (as I did in this case) and the mites, feces, or eggs can be seen. Skin scrapings of fresh papules and burrows using a small blade are
placed on a microscope slide for evaluation.
The treatment of choice for scabies is 5%
permethrin cream, which is applied from the neck down and left on overnight,
followed by thorough rinsing in the morning. Permethrin acts as a neurotoxin for
the mite and has very low potential for toxicity. A second treatment with
permethrin one week later improves the chances of eradication of the mites. It
should be rubbed in well to all skin surfaces (in between finger, toes, groin
arm pits, etc).
Oral ivermectin is also
effective for the treatment of scabies. It works by blocking chemical
transmission across invertebrate nerve synapses that utilize glutamate or
γ-aminobutyric acid (GABA), resulting in paralysis and death.
It is important to be aware that itching may
persist for 2– 6 weeks following successful treatment. This is because the
immune cells remain activated and the mites and eggs (although dead from
treatment) can continue to aggravate the immune system. Thus, the dermatologist
will often prescribe topical corticosteroids and oral antihistamines for
symptomatic relief during this period.
Scabies nodules are red-brown nodules, which
represent a vigorous hypersensitivity response of the host and may last for
several months, even after the scabies infestation has resolved.
In addition to permethrin or oral ivermectin,
environmental decontamination is important. Personal items such as, clothing,
bed linens, and towels should be machine washed in hot water and dried using a
high-heat setting. Items that cannot be washed (i.e., stuffed animals, pillows)
may be stored in clean trash bags for 1 week, as the mite will die when
separated from the human host.
A small group of people who do not have any scabies or other infestation
may actually have a delusion of infestation. This condition is termed, “delusions
of parasitosis” and these patients describe sensations of crawling, burrowing
and even biting from parasites. They may offer elaborate, detailed descriptions
of their parasites and perform obsessive behaviors including picking and the
application of disenfectants and pesticides. These patients may apply topical
anti-scabetic agents such as permethrin cream in an attempt to rid themselves
of “parasites”. Read this article on delusions of parasitosis by Dr. Robles.
Follow David Robles, MD, PhD
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