Salmonella Basics: What You Should Know

Clinically normal horses can shed Salmonella bacteria, especially when stressed. Reports on the percentage of horses shedding Salmonella have ranged from 1.5% to 64.5%, depending on whether the horse was hospitalized (higher numbers). Since stress can cause shedding, transport to the hospital--on top of the ailment requiring hospitalization--often means a high Salmonella shedding rate in clinical settings.

In one session during the 2003 American Association of Equine Practitioners convention, it was noted that in the clinic, a high heat index--the combination of high environmental temperature and high humidity--has been associated with an increase in equine cases of salmonellosis. There is the possibility for "blooms" of Salmonella under these conditions, and it's also possible that heat stress might make horses more prone to Salmonella infections.

What Happens?

Within a few hours of infection by Salmonella, a horse can become critically ill. In foals, Salmonella can cause enteritis (gastrointestinal or GI disease), but it also can result in severe septicemia (systemic infection), organ failure, and death. In adult horses, Salmonella can cause signs from mild fever and malaise to severe enterocolitis accompanied by profuse diarrhea, endotoxemia, and eventual death.

Salmonellae belong to a group of bacteria classified as Enterobacteriaceae that includes many of the bacteria that inhabit the gastrointestinal tract of horses. Some of these bacteria are pathogenic; that is, they cause disease when present in sufficient numbers.

Salmonella bacteria are classified into species and serovars based upon the antigenetic makeup of their flagella (that propel them through fluid) and sugars contained in their outer cell walls. There is tremendous variation in the chemical composition of these sugars. This is important since an antibody directed against one Salmonella serovar will not cross-react with another Salmonella serovar, which is why production of a vaccine against Salmonella bacteria can only be effective against a specific serovar.

Some serovars of Salmonella are more virulent than others. Horses acquire Salmonella infection through ingestion of contaminated material (feed, forage, bedding, etc). The bacteria first attach to the cells lining the GI tract, then invade the tissues. Once they invade, the bacteria can rapidly multiply and release a multitude of toxins. Within hours of invading the GI lining, Salmonella bacteria can infect local lymph nodes of the intestinal tract and spread to other organs via the bloodstream. Thus, within just a few hours of infection, the horse can become critically ill.

How Long Can They Shed?

People infected with some types of Salmonella can be life-long carriers and shedders of the bacteria (thus the phrase "Typhoid Mary").

In many animal species, there are "host-adapted" Salmonellae species that can persistently infect the host. No such host-adapted Salmonellae have been identified in horses, and in most cases, horses will shed Salmonella bacteria for days or weeks, not months or years. However, more sensitive testing techniques might reveal that some horses can shed for longer periods of time.


Animals coming onto a farm should be isolated for a minimum of two weeks to prevent introduction of infections. Routine, rigorous disinfection of stalls with chemicals known to be effective against Salmonella in the presence of organic matter is essential, both in hospitals and on farms.

Since no commercially available vaccine exists against Salmonella, disinfection and biosecurity are the primary preventive measures.

Horse owners need to be aware of the zoonotic potential (horse to human transmission) of any Salmonella-positive horse and take proper precautions (isolation techniques, use of protective clothing, washing hands, etc.).

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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