I saw a horse poo at a show and it was crawling with worms. I hate seeing this stuff...
However, just throwing this in - some horses, for whatever reason, drop weight. My horse Shadow is a prime example of that. I had the vet out time and time again. No diheria(however you spell that), had his teeth floated, wormed him constantly, gave him stradegy and all the food he wanted... on and on. NOTHING made him pick up weight. This was after I brought him to our house in Alabama from my mom's in California and it went on for about 2 years. Took him back to my mom's when I went to stay with her while my hubby was deployed - he was back with his "mimi," my mom's mare, and POOF cured! I really think he was home sick. I had 2 of my other mares at the time but I think they didn't compare to his "mimi." LOL
Anyway, the point is you don't know what could be wrong or not wrong.
But on the other hand - I would never have take Shadow to a show looking all sucked up like he was. I don't think anyone should be taking their horses to shows looking bad and not expect to be confronted about it.