The Collaborative International Dictionary of English v.0.48:

Lyssa \Lys"sa\ (l[i^]s"s[.a]), n. [NL. See Lytta.] (Med.)
   Hydrophobia.
   [1913 Webster]

   Note: The plural (Lyss[ae]) has been used to signify the
         pustules supposed to be developed under the tongue in
         hydrophobia.
         [1913 Webster]

WordNet (r) 3.0 (2006):

lyssa
    n 1: an acute viral disease of the nervous system of warm-
         blooded animals (usually transmitted by the bite of a rabid
         animal); rabies is fatal if the virus reaches the brain
         [syn: rabies, hydrophobia, lyssa, madness]