Something that I would like to address that I didn't see, though it could've been stated, but I'm playing quick catch up so I could've missed it. Blackhead is caused by a protozoa which can be transmitted through a parasite, which can come from bugs or other animals that are near to the bottom of the food chain working their way up. Not saying all earthworms carry blackhead, but there's the possibility that they carry it. Also you need to remember that the animals have an immune system that can fight back, but when exposed to too much then the animal gets sick and starts showing symptoms. You can't go keep putting medicine into them over and over and over. It's wise to space it out, let the animal get an immune system and not rely on the medicine or herb. If you do that you can also shoot yourself in the foot that way too. Remember, what doesn't kill you makes you stronger. I'm not saying don't worm your birds, it's good that you do, you just have to find the right balance. You also have to remember protozoa and parasites are living things.Environment plays a huge role. Usually though parasite eggs can survive the winter. For those that have hatched though, if they're not in a host when it freezes they usually die. In my opinion that's why you need to worm when the ground freezes. That way those that hatched out, the host will not be a safe haven. Rather they die by the wormer or they die by the elements. Either way those that hatched, they die. When the ground thaws, those that didn't hatch, if still alive, will hatch, and need a host. If you worm when the ground thaws, they will die by the wormer, or by starvation. They have to have a food source, that's why they need the host. Without a host they die. This method, in my opinion, and understanding of parasites for right now, makes it so they only way they can get in, is through an intruder. Like a grasshopper, mouse, barnyard cat, dog, etc. Not all parasites though can jump from one species to the next, some are species specific.
I've never really though of parasites being able to pass through the feed that we buy in a feed store. I'm not entirely sure of how true this may be, I've never tested it out, I hope not, because the feed I use for my peachicks has animal based protein in it. Wouldn't be surprised if it had fish in it, the feed has a bit of a fish scent to it. Though I would expect the feed companies to sterilize it during certain parts of the processing it. After all if they didn't that would be one of the easiest ways to have a Salmonella outbreak across large parts of the US and other countries. In my opinion they animals need a little exposure, not enough to make them sick but enough to build up an immune system. If you really want to spend a lot of time thinking on it, we along with animals are in contact with pathogens of some sort. Thinking of the door handle, the ground, everything you buy from the store. For animals it's the ground, maybe a perch or scratching post. But our immune systems, if we take care of ourselves properly is strong enough to fight them off. Most diseased caused pathogens happen when some part of the body is under more stress than the body can hold back. Then it needs help to push back and rebuild. This can be done in multiple ways, wormer, antibiotic, probiotic, vitamin supplement, nutrient supplement, electrolytes, increase the temp of the surrounding area, and one the most people might not like to hear bleeding. Bleeding is a way for your body to push out bad, dirty blood (not really dirty but not clean), and let the body be filled with good, clean blood.
