Introduction: Salmonellae are facultative anaerobic, non-spore forming, gram-negative rods of the family Entero-bacteriaceae. There are more than 2200 serovars of Salmonellae and all are considered possible pathogens for adult horses. The most commonly isolated serovars are Salmonella typhimurium, S. agona, S. anatum, S. krefeld, S. newport, and S. saint-paul. There are no host-adaped serovars for horses.
Epidemiology: The transmission of Salmonella occurs most often by the fecal-oral route, although infection may also take place through the mucous membranes of the eyes and the nose via aerosol droplets. The most common sources of infection are contaminated feed and water and carrier birds, rodents, horses and other farm animal species that excrete the bacteria. Reported prevalence of infection with Salmonella in horses has been variable, ranging from less than 1% to 70%. Variability in prevalence can be explained by differences in methods of detection and populations studied. For example, prevalence will be higher among horses with diarrhea than among apparently healthy horses, and the prevalence will be lower when microbiological culture of feces is used to detect infected horses than when more sensitive methods, such as polymerase chain reaction (PCR) test or culture of intestinal or paraintestinal organs, are used. Risk factors that might enhance the fecal excretion of Salmonella organisms and occasionally precipitate overt clinical disease in carrier animals include transportation, crowding, abrupt change in diet, intensive physical activity, antimicrobial treatment, surgery and gastrointestinal tract disorders. The three most important factors that influence whether a horse becomes ill after exposure to Salmonella are the infective dose of the bacteria, the inherent virulence of the bacteria, and the inherent susceptibility of the host. The infective dose is determined by the amount of bacteria shed in the feces and the environmental conditions that favor (or hinder) the proliferation of the bacteria. Salmonellae possess an array of virulence factors that confer attributes of mucosal adhesion and invasion; secretion of electrolytes and water into the intestinal lumen (enterotoxin-mediated effect), activation of focal immune responses including the recruitment of inflammatory cells and release of their mediators, local cytotoxic effects; and systemic responses (attributable to an endotoxin). The host susceptibility to Salmonella infection is influenced by the composition of the normal intestinal microflora and the competence of local and systemic immune surveillance. Factors that can negatively affect host susceptibility include antimicrobial administration, crowding, or intensive physical activity.
Salmonellae can persist in the environment for protracted periods and have been recovered from contaminated soil after more than 300 days and from water after 9 months. The organisms are very resistant and very adaptable to environmental conditions, surviving for protracted periods in feed. Salmonellae can be killed by desiccation and exposure to sunlight, but can survive in dried fecal matter for as long as 30 months. Freezing will not kill the bacteria, particularly if they are in food or other organic matter. For instance, Salmonella has been isolated from infected ice cream after more than two years.
Fecal shedding of the bacteria: Assuming that the Salmonella cultured from the feces of a horse with diarrhea was the actual cause of the disease, fecal shedding of the bacteria may persist for days to weeks in the animal. Some horses remain consistently positive on fecal culture while they shed the bacteria, but a negative culture does not rule out intermittent or lower level of fecal shedding of Salmonella. In the latter situations, a PCR test for Salmonella may detect persistent fecal shedding of the organism because this test is more sensitive than is the bacteriologic culture. In most cases, the amount of bacteria shed in the manure of convalescing horses will be relatively small and will not pose a serious threat to other animals. However, if the serovar of Salmonella is especially virulent or is able to survive and proliferate in the environment, persistent fecal shedding of the bacteria could pose a risk to other animals or humans.
The surrounding environment is at greatest risk for Salmonella contamination from a diarrheic horse because bacteria will spread over a relatively large area and will be able to "hide" in cracks or stalls, on the surface of water buckets or automatic watering devices, and in bedding materials that may be spread by wind or careless foot traffic. If the horse's manure is formed, however, its removal will facilitate the elimination of pathogenic bacteria from the environment. In such cases, with judicious isolation measures, exposure to other animals and humans to Salmonella infection is reduced.
Clinical findings: Enteric salmonellosis is characterized by an acute colitis that results in profuse diarrhea and, occasionally, abdominal pain. Horses with enteric salmonellosis often have signs compatible with endotoxemia, and suffer from cardio-vascular shock and coagulopathies. Horses are usually febrile, tachycardic, moderately to severely obtunded, and dehydrated. Other clinical signs include fever and leukopenia, colic and proximal enteritis with gastric reflux, dark red or purple mucous membrane, abdominal pain, and abdominal distention. Diarrhea is usually profuse and watery, the feces being often malodorous and variable in color from green to black. Based on clinical and experimental data, it has been suggested that infection with Salmonella produces one of the following eight clinical syndromes in adult horses: peracute diarrhea, acute diarrhea, chronic diarrhea, mild colic without diarrhea, moderate colic with subsequent diarrhea, severe colic with subsequent diarrhea, colic with persistent gastric reflux, and asymptomatic infection. Categorizing the disease into one of these syndromes is not vital, but it is important to recognize the salmonellosis in horses can develop without diarrhea. Complications in animals with enteric salmonellosis include laminitis, bacteremia and septicemia, renal failure, thrombophlebitis, disseminated intra-vascular coagulation, hepatitis, and fungal pneumonia secondary to compromise of the intestinal barrier and the immune system.
Diagnosis: Bacteriologic culture of the bacteria is the only way to make a definitive etiological diagnosis of salmonellosis and of determining the serovar. Multiple fecal cultures for Salmonella should be performed on all horses with diarrhea. It is recommended that at least 3-5 fecal samples be submitted for culture to enhance the chances of isolating Salmonella. Formed fecal samples are more likely to result in a positive culture from infected horses. A 5-10 gram fecal sample should be submitted for culture. Recently, detection of Salmonella using PCR (a highly sensitive and specific test for the detection of Salmonella in fecal samples from horses) has been described as being more rapid and more sensitive than microbiologic culture. Microbiological culture of a punch biopsy of the rectal mucosa or, if the animal does not survive, of the wall of the cecum, large colon and ileum, mesenteric lymph nodes, and spleen may increase the numbers of isolations of Salmonella when compared to the feces.
Differential diagnoses: In adult animals suffering from acute clinical salmonellosis with diarrhea, colitis X, Clostridium perfringens type A and Potomac horse fever (caused by Ehrlichia risticii) should be considered as differential diagnoses. Clinical signs and lesions of colitis X are very similar to those of enteric salmonellosis, and the distinction between the two conditions is dependent on culturing the organism from the content or the wall of the intestinal tract. A history of recent transportation is also often helpful in suggesting the diagnosis of salmonellosis in adult horses. Chronic diarrhea due to salmonellosis in horses may resemble parasitism, granulomatous enteritis, or alimentary lymphosarcoma.
Treatment: In most cases of enteric salmonellosis, aggressive treatment facilitates resolution of the severe diarrhea and associated metabolic disorders within 7-10 days of the onset of illness. Intravenous administration of polyionic fluids is required to replace fluid and electrolyte losses and to augment preload in horses with poor venous return. Plasma may help to alleviate the hypoproteinemia, which develops as a consequence of the enteropathy. Parenteral nutritional support is indicated to provide adequate calories and amino acids during the most debilitating periods of the disease. The use of antimicrobial drugs for adult horses with salmonellosis remains controversial. The use of antimicrobials such as chloramphenicol, trimethoprim-sulfona-mide, gentamicin, and cephalosporin has not appeared to accelerate resolution of signs of colitis. Ceftiofur can be used successfully (2-4 mg/kg IV q12h). Blood samples should be monitored for electrolyte and acid-base status, and the fluid therapy plan should be adjusted according to any changes detected. Horses that have diarrhea for more than 10 days are unlikely to survive since they often have extensive loss of colonic mucosa and chronic severe inflammation within the wall of the colon.
Control and Prevention: Management practices should lessen exposure of horses to possible sources of infection. Measures do not have to be laborious and expensive. Removing potentially contaminated fecal material is the most important measure. Other important practices include thorough cleaning of areas where fecal contamination is likely as well as the prevention of mechanical distribution of contaminated material. Cleaning must include the removal of organic debris which can be accomplished with several products specifically designed for that task. Areas that require particular attention are stalls (including water buckets or automatic watering systems), drains, cracks in floors and walls, stall implements, nasogastric tubes and stomach pumps. Extensive use of disinfectants may not be necessary if cleaning measures are adequate. People handling sick horses should wash their hands thoroughly for at least 30 seconds with a disinfectant soap and take precautions to avoid wearing contaminated clothing from stall to stall. Immuno-suppressed people should not provide care to horses with salmonellosis. Personnel entering the stall should wear disposable plastic boots. Footbaths that contain disinfectants are probably not effective since they quickly accumulate organic material that negates the disinfecting potential of the footbath. A Salmonella vaccine is available, but its effectiveness has not yet been demonstrated in clinical cases. Because immunity to Salmonella is complex, effective and practical immunotherapy may be difficult to attain. Good management practices to minimize stressful situation that may precipitate the disease, such as changes in diet and overcrowding, may be the most effective methods of prevention.
Isolation of affected animals: An affected horse should be isolated from other animals for 10-14 days after returning from the veterinary clinic or hospital. This typically includes stall confinement which is often indicated for a horse convalescing from colitis. When turnout is appropriate, the horse should be confined to an isolated paddock. Some people will elect to continue to isolate a horse as long as it remains positive for Salmonella on fecal culture. An animal which has had enteric salmonellosis should not be returned to contact with other horses until five consecutive fecal samples have proved negative for the organism. Among horses that have recovered from salmonellosis, approximately 2/3 will have ceased shedding after 1 month and approximately 90% will have ceased shedding after 4 months.
- by Briardo Reich, EVFVG student
- edited by Dr. Ingeborg Langohr
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