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Salmonellosis in Horses

By: Dr. Philip Johnson

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Salmonellosis is the most commonly diagnosed infectious cause of diarrhea in adult horses. 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 species ("strains") of Salmonella. Some are much more likely to cause diarrhea than others with most species just innocuous bystanders of the gut. Remarkably, horses normally carry at least one species of Salmonella in their intestinal system, and some horses carry several. As normal residents of the intestine, they do no harm; rather they contribute to digestion.

Salmonella bacteria normally present in the horse's intestine are prevented from overgrowing by the army of normal resident bacteria that constitute the "normal flora" or "microbiota." The Salmonella are simply crowded out, and can't find attachments to the bowel wall. Furthermore, the horse's immune surveillance system also plays a critical role in keeping Salmonella bacteria at bay. Lastly, the normal bowel movements stir up the bacteria, and keep them moving along so they can't get a foothold on the bowel wall. All these factors are essential defenses against Salmonella.

What to Watch For

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. At this stage, 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.

In more severe cases, the diarrhea is projectile, malodorous, brown, profuse and watery. There is no discernible texture or form to the feces. Defecation makes the horse uncomfortable so tail switching, straining and periodic rectal prolapsing may occur. The horse searches in futility for a comfortable stance. Lying down and getting up repetitively is common when the diarrhea starts to pour.

In the most severe cases, there is considerable bloat, colic, flatulence and even bloody diarrhea. In these horses, Salmonella has probably caused considerable damage to the large and small intestine.

Diarrhea rapidly leads to dehydration of the horse. This is the biggest concern initially, because the horse cannot keep up by drinking. The losses are too much and too fast. Diarrhea washes away water and electrolytes, and proteins leak out of damaged blood vessels.

Horses with Salmonella become very toxemic due to a breakdown between the lining of the intestines and the blood stream. Toxic substances ("endotoxins") enter the blood stream and cause the affected horse to appear very sick and depressed. Endotoxins go everywhere in the body, damaging vessels and organs. This appears as a dark or even bright red discoloration of the gums. In the most severe cases of Salmonellosis (peracute Salmonellosis), the affected horse may be found dead.

Signs of colic during Salmonellosis are attributed to the damage inflicted on the lining of the intestinal tract by the invading Salmonella bacteria. The capability of the damaged intestine to move digesta and gas along is reduced leading to accumulations of distending gas that causes colic pain.

Effects on Foals

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.

In many cases, the Salmonella bacteria invade the body of foals and cause septicemia (bacterial dissemination to different parts of the body). Common locations for Salmonella bacterial spread in foals include the joints (lameness attributable to septic arthritis) and the lungs (pneumonia). The most common symptoms of Salmonellosis are diarrhea and fever.

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