Iowa State University Veterinarian Records, RS 22/6/0/9, University Archives, Special Collections Department, Iowa State University, http://www.add.lib.iastate.edu/spcl/arch/rgrp/22-6-0-9.html
Sudden death in young horses, three to six years of age, is frequently observed. The owners usually state the horses were in good condition and had not been observed to be ill. While being worked or exercised the horses suddenly die or show respiratory distress and die after a very brief illness.
Monlux, William S.
"Sudden Death in the Horse Due to Myocarditis,"
Iowa State University Veterinarian: Vol. 24
, Article 8.
Available at: http://lib.dr.iastate.edu/iowastate_veterinarian/vol24/iss1/8