Hi there, I'm Dr B and I'm happy to help you with your questions tonight.

About how large were the worms that you saw in the manure?

Okay, the most likely worm that you were seeing is the large redworm, or large strongyle worm

All horse dewormers except for praziquantel will kill this particular worm, as it is very susceptible to all deworming medications (very little resistance).

It could, depending on the worm load. If the worm load is high enough that you are seeing worms in the manure, it is likely that he has enough worms that the infestation is causing the diarrhea. However, diarrhea in horses can be due to other problems such as salmonella infection. I think that it is smart to start by deworming him (you can dose him by the directions on the label of the deworm, a weight tape might also help you out and can be purchased cheaply at any tack shop), and if the diarrhea does not resolve, I would recommend having a vet out to examine him to see if a cause for the diarrhea can be found.

You may also want to consider having a fecal examination performed by a veterinarian, as there may be other worms that we have not seen in the feces, but that are contributing to the problem.

Do you have any other questions?

Great, if you could please hit the happy face / accept button so I receive credit for my answer, I would really appreciate it. Best of luck with your horse, and please let me know if I can be of further assistance. Have a great weekend! ~Dr B