Mistletoe is poisonous but I'm not sure that abortion is a symptom or result of it. There is a thing called "foothill abortion" that occurs in the situation you describe but I can't remember the exact cause. Wasn't mistletoe poisoning, though. Might be a local cross confusion or assumption from one thing to another like the cows experience foothill abortion when they are on the California foothill ranges with mistletoe in the oak trees, therefore the mistletoe gets the blame. As far as that goes any illness could cause a cow to abort so it indeed could be one result of mistletoe poisoning.