Enteric Salmonellosis is a bacterial overgrowth and infection of the gastrointestinal tract by one or more of the hundreds of species of Salmonella. The bacteria are capable of infecting most mammals including humans and horses, as well as birds and reptiles. Most of the time, horses are exposed by a fecal to oral transmission route. Salmonella-positive feces may contaminate grazing pastures and prepared feeds. The disease may also be contracted from water, especially when an infected bird, rodent, or reptile has drown and decomposed in a horse’s water supply.
Salmonellosis is probably the number one cause of infectious diarrhea in horses. Exposure to the bacteria does not always cause disease, however; the factors that influence the development and severity of symptoms include the bacterial strain of Salmonella, the quantity of the organisms ingested, the immune status of the animal, stress (extreme exercise, overcrowding, diet change, surgery), and antibiotic use (modification of the normal bacterial flora in the gut).
Salmonella causes intestinal disease resulting in diarrhea by invading the lining of the GI tract, releasing enterotoxins which cause the leakage of fluids, protein, and electrolytes through the gut-wall into the stool, and immune-mediated inflammation. Diarrhea is often severe and is accompanied by abdominal discomfort. Overt infection by Salmonella will cause acute symptoms, but a bacterial overgrowth may be responsible for intermittent or chronic diarrhea as well. Systemic toxemia, as seen in advanced cases, can lead to bleeding disorders and cardiovascular shock, represented by petechiae (bruises) and hyperemic (dark red or purple) mucous membranes. Sub-acute cases of Salmonella may cause mild to moderate colic with or without diarrhea.
Horses with Salmonella infections will usually become dehydrated (even without outward signs of fluid loss), have a rapid heart rate, run a fever, and exhibit abdominal distension (bloating). The animal may also experience gastric reflux, the uncontrollable backflow of stomach contents into the esophagus. Serious secondary concerns attributable to Salmonellosis include laminitis, hypoproteinemia, liver and kidney dysfunction, disseminated intravascular coagulation (DIC), and pneumonia.
Salmonella should be confirmed in suspected horses by positive fecal cultures or DNA (PCR) testing for the bacteria. Pending test results, the horse will be treated supportively with IV fluid and electrolyte replacement, plasma or colloid therapy in the case of shock or hypoproteinemia, and possibly parenteral nutrition supplementation. The use of antibiotics in presumed cases of Salmonella (lacking positive test results and verification of antibiotic susceptibility) is not recommended except in the case of immuno-compromised horses. There are many strains of antibiotic-resistant Salmonella, and the indiscriminant use of antibiotics is thought to contribute to the development of these hard-to-kill bacteria. This is a definite concern in human cases of Salmonella infection.
Aggressively treated horses should show good response to supportive therapies within a week. Horses that continue to have severe diarrhea beyond 10 days may not survive the disease.
Preventing Salmonella in unexposed herds is straightforward. General cleanliness is essential. Keeping stalls free of feces, changing bedding materials frequently, and regular disinfection of feed buckets, water troughs, and other equipment will prevent exposure to potential sources of Salmonella. Overcrowding causes stress, unsanitary conditions, and ideal scenarios for the spread of disease, and should always be avoided. Handlers may transport Salmonella from a contaminated area to a susceptible animal on hands, boots, and clothing; they can even become infected themselves. Good hygiene practices and regular hand washing in disinfectant soap is indispensable.
Horses with diarrhea should be segregated from the herd as soon as possible and while testing for Salmonella is pending. All exposed tack, stalls, buckets, water supplies, and equipment must be thoroughly de-soiled, washed, disinfected, and dried before reuse. Exposed feeds, which may have been the original source of the infection, should be disposed of properly or incinerated – never shared with healthy horses.
An infected horse should continue to be isolated from healthy animals for at least 2 weeks following remission. Some veterinarians delay turnout until the horse is no longer Salmonella-positive. A negative PCR test is reliable, but cultures should be repeated and confirmed to be negative five consecutive times.
Salmonella is zoonotic (can be transmitted to humans and cause illness). Sanitary precautions should be taken to prevent fecal oral exposure to humans also.
Equine diarrhea PCR panels are available to ensure that new horses are free from several causes of diarrhea before the introduction into the healthy herd.