Last Updated on March 29, 2022 by Allison Price
Salmonellosis (see Salmonellosis), is the most common cause of diarrhea in adult horses. The clinical manifestations can range from mild to severe diarrhea (subclinical carrier), and even death. It is not common but can become a serious epidemic depending on how virulent the organism is, the level of exposure and other host factors. The environment, water, and feed can all be contaminated. Infections can also occur from contact with bacteria-shedding animals. The pathogenesis of diarrhea is often preceded by stress, such as a history of surgery, transport, or change in feed. Concurrent disease, especially GI disorders (colic), or treatment with broad-spectrum antibiotic drugs are also common.
Salmonella Enterica The serogroup B also includes S enterica serovar Typhimurium, and S enterica Agona. These are two of the most common serovars isolated from horses with clinical diseases. The antibiogram and serovar can be used to track and monitor which serovar of Salmonellae is affecting any particular group or population of horses. This includes tracking nosocomial spread in a veterinary hospital. Multidrug-resistant S enterica is a concern for both nosocomial and zoonosis.
Salmonellosis has been identified in adult horses in three forms. The subclinical carrier is the one that may be actively shedding the organism, but can transmit it to other animals by direct contact, contamination of water and feed sources, or direct contact. Because the organism is shed in the stool intermittently and in small quantities, multiple fecal cultures may prove necessary in order to identify the carriers. The carrier could develop clinical disease if they are stressed. The US’s national prevalence of S Enterica fecal shed by normal horses is 2%. However, hospitalized horses shed a much greater (8%). The two most prevalent serovars among horses were S enterica Muenchen (both serogroups C2) and S enterica Newport.
The second type of the disease has a milder clinical course. It is characterised by signs such as depression, fever, anorexia and soft, but not watery, feces. The absolute neutropenia may occur in affected horses. The clinical disease can last for up to 4-5 days. It is usually self-limiting and S enterica can easily be isolated from the stool. Horses that have been treated for the disease may continue to expel the organism from their feces for several days or months. It is important to isolate the horse, clean the area thoroughly and disinfect it.
A third form of salmonellosis can be characterized by severe depression, anorexia and profound neutropenia. Diarrhea can develop between 6-24 hours after the onset and recurrence of fever. Feces will be fluidy and foul-smelling. Horses with severe dehydration can quickly become severely dehydrated. As the horse deteriorates, metabolic acidosis and electrolyte loss occur. The clinical signs of hypovolemic shock and sepsis can quickly progress. You may experience abdominal discomfort, straining, severe colic, or severe colic due to gas distension, ileus, colonic inflammation, and possible infarction. After a few days of diarrhea, protein-losing enterocolopathy may occur. Plasma protein concentrations can drop to dangerous levels (albumin 2g/dL). Horses can become bacteremic due to bacterial translocations of enteric bacteria. In these cases, coagulation abnormalities can result in disseminated intravascularcoagulation. Salmonellosis can be fatal if it is not treated.
Salmonella Bacteremia can occur in neonatal foals from farms with endemic Salmonellosis (see Bacterial Diarrhea In Foals).
Salmonellosis, equine colon
Diagnosis can be based on clinical signs and severe neutropenia. Salmonellae are isolated from tissues, blood or feces to determine the cause. It has been proven that submitting 10-30g of feces to culture is more effective in diagnosing salmonellae than culturing fecal samples. It is important that you collect and submit feces according to the recommendations of the laboratory performing culture. It is recommended to work with a laboratory that uses enrichment methods and agars specially selected to maximize recovery of S enterica. Salmonellae cannot always be cultured from feces. Therefore, it is important to collect multiple samples daily from each horse. The possibility of isolating salmonellae is increased by the collection of rectal mucosal biopsies. However, this technique can pose a risk to horses. Fecal samples should be sent by mail in appropriate transport media for enteric pathogens. A PCR test can be used to detect salmonellae. It is more sensitive than routine bacterial culture, depending on which primers were used.
The treatment of severe salmonellosis involves IV fluid and electrolyte exchange, as well as efforts to control host responses that are triggered by the systemic inflammation response. Volume replacement is done with a polyionic isotonic liquid. An IV fluid volume of 40 to 80 L/day may be required due to active secretion fluid and electrolytes into your intestine lumen. Acid-base and electrolyte deficiencies are common. They can be corrected with IV fluids and oral supplements. It is not possible to predict the horse’s electrolyte status. It is possible to determine the electrolyte status of horses affected by a serum biochemical analysis. Supplementation with sodium, potassium, magnesium sulfate and calcium gluconate might be recommended.
The use of antimicrobial treatment for salmonellosis in adult horses is controversial. It does not seem to alter the course or reduce the shedding of salmonellae. However, it can help reduce the risk of developing bacteremia. It is difficult to choose an antimicrobial and it should be determined based on the organism’s sensitivity. There are many resistance patterns to Salmonella strains, and they can change throughout an outbreak. Aminoglycoside antibiotics can cause nephrotoxicosis in horses with low volume. Therefore, it is important to consider the horse’s hydration when choosing an antimicrobial. Ideal antibiotics should also be lipid-soluble.
GI protectors (e.g. biosponge or bismuth subsalicylate, activated carbon) can be very beneficial. These substances can bind to bacterial toxins. Flunixin meglumine and other NSAIDs like flunixin meglumine can be used to counter the effects of endotoxin and control pain. They also help to prevent laminitis. There has been a wide range of NSAID dosages. NSAIDs can cause serious adverse effects such as colonic and gastric ulceration, and renal nephrotoxicosis. Therefore, it is important to use the lowest effective dose. To correct hypoproteinemia, supply coagulation factors, and specific antibodies to endotoxin or Salmonella may be given to Equine Plasma. Hetastarch and colloidal plasma substitutes may be required to maintain oncotic pressure in horses suffering from significant protein loss. Some horses may tolerate these colloidal plasma substitutes better than equine. Horses with colitis or hypoproteinemia may use both equine and colloidal substitutes.
To bind the circulating endotoxin, low-dose polymyxin (6,000 units/kg) has been recommended. Polymyxin B was shown to reduce some of the known side effects of endotoxemia on horses in controlled trials. The antimicrobial doses for polymyxinB are significantly higher than those used to bind the endotoxin, and could be nephrotoxic. In horses that are adequately hydrated and receiving IV fluids, low-dose polymyxin B treatment is unlikely to cause kidney damage.
Salmonellosis can be prevented, but it is hard to prevent. The organism is also present in the feces and environment of healthy animals. Salmonellae-infected horses should be identified and isolated in a hospital setting where horses may be stressed. It is also recommended to use biosecurity measures to reduce cross-contamination among horses in hospital. To determine if isolates are genetically related, as well as whether they are nosocomial, serotyping, antimicrobial susceptibility profils and genotyping with pulse field electrophoresis and phage typing, can be used.