worms

They are probably pinworm (like beansprouts?), hard to get rid of, unpleasant but not really harmful. Ask your vet for help, maybe worm with Strongid p which seems to reach the hind gut better than others.
 
If you kept a sample for your vet that could be of use.

You may need to do the 5 day Equine Guard for encysted redworm

Definitely speak to your vet as resistant worms could be involved and your current routine worming programme may not be changing chemical groups annually/biannually as is currently recommended

Worm egg counts are useful but do not show up tapeworm eggs or encysted redworms.

As above pinworms seem to be causing problems but you would probably have noticed your horse rubbing her backside as these cause irritation around the anus.
 
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