I looked hard for that one. The only mention of it I can find is on forums or blogs in the context of, "I heard that...". I can't imagine that a major animal rights group poisoned dogs and the media never reported it anywhere.
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"If you have men who will exclude any of God’s creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men." — St. Francis of Assisi, 1181-1226