Chickens are infected by eating roundworm eggs (called A. galli eggs), that are passed out of another chicken
Or any other number of hosts.. like wild bird visiting the area.. say hawks that just consumed a bird loaded maybe passing them in feces flying over one's pasture... dropped on the ground.. it does NOT have to come from another infected CHICKEN.. I see chickens dig through dog poo, pig poo, and others looking for tasty treats including maggots.
Round worms and others do not equal a bad keeper.. they equal a keeper who birds have access to nature, they are part of the environment and some places more than others..
Quite the opposite of what someone indicated that it may be LESS likely.. I say more likely to get roundworms free ranging than confined.. IF the flock is roundworm free.. it will come from outside sources.
Although I agree a microscope is the best way to determine parasite species load count.. I will also note that you could get clear test all day long and it may not be the whole picture.. it's just one tool.
The dog which I witness round worms come out.. the end/plug portion of the dropping was worm free and maybe denser.. farther up, it was softer and the worms were visible.. Since I saw it happen, I know it wasn't maggots. And despite my chicken float coming back clean.. less than a week later, I saw a broody pass a load of round worms.. since only round worm and tape worms are visible. One rescue dog had tape worms and it look like rice crawling around the anus.. said by the veterinarian to have been caused by ingesting a flea.
Earthworms can host roundworm eggs.. it has NOT been an issue with MY stock density, sandy acidic soil with wet mild year round climate. I don't fear my birds eating earth worms or catching round worms..
One thing not mentioned by the OP is
follow up treatment.. Usually anything you use has a recommended follow up time line to catch anything newly matured and put an end to the life-cycle. Otherwise treating on occasion is knocking SOME load down and that may be it.. possibly even creating resistance to that drug by the next generation. Get familiar with YOUR internal parasite load.
Another worming alternative though I know not which species.. is Ivermectin.. used off label, (pour on) it's also effective at treating many mites and lice. though also requires individual treatment..
droppings, contaminating the run, feed, water, etc. Moving your chickens to fresh ground and keeping water and food dishes clean, are strategies for combatting reinfection.
These are good basic suggestions!
Noting that my birds usually access an acre also and I still remove whatever waste I can using a dog poo shovel daily on my normal rounds as part of my husbandry practice..
ETA: Worms and tape worms don't kill a bird in healthy condition. There were other issues at play, consider getting a necropsy if you have another unexplained death. Sorry for your loss!
