My gelding, Sarge, got infested with those, even though he was routinely wormed. When I got Biscuit he was pretty wormy - I had him tubed and kept him separate but Sarge went down hill...his coat was rough, dull, dry and he lost weight and muscle mass.
I had him tube wormed 3 times with literally no success. Success came from using Panacur Power Pack 3 weeks after the last tube worming. I called and spoke with a vet at Panacur and they were very helpful.
Hope your filly gets better soon....those worms can really drag them down.