Diagnosing Salmonellosis in Horses
Salmonellosis is an important problem in horses. Infection with Salmonella can cause disease ranging from fever and/or mild diarrhea, to severe diarrhea (see picture right), bloodstream infection and death. Horses can also carry Salmonella in their intestinal tract without any signs of illness at all! Testing of horses with diarrhea for Salmonella is commonly performed, but diagnosing Salmonella infection is not as straighforward as you may think:
- The primary means of diagnosing salmonellosis is to culture the bacterium from the animal's feces, but fecal culture is far from a perfect test. Isolating Salmonella from all the other bacteria that are present in the manure can sometimes be difficult, and often takes at least 48-72 hours. Multiple cultures may need to be performed over several days, because sometimes horses with Salmonella only shed the bacterium intermittently in their feces. It is generally recommended that a horse have five negative fecal cultures in a row before it can be safely called "Salmonella-negative". If a horse has diarrhea caused by Salmonella, it's uncommon to need five samples before a positive culture is obtained, but it can happen.
- Culture of a rectal mucosal biospy is more likely to detect of Salmonella than fecal culture, but this procedure is rarely performed. Rectal mucosal biopsy involves removing a small piece of tissue from the lining of the rectum using a pair of biopsy foreceps. Culture of such a biopsy may be more useful for identifying horses that are carrying Salmonella but have no clinical signs of illness. Serial fecal cultures are usually adequate for finding Salmonella in horses that have diarrhea.
- Molecular tests (such as PCR) for Salmonella are available from some laboraties. These tests tend to be faster and may be more sensitive, but there are also numerous potential problems with them, as discussed in the equIDblog post entitled "Molecular Diagnostic Testing: Pros and Cons".
- Clinical signs are not adequate for diagnosing salmonellosis. Other microorganisms (e.g. Clostridium difficile) can cause disease that appears exactly the same.
Despite these challenges, trying to diagnose salmonellosis is important. Salmonella can cause serious outbreaks of disease - it's important to know with what you are dealing, especially since Salmonella can also be spread to people and other animal species. Additionally, isolating Salmonella allows the bacterium to be tested for its susceptibility to antibiotics. Antibiotics are rarely indicated for the treatment of salmonellosis, but when they are needed, it is much better to know which ones are more likely to work.
