I read an article once where a scientist who had experienced redworm poisoning. In her lab animals said that the toxin is only sporadically produced. You can get dozens of good batches, then suddenly a batch from the same supplier under the same husbandry and conditions, will cause convulsions and death. They had no clue what triggered production of the toxin, so chose to avoid red worms to prevent future loss.

You may have got lucky with this batch. I sure hope they are fine.