parasitic skin infections

Dermatology Dr. Hussein A. Al-Sultany
2015-2016

Parasitic Skin Infections

A parasite is an organism that depends on a living host, for one or more of its essential metabolic requirements.
Scabies
Cause:
Human scabies is a highly contagious infestation, caused by a mite (sarcoptes scabiei var hominus).


Life Cycle:
Adult mites are 0.3 mm long. The mite infestation begins when a fertilized female mite arrives to skin surface, then burrow through the stratum corneum at a rate of 3 mm per day, it lays about 3 eggs per day, eggs hatch within 3 days into larvae, which reach maturity in about 3 weeks. The mite can survive for about 3 days outside the human skin (Role of 3).


Transmission:
Transmission of the mite occurs usually through prolong (5-20 min) close contact, some infections can be from exposure to fomites.


Presentation:
Scabies affects all races and all social classes. Occurs at any age, with equal sex incidence.
The IP: weeks to months after the initial exposure to the mite, subsequent exposure results in rapid eruption (the IP is few days), probably due to prior sensitization (the eruption caused by sensitization to the mites or their products).
The itching: is severe and more at night (nocturnal pruritus is characteristic).
The lesions: the pathognomonic lesion is a burrow, which is a gray white, slightly elevated, tortuous or zigzag like linear lesion that is 1 to 10 mm in length. Scratching may destroy the burrows; therefore they don’t appear in some patients.
Other types of lesions are vesicles and papules (red, excoriated, and urticarial).
The predilection sites: interdigital webs, sides of fingers, sides of hand, wrists, elbow flexure, axillae, areola, umbilicus and genitalia (Circle of Hebra; which is an imaginary circle intersecting the predilection sites).
In infants, the face and scalp can be infested (very rare in adults), and there is a characteristic palm and sole involvement.
The average number of adult female mites in an infected patient, is about 12. Postscabitic allergic nodules: intensely itchy, red nodules that may persist for weeks or months after successful treatment of scabies, treated by steroid.

Scabies incognito: is a modified scabies due to inappropriate use of topical steroids.
Crusted (Norwegian) scabies: is a widespread crusted eruption with minimum or no itching with vast number of mites. It affects people with neurological disorders, senile dementia, or immunosuppressed, and can be a source of epidemics of ordinary scabies.