Mouth ulcerations can be seen with moxidectin or ivermectin due to the killing of bot fly larva that are encysted in the gums/tongue. However this is more common with one specific ivermectin product-Zimectrin Gold.
Quest Plus doesn't cross the blood brain barrier unless there is a problem such as over-dosing or some damage the the central nervous system or organ dysfunction. Head/neck trauma, inflammation of the central nervous system, EPM, etc can all affect the functioning of the blood-brain-barrier and lead to neurotoxicity from allowing substances that should not pass into the CNS to do get there.
I've been party to hundreds of doses of moxidectin being given---even in senior horses, pregnant mares, etc and have seen no adverse reactions. However, you are right, I wouldn't dose it in lactating mares.
I simply cannot understand the logic behind giving a drug to kill encysted larva that are doing no damage and with which the major issue is the severe GI inflammation that they can cause if they all emerge at once when you know that the drug you are giving causes the very severe inflammation that is the main reason for getting rid of encysted strongyles.
Licensed Veterinary Technician