Oh no! I hope that your juvenile recovers! Have you tried posting on the diseases and emergencies forum? Disease and injuries of chickens and guineas are similar. Coccidiosis is one disease that could explain feeling poorly with diarrhea. Can you submit a fecal sample to your vet or diagnostic lab? May be worthwhile to try high dose amprolium (Corid) while awaiting results.
IMHO, there are only so many diseases that you can treat for, so I would start with the treatable ones. You have lethargy and diarrhea, so intestinal parasites are one of the major possible causes that can be treated for. The amprolium in feed is “low dose” amprolium, and coccidia may be or become resistant to that. Sometimes high dose still works though (I had that situation in a group of 2 mo old chicks, so I don’t use medicated feed anymore). A fecal test would help you figure out if parasites are really the problem and what the best treatment would be. Unfortunately, there are a lot of other diseases that could also cause the signs you’re seeing. Do you have any pictures of the poop? Also, keep trying on the diseases forum - it’s a busy place and it can be hit or miss who responds.
