References

Definition Of A Painful Or Distressful Procedure On An Animal

From the Guide, 
“In general, unless the contrary is known or established it should be assumed that procedures that cause pain in humans also cause pain in animals (IRAC 1985).” 
http://www.nap.edu/catalog.php?record_id=12910 
and 
From the U.S. Government Principles For The Utilization And Care Of Vertebrate Animals Used In Testing, Research, And Training: 
“IV. …Unless the contrary is established, investigators should consider that procedures that cause pain or distress in human beings may cause pain or distress in other animals.” 
http://grants.nih.gov/grants/olaw/references/phspol.htm#principle