Worming

I recently wormed my 24 year old horse, but still find dead worms in each poo

It is not uncommon for stray dead and dying worms to continue to appear in the faeces for some days after worming, especially if there has been an apparent heavy burden. It sounds like the worms you are seeing are strongyle worms which are the most common and most dangerous in modern adult horses.

I would worm her with Equest every 3 months for the next year and then check her faecal egg count 4 months after the final worming. If she looks really healthy and has a faecal egg count under 200 then worming in early spring and early autumn each year with Equest should be adequate. If her egg count is higher than 200 then continue worming 3 or 4 times a year with Equest.