Fortunately, anthrax is rare in the United States. Contact with these generally rare species can lead to swelling of the head and legs, erratic movements and loss of appetite. A variety of equine diseases, including rhinopneumonitis, influenza and strangles, can be transmitted via communal watering troughs. As a precaution, use your own buckets when visiting an unfamiliar farm or when your horse travels to shows. Horses can be picky about unfamiliar water sources, but they usually won't allow themselves to become dehydrated.
Muddy water is harmless to horses, though they probably prefer clean water.
Horses in Water Horses find water a great source of amusement. A bored horse may pass the time by blowing bubbles in his bucket or flipping his nose on the surface of the trough, splashing anyone who happens by. In summer, horses with access to natural bodies of water make the most of it by pawing, rolling and wallowing in shallow spots, enjoying the cool water and the respite from insects. More adventurous horses will actually swim through deeper bodies of water.