The Collaborative International Dictionary of English v.0.48:

Backworm \Back"worm`\, n. [2d back, n. + worm.]
   A disease of hawks. See Filanders. --Wright.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Filanders \Fil"an*ders\, n. pl. [F. filandres, fr. L. filum
   thread.] (Falconry)
   A disease in hawks, characterized by the presence of small
   threadlike worms, also of filaments of coagulated blood, from
   the rupture of a vein; -- called also backworm. --Sir T.
   Browne.
   [1913 Webster]