Here is the abstract of the article I was thinking of. They tested coelomic fluid (this is presumably the liquid that leaks out when you cut them) on tadpoles.
I find this thread very interesting, as I've been feeding E. foetida to both tadpoles and salamanders, getting mixed results. I will share my experiences, and then my hypothoses. First I fed unrinsed E. foetida to my A. mavortium, who became sick and died a month later, but I also fed him Tenebrio molito, which I didn't realize, could also have killed him. Then I fed unrinsed E. foetida to A. mexicanicum, who eagerly took, but later developed long-term sickness, and refused to touch them. Next, I began letting them soak for 20 minutes in warm water before feeding them to R. ventrimaculatus tadpoles, S. algira adults and juveniles, T. marmoratus adults, N. crocatus juveniles and Cynops ensicauda adults. The S. algira adults got slightly and the T. marmoratus adults got seriously sick, and were not the least bit eager for them the 2nd time as they were the 1st. I never cut any of these worms, but gave the according to the size of the animal. I repeatedly fed the tadpoles tiny, white baby E. foetida when I was not giving daphnia or blood worms. 4 out of 6 tadpoles are now healthy froglets. The S. algira juveniles, N. crocatus juveniles took very small E. foetida which had developed coloration, and would take them meal after meal. The Cynops ensicauda adults took medium sized E. foetida repeatedly - do they perhaps have an immunity to E. foetida's coelomic fluid? Soaking in warm water first seems to remove some of the slimey excretions(perhaps making them slightly less dangerous, but I would by no means say safe). From my trial and error (this wasn't an animal experiment), it appears that E. foetida develops toxicicity with maturity. I don't know of any studies to suggest that other of the Eisenia genus such as hortensis have this toxic coelomic fluid.