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"Poisonous" does not mean deadly. Some manifestations of toxicity are subtle. The dose, as always, determines if a plant is safe source of nutrients or a toxic hazard.

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BRIEF: Are there any poisonous plants that can cause death in dogs, horses or humans without symptoms?

I go on hikes a lot with my dog; I also pasture two horses, and I was just wondering are there any plants that cause death in dogs, horses or humans without any symptoms?


No, but you might not notice the symptoms or associate them with the ingestion of the plant. For example some of the deadliest amanita mushrooms have two sets of ver different delayed symptoms before collapse and death (a third kind). Liver toxins from pyrrolizidine alkaloid-bearing plants might take months to accumulate damage, and progressive brain damage from yellow star thistle in horses can take a long time and be misdiagnosed, as well.