Can you clean off the ones with a lot of droppings, and look for any maggots? If you have flies, they may lay fly larvae or maggots on the poop or any area that has a cut or injury. Flystrike is what it it is called when there is a maggot infestation. I don’t know if that is what is happening, or if it is just an outbreak of diarrhea from something like coccidiosis or worms. Vent gleet might be possible, but that is a fungal infection, and there would be raw vents with a bad odor.
If you see any maggots, get the chicken into a pan of warm soapy water and get them off as quickly as possible. The maggots can hatch out for a day or so afterward.
Hopefully, this is just runny droppings. You can then try treating for possible coccidiosis with Corid, and possibly worm them all with SafeGuard Liquid Goat Wormer or Valbazen. But the best way to know for sure is to get some droppings checked at your vet for coccidia and worms.
Last night we lost one of our leghorns to what we thought was just vent gleet. The leghorn stopped going outside with everyone else when it was time to free range and always stayed in the nesting box. She ate very little and we noticed how bad the vent gleet was. We tried giving her baths and apple cider vinegar in the water but sadly nothing helped and we lost her last night. The other leghorns have this “vent gleet” but it isn’t nearly as bad as the original. We don’t know how to treat it and can’t find anything that really helps reduce it. I’m cleaning their coop monthly as well. Below I’m showing two leghorns, one of them has it the worst out of all 4 of them while the other one is the least infected out of all of them
