Residual chemicals in horse manure?

There's free horse manure to be found, but I worry about whether the wormers and other chemicals people give their horses here might be bad for the soil?

Do the wormers kill off earthworms?

Do any other chemicals pass through the horse manure and do nasty things to soil and plants?

Is there anything else to consider about free horse manure?

Actually the worst thing you can find in horse manure is residual herbicide, Aminopyralidand and clopyralid  pass right through the horse/cow/sheep and will then kill anything other than grass you use the manure on. Beans are very susceptible to this herbicide so the best and cheapest way to check for it is to sprout beans in the manure.