MICROBIOLOGY

• Sarcoptes scabiei var hominis: a mite (an ectoparasite); this mite is specific to humans
  • Standard taxonomy divides mite based on host origin.
  • Recent data suggest mites are distributed into three genetically distinct clades, with most mites in the B and C clades. Therefore, all human scabies is not caused by a homogeneous population. It is currently unknown if different clades are associated with differential gene expression.
• The active life cycle process is on the skin.
  • Once on the skin, the male mite searches for unfertilized females, mates on the skin surface, and then dies soon after that.
  • Fertilized female mites secrete proteolytic enzymes that allow them to dig burrow-like tunnels into the stratum corneum in about 20 to 30 minutes; after that, they continue to burrow up to 5 mm per day for the rest of their 4- to 8-week lives.
    • They lay up to 4 eggs per day or about 40 to 50 eggs during their lifetime; fewer than 10% of eggs develop into adult mites.
    • The larvae also burrow and molt to nymphs and then adult mites over a 10- to 17-day period and then climb onto the skin surface to begin the process anew.
  • Mites can crawl 2.5 cm/min on warm skin but cannot jump or fly
  • Mites are resistant to alcohol and soap.
  • If dislodged from a human host, the mite can live at room temperature with 40 to 80% humidity for 24 to 36 hours. However, the mite will use odor and temperature gradients to find a new human host.
  • Scabies can be a major initiating factor in streptococcal pyoderma, and linked to post-streptococcal sequelae such as acute rheumatic fever and acute post-streptococcal glomerulonephritis.