Headline stories about human sickness and even deaths related to food poisoning caused by Salmonella bacteria are quite common.
Thus, many people know the word Salmonella, but only a few realize that it can be a severe problem in horses, too.
“Salmonellosis is the most commonly diagnosed infectious cause of diarrhea in adult horses,”according to Dr. Keith Taraba at Taraba Equine Sports Medicine, Inc., Los Angeles, Calif.
It is an infection of the intestinal tract by a bacterial pathogen called Salmonella, named after pathologist Daniel Elmer Salmon (1850-1914), who recognized Salmonella as a major cause of diarrhea in livestock. There are over 100 different strains of Salmonella. Some cause diarrhea, but most species are just bystanders in the gut.
“Remarkably, horses normally carry at least one species of Salmonella in their intestinal system, and some horses carry several,” Taraba explained. “As normal residents of the intestine, they do no harm; rather they contribute to digestion.
“Clinical disease attributable to Salmonellosis occurs almost exclusively when either theimmune system of the horse is compromised or when the normal flora has been disturbed, often by the use of antibiotics,” Taraba pointed out.“The injudicious use of antibiotics in the equine species carries the risk of letting Salmonella overgrow.
“Some antibiotics such as lincomycin and clindamycin are simply never used in horses because they carry an exceptionally high risk for causing serious shifts in
intestinal microflora, precipitating severe diarrhea,” Taraba continued.
“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,”Taraba emphasized.
Stressful factors leading to increased risk for Salmonellosis include: general anesthesia and surgery, severe pain, transport, long-term medication, other diseases, mixing with other horses, heavy parasitism, abrupt diet change, fasting, hospitalization in a veterinary facility, inclement weather, overcrowding, deworming and weaning.
“The reason some of these factors may increase risk for Salmonella is unknown,” Taraba admitted.“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 plays a role.”
In the mild form, and in the early stages of infection, there may be little or no diarrhea, but fever is prominent in most cases and there may be some mild colic signs. “Fever and mild colic should suggest that Salmonella might be brewing. This is
the stage when Salmonella is often wrongly disregarded,” Taraba commented.
In more severe cases, the diarrhea is projectile, malodorous, brown, profuse and watery. Lying down and getting up repetitively is common when the diarrhea starts.
In the most severe cases, there is bloat, colic, flatulence and bloody diarrhea.
Diarrhea rapidly leads to dehydration of the horse. This is the biggest concern initially, because the horse cannot keep up by drinking. Diarrhea washes away water and electrolytes, and proteins leak out of damaged blood vessels.
Young foals may also develop diarrhea associated with infection by Salmonella bacteria. Certainly, Salmonella infection should be considered as a possible cause in any foal presenting with diarrhea. “Unfortunately, Salmonellosis in young foals appears to have a relatively unfavorable prognosis,” Taraba said.
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. Similar Salmonella outbreaks occur on big farms where these are a lot of animals, particularly broodmares, which shed Salmonella in late pregnancy or after.
Since horses are always mixing on these farms, outbreaks can occur because horses without immunity are exposed,” Taraba noted. “Some horses are left as carriers in the
end, allowing the cycle to begin another year when conditions are right.”
After the affected horse has been isolated, the primary goal of treatment is to support the horse’s circulation until the immune system is able to re-establish normality in the intestinal tract. “Therefore, the major component of treatment is intravenous fluid therapy, paying attention to abnormalvacid-base and electrolyte status,” Taraba stated.
Other drugs, including flunixin meglumine and polymyxin B, are used to ward off the effects of endotoxins that are absorbed from the diseased intestinal tract.
“The affected horse must be rested, protected from stress, and observed critically throughout the treatment period,” Taraba recognized.
Development of various complications is common during the treatment of Salmonellosis. Specific additional treatments must be provided to alleviate the effect of these complications.
“Some horses simply cannot resolve the damage caused by invasive and destructive Salmonella bacteria, and the diarrhea persists until the horse has lost sufficient weight to justify euthanasia,” Taraba admitted.
With aggressive treatment, other horses are able to reverse the damage and produce normal manure. “Whenever horses recover from Salmonellosis, they may be at risk for further bouts of loose stool and colic because their intestinal system might not fully recover,” Taraba related.
Recovered horses should be tested occasionally to determine whether they are shedding Salmonella bacteria or not. Horses may carry the offensive Salmonella for several weeks to months.
“The actual duration of carriers depends on the severity and nature of the original infection, so there are no hard and fast rules how long horses will be contagious,” Taraba concluded.