Causal Agent:
Sarcoptes scabei, human itch or mange mites, are in the arthropod class Arachnida, subclass Acari, family Sarcoptidae.  The mites burrow into the skin but never below the stratum corneum.  The burrows appear as raised serpentine lines up to several centimeters long.  Other races of scabies may cause infestations in other mammals such as domestic cats, dogs, pigs, and horses.  It should be noted that races of mites found on other animals may establish infestations in humans.  They may cause temporary itching due to dermatitis but they do not multiply on the human host.

Life Cycle:

Sarcoptes scabei undergoes four stages in its life cycle; egg, larva, nymph and adult.  Females deposit eggs at 2 to 3 day intervals as they burrow through the skin .  Eggs are oval and 0.1 to 0.15 mm in length and incubation time is 3 to 8 days.  After the eggs hatch, the larvae migrate to the skin surface and burrow into the intact stratum corneum to construct almost invisible, short burrows called molting pouches.  The larval stage, which emerges from the eggs, has only 3 pairs of legs , and this form lasts 2 to 3 days.  After larvae molt, the resulting nymphs have 4 pairs of legs .  This form molts into slightly larger nymphs before molting into adults.  Larvae and nymphs may often be found in molting pouches or in hair follicles and look similar to adults, only smaller.  Adults are round, sac-like eyeless mites.  Females are 0.3 to 0.4 mm long and 0.25 to 0.35 mm wide, and males are slightly more than half that size.  Mating occurs after the nomadic male penetrates the molting pouch of the adult female .  Impregnated females extend their molting pouches into the characteristic serpentine burrows, laying eggs in the process.  The impregnated females burrow into the skin and spend the remaining 2 months of their lives in tunnels under the surface of the skin.  Males are rarely seen.  They make a temporary gallery in the skin before mating.
Transmission occurs by the transfer of ovigerous females during personal contact.  Mode of transmission is primarily person-to-person contact, but transmission may also occur via fomites (e.g., bedding or clothing).  Mites are found predominantly between the fingers and on the wrists.  The mites hold onto the skin using suckers attached to the two most anterior pairs of legs.

Geographic Distribution:
Scabies mites are distributed worldwide, affecting all races and socioeconomic classes in all climates.