I am going to bet it was goose poo.
The National Post recently published a warning issued by the Toronto Police Department concerning a spate of dog-poisoning in High Park. I run through the park 3-4 times a week, and my money is on goose-poo being the culprit. At this time of year, there are so many geese wandering the park and the area around Grenadier Pond that the path is, in some places, covered with a slick green slime.
I feel really bad for the owners of the dogs, since it is impossible to watch your dog for every minute of the walk. It is especially impossible- and, I would argue cruel- to keep them from sniffing (and thereby licking) at the things that interest them. No I don't mean that dogs sniff by licking, instead, I am trying to say that if one is to prevent a dog from licking foreign objects/substances, one must prevent them from sniffing it since their tongue and nose will be equally close to the object of inquiry.
Here's to hoping that a pathologist or veterinarian figures out the vector for the poison. Once again, I am betting it is a massive bacterial infection caused by ingesting goose feces.
Update: According to this story the dogs have been poisoned in what appears to be a deliberate attack, against them and their owners. Personally, I don't buy the "person disgruntled against the off-leash areas" statement made by the police. However, I guess that I am not quite willing to accept that someone would be willing to do something so cruel and petty over such a tiny matter. Then again, one would have to be the sort who regards an issue like off-leash areas to be quite important to be willing to lay such a cruel and petty trap. Not to mention pathetic, tiny (morally and literally), and weak.