horse-leech

From Middle English horseleche, horse leche (“horse doctor; bloodsucker, leech”), equivalent to horse +‎ leech. So called because it commonly attacks the membrane that lines the inside of the mouth and nostrils of animals, such as horses, that drink at pools where it lives.

horse-leech (plural horse-leeches)

1. (obsolete) A veterinarian for horses.
2. A type of sucking worm, Haemopis sanguisuga, larger than the common leech.
   • 1621, Democritus Junior [pseudonym; Robert Burton], The Anatomy of Melancholy, Oxford: Printed by Iohn Lichfield and Iames Short, for Henry Cripps, OCLC 216894069; The Anatomy of Melancholy: […], 2nd corrected and augmented edition, Oxford: Printed by John Lichfield and James Short, for Henry Cripps, 1624, OCLC 54573970, partition II, section 5, member 1, subsection vi:

     Some use horse-leeches behind the ears, and apply opium to the place.

• horse-leechery