By Beaudoin, DVM & S. Valberg, DVM,
Courtesy of University of Minnesota Extension
Salmonellosis is a bacterial infection that affects humans, horses, most mammals, birds and reptiles. Though there are multiple types of the Salmonella bacteria, and while all can cause disease in horses, most equine infections are caused by S. Typhimurium. In adult horses, Salmonellosis is usually a gastrointestinal disease and may cause severe diarrhea. In some cases, usually foals, it can cause systemic disease (septicemia) as a result of movement of the bacteria throughout the body. Salmonellosis can cause abortion, though this is rare in horses.
Salmonella is transmitted via oral ingestion of contaminated fecal material. The bacteria are shed in the manure of infected horses, and then ingested by other horses as a result of environmental contamination. A horse can become infected with Salmonella by ingesting grass, hay, and other feeds that are contaminated with the bacteria. In addition, things such as boots, water buckets, tack, shared grooming tools, and unwashed hands can carry the bacteria from an infected horse to a susceptible one.
Most adult horses that are infected with Salmonella develop a fever and severe watery diarrhea. There is often a foul smell associated with the diarrhea, and it can sometimes be bloody. The horse is typically weak and lethargic and may stop eating. Foals that are septicemic with a Salmonella infection are usually dull and depressed and have a high fever. Such foals may also display signs of lameness and joint swelling.
Many horses may have Salmonella in their systems but do not shed it, and most of the time they are not ill. A United States Department of Ag study documented that 1% of US horses were found to be shedding Salmonella on farm. A study at a large veterinary hospital in the United States found that 13% of horses admitted for colic were shedding Salmonella. Other studies have showed lower prevalence (3- 9%), especially among horses admitted for reasons other than colic. At the Univ of Minn College of Vet Med, 1 to 5 horses are documented annually with Salmonella infections.
An infected horse sheds more bacteria when it is showing clinical signs of diarrhea. A horse is more likely to develop illness if it ingests a large amount of Salmonella organisms at once, or if the horse is already sick or stressed. There are some horses which shed Salmonella without showing the typical signs of infection. This usually occurs when the animal has a small amount of Salmonella present within its intestines and is then exposed to a stressful situation, such as trailering, movement to a new facility, illness, or surgery. Shedding decreases over time and many horses will cease shedding altogether.
People can, and do, become infected with Salmonella from horses. Salmonellosis is a zoonotic disease, meaning that it can be transmitted between animals and people. If a horse is shedding Salmonella, it can cause illness in people by the same oral ingestion of contaminated feces. Signs of Salmonella infection in people can include diarrhea, abdominal cramps, and fever. It is important to always use good hygiene practices, including hand-washing before eating and not touching objects with manure contaminated hands.
Shedding of Salmonella organisms can be stimulated by stress or illness. Hence, horses seen at veterinary hospitals are more likely to shed Salmonella than are horses seen in their own barns.
Due to this fact and the risk of people becoming infected with Salmonella from an infected horse, equine hospitals have personal and equine patient hygiene steps in place to help prevent disease transmission.