As dawg says, tapeworms are transmitted by insects. Basically, insects eat chicken droppings that contain tapeworm bits, the tapeworm grows inside the insects, the chicken eats the infected insects, and the tapeworm reaches maturity inside the chicken. The mature tapeworm then sheds more bits with the chicken droppings and the cycle repeats.
Absent completely confining your birds so that cannot access insects, tapeworms can be very difficult thing to get rid of. Some things that
may help:
--Time your deworming with the onset of cold weather. Insects seem to be more active in the spring/summer, so by deworming when the insect population is naturally low, the chickens may not get reinfected so quickly.
--Move the chicken run to fresh ground if possible. You can also rake up the existing run of droppings to the best of your ability and dispose the droppings far from the chicken run. This may be a bit of an exercise in futility, but it made me feel better to be trying something.
--Plan on a regular deworming schedule. It may be once a year or more often depending on your chicken's health and perceived worm load.
Scrubbing out the coop, etc. really won't help. The most common insect carriers are beetles, ants and earthworms -- so nothing you are going to be able to defeat by cleaning.
The only thing I will add, is that when my flock got tapeworms a few years ago, I eventually decided against giving a dewormer. My reasons for this are detailed in a thread I wrote back then (
https://www.backyardchickens.com/threads/the-case-of-the-disappearing-tapeworms.1337076/ )
I kept the run clean, I gave some natural remedies (that in all honesty may have done nothing), but the tapeworms went away on their own. Since then, I've noticed a small amount of tapeworms each spring, but they are gone by late summer. Huge caveat in that this was only my experience and your situation may be different.
Tapeworms are gross little things, sorry you are dealing with this.
