Salmonellosis is an infection caused by Salmonella bacteria, which is one of the most common infectious causes of diarrhea in adult horses. Salmonella are gram-negative, rod-shaped bacteria which contain two species, S. enterica and S. bongori. The majority of cases are associated with a single subspecies, S. enterica. The severity of diarrhea can range from soft but not watery feces to foul smelling fluid.

Transmission
Horses can become infected by consuming contaminated water or feed and oral contact with the feces of the infected animals, contaminated environmental surfaces, or from contaminated hands or equipment of barn personnel or riders.
Horses can also be carriers of salmonella and intermittently shed the organism in their feces; these horses sometimes show no clinical signs until stressed.