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Salmonellosis in Horses

By: Dr. Philip Johnson

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Clinical disease attributable to Salmonellosis occurs almost exclusively when either the immune system of the horse is compromised or when the normal flora has been disturbed, often by the use of antibiotics. Antibiotics are used to kill bacteria in the treatment of many diseases. However, the injudicious use of antibiotics in the equine species carries the risk of letting certain bacteria, for example Salmonella, overgrow and gain attachments to (and infect) the bowel wall, causing diarrhea.

Some antibiotics (lincomycin, clindamycin) are simply never used in horses because they carry an exceptionally high risk for causing serious shifts in intestinal microflora, precipitating severe diarrhea. Antibiotics should never be used without thoughtful and proper justification in horses.

Any and all diet changes should be undertaken very gradually to allow time for floral adaptation to a new diet and thereby reduce the risk for Salmonellosis.

Well recognized stressful factors that have been implicated in the suppression of the immune system leading to increased risk for Salmonellosis include the following:

  • General anesthesia and surgery
  • Severe pain
  • Transport
  • Long-term medication
  • Other diseases
  • Mixing with other horses (especially with other dominant horses)
  • Heavy parasitism
  • Abrupt diet change
  • Fasting
  • Hospitalization in a veterinary facility
  • Inclement weather (very hot or very cold temperatures)
  • Overcrowding
  • Deworming
  • Weaning

    The reason some of these factors may increase risk for Salmonella is unknown. It is thought that anything that causes a big shift in the motility, immunity, the breakage or irritation of the wall, a change in the surface of the intestine, and hormonal factors that influence the normal defenses of the bowel wall may play a role.

    Other Causes of Diarrhea

    It is often not possible to define the cause of adult horse diarrhea, even after completion of a battery of diagnostic tests. If the cause of diarrhea cannot be determined, the diarrhea is referred to as "undifferentiated" diarrhea. In light of the fact that it is often difficult or impossible to rule out Salmonellosis with a high level of confidence, horses with "undifferentiated" diarrhea are also treated (and isolated) in a manner similar to those known to be affected with Salmonellosis.

    Other important causes of diarrhea in adult horses include:

  • Infection of the intestinal tract by other bacterial pathogens (notably, Clostridium difficile and Clostridium perfringens)
  • Endoparasitism
  • Rapid changes in the diet (leading to perturbation of the colonic flora)
  • Adverse effect of antibiotic administration
  • Potomac Horse Fever
  • Presence of sand or gravel (geosediment) in the intestine
  • Peritonitis
  • Liver disease
  • Heart failure
  • Drug toxicity
  • Cancer

    Salmonella Outbreaks

    Some species of Salmonella may be substantially more dangerous, in terms of potential disease, than others. Many horse hospital facilities have had to close down their operations for a period in order to depopulate and clean the hospital accommodation for horses. Hospital facilities are at particular risk because "sick" horses that are being treated with antibiotics are much more likely to be shedding large numbers of Salmonella bacteria than healthy horses at pasture. Consequently, "healthy" horses that are admitted to a contaminated clinic for a routine procedure (such as castration) may be infected and develop severe (life threatening) diarrhea during their stay at the facility. Horse owners should inquire about the history of Salmonellosis in a hospital barn prior to taking their horses there.

    Similar Salmonella outbreaks occur on big farms where these are a lot of animals, particularly broodmare, since they shed more Salmonella toward the end of pregnancy and shortly after. Since horses are always mixing on these farms, outbreaks can occur because horses without immunity ("naïve horses") are exposed to some really hot bugs. Some horses are left as carriers in the end, allowing the cycle to begin another year when the conditions are right.

    Whenever a horse develops diarrhea, a veterinarian is likely to consider that Salmonellosis is a possible cause. Failure to recognize Salmonellosis may lead to the unnecessary exposure of other horses in the same environment, and one of the management priorities for horses affected with diarrhea is isolation form other horses. If the diarrhea is actually a result of Salmonellosis, the affected horse sheds a substantial number of Salmonella bacteria in to the environment and this contamination represents a big risk for any other horses that are being treated with antibiotics or are being stressed (reduced immunity) in any way.

    In the earliest stages of Salmonellosis, actual diarrhea may not yet be evident. The earliest signs of Salmonellosis are nonspecific and often include fever, lethargy, depression and inappetance. If horses presenting with these nonspecific symptoms are treated with antibiotics, the clinical course of Salmonellosis may be very much more severe. Therefore, the use of antibiotics to treat horses with simple signs of fever, lethargy, depression and inappetance is not recommended before a diagnosis has been established.

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