Oops,sorry forgot the links, check it out yourself.
I also forgot to mention that animals can be sensitive to ground differentials as small as 0.5 to 1 volt depending upon conditions.

This engineer testifies under oath citing research performed by Doug Reinmann at University of Wisconsin stating those same facts.

Doctor Reinmann's research paper can be found here
(pdf reader required for some links)
http://www.uwex.edu/uwmril/stray_voltage/s vmain.ht m
http://www.strayvoltage.org/stories/index.php3? Sto ry=20010221_utility.inc