Phoebe
Mine had the same symptoms as yours after being on Trifexis for 4 months: lethargic, loss of appetite, no interest in playing or moving. My happy active 8 months old puppy just laid in a heap for almost 4 days and was a sack of bones. Thankfully, he came around on the 4th day. Never touched that stuff or any chemical pesticide (topical or oral) since then