Salmonella and some strains of E-coli as well as some strains of Coccidia can cause Coccidiosis but usually only young or debilitated horses are sickened.
I have seen most cases come from bird contamination. I won't let chickens roost in a barn or let horses graze with geese and ducks. I have had one weanling die from Salmonella when geese invaded a winter wheat pasture to eat the grass. We do not let them clean up around horse feeders, especially where foals, weanlings or yearlings are kept.
E-coli is less pathogenic than Salmonella and coccidia are usually species specific, but again, it will be young horses and debilitated horses that are most susceptible.
I have not known of any horse getting sick from a water source, but I suppose it is possible. Horses that live in natural pastures and drink from ponds probably have a good immunity to everything. Horse that go from years in stall to being turned out where there are low and nasty ponds might be susceptible, but I have not seen it.
I have seen horses and cattle killed by botulism in hot summer months when there is a drought and the ponds get really nasty, hot and low. We had 50 horses drinking city water when our ponds started to go dry last summer. I had to move some of them because they kept wanting that nasty water and would wade knee-deep mud to drink the tiny bit of water that was left in a pond.
Last edited by Cherie; 03-19-2013 at 08:23 AM.