Salmonella Infection in Horses

Introduction and Nomenclature Surrounding Salmonella and Horses

Salmonella is a large group of Gram-negative, rod-shaped bacilli, and a member of the Enterobacteriaceae family. The nomenclature of Salmonella is complex and continuously changing. For example, the serotype that commonly affects horses is Salmonella enterica subspecies Enterica serotype Javiana, which is generally abbreviated to Salmonella Javiana (USDA National Veterinary Services Laboratory). S. enterica ser. Typhimurium is the second most common type to affect horses, but probably the more recognized name. There are many other serotypes that cause intestinal disease in horses.

An interesting historical detail: Salmonella was named after an American veterinarian, Daniel E. Salmon, who in 1884 cultured the bacteria from pigs with hog cholera.

Epidemiology of Salmonella in Horses

Salmonella is a widespread organism that is commensal to the intestinal tract of many animals. Horses, however, do not harbor any host-adapted enteric strains of Salmonella, and are therefore susceptible to most serotypes of Salmonella (Lester 2003).

A variety of horse populations have been studied, both healthy and diseased, to establish the likelihood of fecal shedding of Salmonella. The occurrence is usually low, around 1% of the population, but can sometimes be up to 8%. One study found that Warmbloods and Arabs with colic symptoms had increased odds for Salmonella shedding (Dallap Schaer, B.L. et al. 2012).

Horses can acquire Salmonella in many settings. Salmonella can be accessed anywhere birds, and particularly poultry, are present– on pastures, in feeding bins, water troughs or stagnating creeks. Rodents, cattle, wildlife and many other animals can be carriers of Salmonella as well.

The virulence of the Salmonella strain and the dose ingested by the horse will influence the severity of disease, along with the horse’s immune status, systemic health and nutritional status.

Clinical cases of Salmonella colitis in horses tend to occur more frequently in the warmer months of the year.

Hospitalized horses are at risk of infection with Salmonella. Predisposing factors include dietary changes, any form of stress, gastrointestinal disease, anesthesia, surgery, and antimicrobial use.

Pathophysiology of Salmonella in Horses

The mechanism for Salmonella-induced intestinal disease is still being researched. What is known is that some serotypes of Salmonella have virulence factors that have a variety of effects on the intestinal mucosa, including assisting the bacteria to invade the intestinal mucosa.

The inflammatory response of the host to this invasion plays an important role in triggering hypersecretion by the enterocytes, which leads to diarrhea.

If the bacterial invasion and subsequent inflammation are severe enough, the mucosa becomes permeable to toxins, which, if they bypass or overwhelm the capacity of the liver, can induce a systemic response that results in endotoxemia and potentially systemic inflammatory response syndrome (SIRS).

Proteins, particularly albumin, will leak from the inflamed and damaged mucosa. This leads to a decrease in the colloid oncotic pressure, and subsequent tissue edema, including intestinal edema, which further exacerbates the intestinal dysfunction. Villous blunting and loss of absorptive surfaces can be severe and result in malabsorption.

Necrotizing enterocolitis is the terminal outcome of severe disease.

Clinical Signs of Salmonella Colitis in Horses

Clinical signs may be restricted to a mild self-limiting diarrhea, often with a fetid odor. In more severe acute cases, colic is the first clinical sign, which generally subsides as the diarrhea develops. Hematochezia is a rare, but highly concerning sign (Lester 2003).

Intestinal sounds are often fluidy and overactive, but can vary depending of the type and severity of the condition.

In acute cases of colitis, the horse may develop endotoxemia with a fever, elevated heart and respiratory rates, and congestion of the mucous membranes.

If the disease progresses, or is acutely severe, further complications can arise such as hypercoagulation (causing venous thrombosis), laminitis, and edema of the distal limbs and other ventrally dependent areas of anatomy.

Necrotizing enterocolitis is the most severe form of the disease and presents with an acute, brief clinical course with profound metabolic abnormalities and terminal severe abdominal pain that is refractory to analgesics. One study found the median time to death in these cases was 45.5 hours (Saville, W. et al. 1996).

Salmonella infections can be particularly severe in foals, as along with the diarrhea they can develop septicemia, osteomyelitis, synovitis, arterial thrombosis, laminitis, meningitis, abscesses, pyelonephritis, pneumonia and ocular infections.

Salmonella can also cause other forms of intestinal disease such as impaction of the small colon and cecum, as well as diffuse or localized small intestinal inflammation. If colic pain is persistent and diarrhea is not evident, then the horse should be checked for gastric reflux by nasogastric intubation, and for an impaction with a rectal exam. The rectal exam can irritate an already inflamed small colon and rectal mucosa, so the exam should be as gentle as possible, and the rectum infused with lidocaine prior, to minimize inflammation and straining.

Hematology and Biochemistry

A neutropenia with a shift to immature neutrophils and toxic changes is common in the acute phase. As the disease progresses, this may change to a mature neutrophilia with elevated fibrinogen.

Electrolyte derangements will likely occur and include hyponatremia, hypochloremia and decreased bicarbonate.

If the horse is dehydrated or has circulatory problems due to endotoxemia, the lactate will be elevated, as will the blood urea nitrogen (BUN) and creatinine. These derangements should resolve within 24-48 hours with appropriate fluid therapy, unless there is a secondary problem.

Hypoproteinemia can be profound, and progressively worsen. The severity can be masked if the horse is dehydrated.

Liver enzymes may show mild elevations secondary to endotoxemia or ascending infections.

Diagnosing Salmonella Colitis

The gold standard for Salmonella testing is aerobic culture of five fecal samples collected at 24 hours intervals. Early diarrhea tends to be watery and dilute, so the likelihood of positive culture will improve as the fecal consistency becomes more solid; however, this often means a significant delay before diagnosis is confirmed.

More rapid methods of testing include:

1. Lateral Flow Immunoassays (LFI), such as Reveal® for Salmonella Test System, and RapidChek® SELECTTM for Salmonella. These tests offer the advantage of rapid point-of-care testing, and have a low occurrence of false positives, however the sensitivity is not high and repeat testing may be required. Also, the accuracy of the test varies with the strain and/or serotype. The test takes at least 18 hours to complete, as an enrichment period is required prior to running the test.
2. Real-time PCR (qPCR) and DNA hybridization assays (DNAH). These tests can be highly sensitive, however they are more likely to provide a false positive as they can detect DNA of non-viable bacteria. Many diagnostic laboratories are now running PCR diagnostic profiles for diarrhea that include Salmonella.