I did try to simplify it. Just trying to get the basics across.
This is a fairly good write-up on it. Like a lot of things on chickens this is pointed more toward commercial operations than us, but the basic principles hold.
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex4616
You’ll see where it says that younger birds are more likely to be affected, but further down it says that’s because the older birds are probably already immune.
I’ve seen that comment about medicating chicks pointed toward broilers a lot more than hens that will join a laying flock. Broilers, Cornish Cross, Cornish X, whatever you want to call them are usually given something that totally eliminates the cocci protozoa so they never build immunity. They won’t live long enough for that to be an issue. Chicks that will become layers are generally given something that limits the numbers of the protozoa but doesn’t kill them all. It keeps the numbers in check so they are less likely to get ill yet can develop immunity.
I purposely feed my chicks in the brooder dirt from the run on about their second or third day in the brooder to introduce the cocci I have in my flock. I don’t feed medicated feed but rely on keeping the brooder pretty dry. Some strains are worse than others so this doesn’t always work for everyone. Some people on this forum have said their problems with cocci greatly decreased when they started doing this.
Feeding medicated feed does absolutely no good if cocci is not present. The biggest risk time is after they first come in contact with the ground. Another too common occurrence is that people feed medicated feed while they are isolated in the brooder and never come in contact with cocci but stop feeding medicated feed when they take them out of the brooder and put them in the run. If the run is wet, they can get pretty sick.
I don’t see anything wrong with feeding medicated feed. It does not hurt them at all. In some cases it is really helpful but often it’s just not needed.
The medicine in practically all medicated feed intended for chicks that will be layers is Amprolium. That’s the stuff we normally buy. In the dosage in medicated feed, it does not kill all the protozoa but just helps keep the numbers in check so they can develop immunity. If the brooder, coop, or run is wet, they can still get deathly sick from cocci. The medicated feed does not cure cocci but can help improve your odds so they don’t get sick.
Amprolium is the active ingredient in Corid, which is used to treat cocci. The dosage in Corid is much higher than the dosage in medicated feed.
Some medicated feed intended for broilers is something other than Amprolium or sometimes Amprolium plus something else. It’s always a good idea to read the label to see what the actual medicine is in medicated feed.
It’s been a few years since I checked, but last time I checked, the vaccine for cocci only covered a few of the strains. It’s still possible for them to get sick from another strain even if they are vaccinated. If you feed medicated feed or antibiotics to vaccinated chicks in the three weeks after the vaccination you can negate the vaccination. It generally takes them three weeks to develop that immunity after exposure, whether from the vaccine or from contact with the ground.
I’m sure I’m forgetting some stuff but that’s enough. Have a good one.