After a little more research, I realize ALL the drugs involved are macrocyclic lactones. They fall into three groups
The first group is the avermectin group which includes the drug added to Advantage to make it Advantage Multi, which also kills worms and protects against heartworm. The "multi" part of Advantage Multi has nothing to do with the fleas. It is Moxidectin. Revolution also contains a macrocyclic lactone, Selamectin, which is considered to be a 'semi-synthetic' avermectin. The semi-synthetic variants of the avermectin group were created to produce improvements in many features of the macrocyclic lactone family including: safety (e.g. Selamectin is not thought to cross the blood-brain barrier as easily, resulting in improved safety for mammals, in particular ivermectin-sensitive breeds like collies.) Some dogs and even fewer cats have a genetic mutation that allows these drugs to pass through the blood-brain barrier and if that happens, they get neurological symptoms that are extremely alarming and can be life threatening. I had a cat who had a neurological reaction to the moxidectin in Advantage Multi.
The second group of macrocyclic lactones is the milbemycins (the main ingredient in Interceptor and Sentinel). In addition it is used in Trifexis, in combination with a drug from the third and most recent group of macrocyclic lactones, the Spinosyn group. I suspect that the cats and dogs who have died when given Comfortis, Trifexis, Assurity and now the new Cheristin have the same genetic mutation, one that allows the chemical to cross the blood brain barrier and thus work on the central nervous system of the host just as it does the insect.
Edited by Red Top Rescue - 7/23/16 at 5:17pm