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.
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:
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:
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.
Some laboratory work, showing a low white blood cell count and electrolyte derangement, can be useful to establish the diagnosis even before diarrhea begins. Of course, it's better to diagnose a highly contagious disease like Salmonella as early as possible, so you can isolate the horse from others.
The diagnosis of Salmonellosis must be based on demonstration of Salmonella bacteria in the horse's feces. This is typically accomplished by bacteriological culturing of stool samples from affected horses. Your veterinarian will collect a substantial volume of feces and submit the sample to a microbiology laboratory. It takes several days for the identification of the presence or absence of Salmonella bacteria in feces, so the diagnosis is not usually very rapid.
It is also known that horses affected with Salmonellosis do not always shed the bacteria all of the time. Therefore, the current recommendation requires that stool samples be tested from at least five different days of the disease. If all five samples are negative, it is concluded that (with 95 percent confidence) the diarrhea is probably not a result of Salmonellosis.
Your veterinarian may also undertake specific blood tests during the early phases of diarrhea. Some changes in the white blood cell count are very strongly associated with Salmonellosis and these changes are easy to detect prior to the completion of fecal culturing in the laboratory.
Another new test for the presence of Salmonella bacteria in feces is the "PCR" test. The PCR test is more rapid than routine bacteriological culturing but it is so sensitive that Salmonella bacteria may be detected, even if they are not contributing to the cause of diarrhea. However, a negative PCR test result in the face of diarrhea suggests that the diarrhea may have been caused by something other than Salmonellosis.
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.
The very best IV fluid treatment should be formulated based on the results of analysis of the composition of the affected horse's blood, paying special attention to abnormalities of acid-base and electrolyte status. For the best success in the management of Salmonellosis, serial analysis of the blood may need to be done 3 or 4 times daily. To this end, a case is often made that the best success is attained when affected horses are treated in a hospital facility with access to immediate blood testing.
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.
The development of various complications is common during the treatment of Salmonellosis. Specific additional treatments must be provided to alleviate the effect of these complications. Antibiotics are rarely used in the treatment of Salmonellosis unless there is evidence that invasion of the blood stream is occurring secondary to the intestinal damage inflicted by Salmonella bacteria (rare in adult horses).
A very common and important complication that arises during the treatment of Salmonellosis is the loss of protein from the blood through the damaged lining of the intestinal tract. When protein loss occurs, the ability of the blood to hold water (volume) in the blood vessels is lessened and contributes to reduced blood delivery to vital organs. Additionally, the water normally contained in blood tends to leak into the tissues causing subcutaneous edema. Subcutaneous edema can be seen as swelling under the belly and between the forelimbs. The same edema occurs in the intestine and precludes intestinal healing. Therefore, the low protein status must be treated in order to promote healing of the intestine. A plasma transfusion is the only effective treatment for low protein status, but plasma transfusion is usually very expensive in adult horses.
In addition to low protein status that occurs as a result of the damage imposed on the lining of the intestine by Salmonella bacteria, other noteworthy complications may arise during the treatment of Salmonellosis. Common and important complications include laminitis ("founder"), infection of the IV catheter used to provide fluids, kidney failure, colic (the diseased intestine does not work properly to move gas and digesta along its length) and abnormal coagulability of the blood (may lead to other organ failure).
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. 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. Recovered horses should be tested occasionally to determine whether they are shedding Salmonella bacteria or not. If a recovered horse is shedding Salmonella bacteria, it should probably be kept separate from weakened horses and foals.
In those horses that survive the beginning onset of Salmonellosis, the diarrhea may persist for several days or for several weeks. Persisting (chronic) diarrhea is also associated with marked reduction in the horse's body weight. In some cases, intractable diarrhea is attended by such marked weight loss that euthanasia must be considered. During the treatment of Salmonellosis, there is risk for notable complications that might also necessitate consideration for euthanasia.
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.