Mystery worms

Worms in the bottom of a water trough. Horses using it.  Mid-winter. What the...?

My first thought is that they are hookworms ... not a good thing.  I can't say I have ever seen one and don't want to.  Maybe someone else will chime in with something better.

https://en.wikipedia.org/wiki/Hookworm

https://www.petsandparasites.org/dog-owners/hookworms/

Anne, these look more like regular earthworms. Guessing maybe they climb up the trough and fall in?  Or birds drop them in?  Anyway, hopefully not hookworms!

Earthworms, the whitish ones have been in the water long enough to start decaying.  During rainy weather earthworms will crawl around and get into all sorts of strange locations such as  watering troughs, rain gutters 2 stories up that have accumulated soil in them, etc.

Hookworms are pale white and less than 1/2 inch long, attached to the inside of the small intestine and aren't found in their adult form outside of the host's body as is the case with roundworms, whip worms, and tapeworm segments which can be found in the feces.