Salmonellosis in Horses

Three forms of salmonellosis have been recognized in adult horses:

First is the subclinical carrier, which may or may not be actively shedding the organism but has the potential to transmit the bacteria to susceptible animals either by direct contact or by contamination of the environment, water, or feed sources. Multiple fecal PCR assays and/or cultures may be necessary to identify carriers because the organism is shed in the feces intermittently and in small numbers. If stressed, the carrier may develop clinical disease. The national prevalence of fecal shedding of S enterica by healthy horses in the US is estimated to be < 2%;1 however, the proportion of hospitalized horses shedding is much higher (~13%).2 The most common serovars identified among the general population of horses were S enterica Muenchen and S enterica Newport (both serogroup C2).

The second form of the disease is characterized by a mild clinical course, with signs of depression, fever, anorexia, and soft but not watery feces. Affected horses may have an absolute neutropenia. Clinical disease may last 4–5 days and usually is self-limiting; S enterica can be isolated from the feces. Recovered horses may continue to excrete the organism in their feces for days to months; therefore, isolation of the shedding horse and thorough cleaning and subsequent disinfection of contaminated areas are recommended.

The third form of salmonellosis is characterized by an acute onset, severe clinical course, featuring depression, anorexia, profound neutropenia and, frequently, abdominal pain. Diarrhea develops 6–24 hours after the onset of fever; feces are fluid and foul smelling. Affected horses dehydrate rapidly, and metabolic acidosis and electrolyte losses occur as the horse deteriorates. Clinical signs of sepsis and hypovolemic shock can progress rapidly. There may be signs of abdominal discomfort, straining, or severe colic secondary to ileus, gas distension, colonic inflammation and possible infarction.

Protein-losing enteropathy can occur with plasma protein concentrations becoming dangerously low (albumin < 2 g/dL) after a few days of diarrhea. These horses can become bacteremic because of translocation of enteric organisms, and coagulation abnormalities resulting in disseminated intravascular coagulation can occur. If untreated, this form of salmonellosis is often fatal.

Salmonella bacteremia can occur in neonatal foals, especially from farms with endemic salmonellosis. Neonates, especially those with failure of transfer of passive immunity, are at high risk of bacteremia from ingestion of bacterial pathogens prior to gut wall closure. Salmonella bacteremia often with subsequent localization in bone physes and joints can occur.  Foals may present with lameness and no other clinical signs of sepsis. Extreme elevations in serum fibrinogen concentration and other indicators of inflammation such as low serum iron, elevated serum amyloid A, neutrophilia, and hypergammaglobulinemia in a lame foal are suggestive of osteomyelitis.

• Daily fecal samples

• PCR assay

Salmonellosis, colon, horse

Image

Courtesy of Dr. John Prescott.

Diagnosis of salmonellosis is based on clinical signs, severe neutropenia, and isolation of salmonellae from feces, blood, or tissues.

Submission of 10–30 g of feces for culture has been more successful in identifying salmonellae than has culturing fecal swabs. It is important to collect and submit feces based on the recommendations of the laboratory performing the culture. Working with a diagnostic laboratory that uses enrichment techniques along with agars specifically selected to optimize recovery of S enterica is advisable. Because salmonellae cannot be consistently cultured from feces, multiple daily samples (generally 3–5) should be collected from each horse. Culturing of rectal mucosal biopsies increases the probability of isolating the organism; however, the technique is not without risk to the horse.

Fecal samples that must be mailed should be placed in transport media suitable for enteric pathogens at the time of collection and shipped on ice. A PCR assay is available and, depending on the primers used, may be more sensitive than routine bacterial culture for detection of salmonellae;1 however, there may be variation between countries and hospital settings. Culture or PCR assay of environmental samples can determine if facility cleaning and disinfection has been adequate.

PCR assay is useful in situations of low prevalence of disease but can be too sensitive in high-prevalence areas, with a higher proportion of horses testing positive even in the absence of clinical disease. Two positive PCR results correlate well with positive fecal cultures for Salmonella. Because the Salmonella PCR assay does not provide antimicrobial sensitivity results or permit serotyping or speciation of isolates, which are important for surveillance purposes, fecal cultures are still recommended.

• Crystalloid fluids

• Colloids

• Antimicrobials

Treatment of the severe form of salmonellosis is based on IV fluid and electrolyte replacement and efforts to control the host’s responses initiated by the systemic inflammatory response. A polyionic isotonic fluid is used for volume replacement. Because of active secretion of fluid and electrolytes into the lumen of the intestine, IV fluid volumes of 40–80 L/day may be necessary. Electrolyte and acid-base deficiencies are common and are corrected with IV fluids supplemented with electrolytes. It is difficult to predict the electrolyte status of affected horses. Deficits should be determined by serum biochemical analysis; supplementation with potassium chloride, calcium gluconate, magnesium sulfate, and occasionally sodium bicarbonate may be indicated.

Equine plasma may be administered to correct hypoproteinemia and to supply coagulation factors and, depending on the source of the plasma, specific antibodies to endotoxin and Salmonella. Synthetic colloidal plasma substitutes such as hetastarch may also be used to maintain colloidal oncotic pressure in horses with substantial protein loss into the GI tract. These colloidal plasma substitutes may be less expensive than equine plasma but do not contain other natural beneficial components of plasma such as clotting factors and antithrombin III (ATIII). Often, equine plasma and colloidal plasma substitutes are simultaneously used in horses with hypoproteinemia due to colitis.

Antimicrobial treatment in adult horses with salmonellosis is controversial and does not appear to alter the course of the colitis or decrease shedding of salmonellae; however, it may decrease the likelihood of bacteremia. Selection of an antimicrobial is not easy and should ideally be based on the sensitivity of the organism isolated. Resistance patterns vary among Salmonella isolates and can change over the course of an outbreak.

There is potential nephrotoxicosis from aminoglycoside antimicrobials in volume-depleted horses; therefore, the hydration status of the horse should be considered when selecting an antimicrobial. The ideal antimicrobial should also be lipid soluble. Enrofloxacin may be useful to treat the salmonellae; however, a combination of broad spectrum antimicrobials such as penicillin, gentamicin, and metronidazole is often administered to neutropenic horses to provide some treatment of bacteremia induced by bacterial translocation across damaged intestinal epithelium.

NSAIDs, such as flunixin meglumine, meloxicam, and cyclooxygenase-2 (COX-2) selective firocoxib, help counteract the effect of endotoxin, control pain, and possibly prevent laminitis. NSAID dosage has been quite variable. Serious adverse effects, such as gastric and colonic ulceration and renal nephrotoxicosis, can result from NSAID treatment, so the minimum effective dosage should be used.

Administration of low-dose polymyxin B (6,000 units/kg, every 12 hours) has also been advocated to bind circulating endotoxin. In controlled trials, polymyxin B ameliorated some of the known effects of endotoxemia in horses. Antimicrobial doses of polymyxin B are substantially higher than the dose used to bind endotoxin and may be nephrotoxic. Low-dose polymyxin B treatment is unlikely to be nephrotoxic in adequately hydrated horses receiving IV fluids.

The use of GI protectants such as bismuth subsalicylate and activated charcoal have shown little benefit. Di-tri-octahedral smectite has been shown to bind clostridial toxins, and the yeast Saccharomyces boulardii has shown benefit in treating colitis in humans.

Prevention of salmonellosis is difficult, because the organism is present in the environment as well as in the feces of some healthy animals. In a hospital environment where horses are stressed, perhaps off feed, and often receiving antimicrobial treatment, aggressive identification and strict isolation of salmonellae-infected horses is indicated. Biosecurity practices to minimize cross-contamination between hospitalized horses are also advisable. Serotyping, antimicrobial susceptibility profiles, and genotyping by pulse field electrophoresis, plasmid profile analysis, and phage typing can be used to determine whether isolates are genetically related and help determine whether infection is nosocomial.

Owners should be made aware of the zoonotic risk of S enterica infection. Humans working with infected animals should adhere to strict biosecurity protocols.