I know most of the vet meds, but I cannot figure out what that word is? Tetracycline? Not that- won't do a thing for histomoniasis (blackhead). Metronidazole is what you need for this disease if this is what he has. If you have a ruffled punky turkey with diarrhea (yellow especially) and you have or have had chickens- this is blackhead until proven otherwise- you will need to get it from the vet if you want it NOW- which you really need to do- as they die quickly. BY the time they look sick, they are usually septic and in liver failure. If you want to have some on hand for the future, you can get fish-zole on line which is OTC metronidazole.
Denagard (tiamulin) may have some effect in blackhead treatment and control- though the why of it is unknown, and unless you live in the UK- it will be harder to get than metronidazole. Also for control- you need to deworm everyone with something that will kill cecal worm (ie NOT wazine).