The hospital was closed for about six weeks. Similar problems have occurred with other strains of the bacteria. Many horses were affected by Salmonella anatum at an Australian university veterinary hospital. Not all infected horses get sick, or even experience diarrhea. Some horses become carriers of the organism, and shed it into the environment, infecting other horses.
Some carriers have a natural immunity to the disease.
Others are resistant to infection most of the time, but can develop salmonellosis when their resistance gets down. Nearly all horses have a few Salmonella in their gut, along with many other types of bacteria, which collectively are known as intestinal flora. Under normal circumstances these intestinal bacteria do the major work of digestion of the feedstuff so that nutrients can be absorbed. So there are good bacteria in the gut and also bad bacteria. Normally, the good bacteria keep the bad bacteria in check.