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Salmonellosis in Horses

By: Dr. Philip Johnson

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Diagnosis

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.

Treatment

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.

Complications

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.

Residual Effects

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.

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