The Collaborative International Dictionary of English v.0.48:

Jigger \Jig"ger\ (j[i^]g"g[~e]r), n. [A corrupt. of chigre.]
   1. (Zool.) A species of flea (Tunga penetrans, or
      Sarcopsylla penetrans, or Pulex penetrans), which
      burrows beneath the skin; called also jigger flea. See
      Chigoe.
      [1913 Webster]

   2. (Zool.) Any one of several species of small red mites
      (esp. Tetranychus irritans and Tetranychus Americanus)
      of the family Trombiculidae, which, in the larval or
      leptus stage, burrow beneath the skin of man and various
      animals, causing great annoyance. Also called chigger.
      [Southern U. S.]
      [Webster 1913 Suppl. +PJC]

The Collaborative International Dictionary of English v.0.48:

Chigoe \Chig"oe\, Chigre \Chig"re\, n. [Cf. F. chigue, perh. fr.
   Catalan chic small, Sp. chico; or of Peruvian origin.]
   (Zool.)
   A species of flea (Tunga penetrans, formerly Pulex
   penetrans), common in the West Indies and South America,
   which often attacks the feet or any exposed part of the human
   body, and burrowing beneath the skin produces great
   irritation. When the female is allowed to remain and breed,
   troublesome sores result, which are sometimes dangerous. See
   Jigger. [Written also chegre, chegoe, chique,
   chigger, jigger.]
   [1913 Webster]

   Note: The name is sometimes erroneously given to certain
         mites or ticks having similar habits.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

chigger \chigger\ n.
   1. a small tropical flea (Tunga penetrans, formerly Pulex
      penetrans); same as chigoe. [Also spelled chigre.]

   Syn: chigoe, chigoe flea, Tunga penetrans.
        [WordNet 1.5]

   2. a larval mite of the family Trombiculidae that sucks the
      blood of vertebrates including human beings, causing
      intense irritation, and serving to transmit several
      infectious diseases.

   Syn: harvest mite, jigger, redbug.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

chigger
    n 1: small tropical flea; the fertile female burrows under the
         skin of the host including humans [syn: chigoe,
         chigger, chigoe flea, Tunga penetrans]
    2: larval mite that sucks the blood of vertebrates including
       human beings causing intense irritation [syn: harvest mite,
       chigger, jigger, redbug]