Hello and
Worms are one method of carrying BH, hence the fact that it often shows up after rains when the worms come to the surface. They ingest the oocysts that are carrying BH and the poultry can pick it up by eating the worms. Obviously you're going to have a hard time keeping worms out. The best bet is to treat them via diet. Cayenne pepper regularly in their food will burn the oocysts and can prevent BH before it starts. poultry in general love hot things in their foods and will thrive on them. Raw garlic, onion, black pepper, cayenne, curry, they enjoy lots of hot things.
If you do get a bird with BH, the treatment I found worked was to give the sick bird (once it's fasting itself, usually lying down by that point) a cupful of raw (unpasteurized and unhomogenized) cow's milk with the fat scraped off the top (let it sit for an hour or two to get the fat off), with a teaspoon of honey mixed in. The vast majority recover completely from that and an ever greater majority develop lifelong resistance or immunity to BH after that. Giving it to them before they're in the latter stages of the disease can prolong their illness and cause them not to respond to the treatment when it's given, though, so don't try to be preemptive with it. When the yellow in their feces is about banana yellow, and they have diarrhea, is a good time. When it's just a light shade of yellow, and the poops are still somewhat firm, it's too soon.
Some need their beaks dipped in to make them drink it but most will drink it without help. I still don't know why it works, it seems counter-intuitive since the fat is dangerous to them at that point, hence not using goat's milk or storebought milk which are both homogenized, one naturally and one artificially, but rather using cow's milk with the fat scraped off the top. A few times I gave it to them too soon, and some dug holes in the fat (without eating it) to get to the milk beneath, whereas others would not touch it at all until I removed the fat. At least one was foolish enough to eat the fat and suffered longer for it. It's very beneficial --- as long as the excess fat is removed. Using cooked milks would probably kill them.
Some birds are very susceptible to BH and will come down with it again. I would cull those out, either rehoming or killing, and breed only those that are resistant and either respond well to treatment or never require it. Within a few generations I bred BH susceptibility out of my turkeys, and while I wouldn't think they're totally immune, I stopped having any problems with it. Different individuals within the same family showed different tolerances for it, with some not needing treatment and some full siblings of theirs being highly susceptible.
Originally my turkeys had TB and I found that a diet involving mainly green grass, millet, and hard boiled eggs with dandelion or raw onion if there's no dandelion prevented them from suffering or dying from TB. Soon after they seemed to develop resistance to it and I put them back onto the normal diet, almost identical to the chickens, and never had problems with it again. Turkeys are highly green feeders and you can keep them without grass but they will do so much better with it.
Best wishes with them.
