Peritonitis in the Horse
Dr. Melissa Mazan
The peritoneum lines the whole of the abdominal cavity and the pelvic cavity and covers all of the internal organs. The peritoneum is a remarkably tough and resilient layer, even though it is only one cell layer thick. Young horses are prone to exploring their environments with their mouths, and so may ingest a foreign body (this refers to anything other than food that the horse eats). If the foreign body is sharp, it has the potential to penetrate the intestine and allow bacteria to leak into the peritoneum.
It secretes a fluid, called the peritoneal fluid that has a number of functions: it lubricates the internal organs so that the stomach, for instance, doesn't stick to the small intestine, and it helps to prevent infection of the peritoneal cavity. The peritoneum itself also serves as part of the abdomen's defense system – cells such as macrophages and mast cells that inhabit the peritoneum help to ward off infection. The peritoneum also regulates the way that fluid, proteins, electrolytes, and other molecules are able to pass between the blood vessels and the peritoneal fluid. When the peritoneal space develops an infection, the peritoneal lining becomes inflamed – thus we call this infection peritonitis.
Peritonitis is most commonly caused by bacterial infections – especially gram negative bacteria. Gram negative bacteria, such as E.coli, contain a product called endotoxin which is part of the bacterial cell wall. Endotoxin causes symptoms such as fever, high heart rate, and pain in most species, but horses are particularly susceptible to the effects of endotoxin – much more so than are humans or dogs, for instance. This is one of the reasons that laminitis, or inflammation of the sensitive structures of the hooves, is one of the possible complications of peritonitis.
There are many different causes of peritonitis:
External injuries that penetrate the abdomen can also cause peritonitis, although they are rare.
Strangles, caused by the bacterium S. equi, can occasionally infect the abdomen, resulting in peritonitis – this is commonly called bastard strangles.
In foals, and occasionally older horses, gastric ulcers can develop pinpoint leaks, or ruptures, that then cause peritonitis.
Umbilical infections in foals can sometimes rupture and cause peritonitis. Mares who have had a difficult foaling (dystocia) can, rarely, suffer a rupture of the uterus, which in turn contaminates the abdomen.
Horses with rectal tears often develop peritonitis, due to contamination of the peritoneal cavity.
In rare cases, ascending infections from the urinary tract can cause peritonitis, as can systemic infections.
Non-infectious peritonitis can be caused by certain types of cancer.
Horses with liver problems may develop peritonitis due to irritation of the peritoneal cavity from bile.
Any inflammation in the abdominal cavity – for instance, an inflammation of the spleen or the pancreas – while rare, can cause peritonitis.
Although most horses these days are dewormed adequately, cases of peritonitis that are caused by intestinal parasites migrating through the walls of the intestines and the peritoneum still occur.
Acute or Chronic
Peritonits can appear very different, depending whether it is acute or chronic. Horses with acute onset of peritonitis – for example, from a rectal tear – often have severe colic signs. They may be sweaty, their extremities may be cold, their mucous membranes may be gray or bluish from shock, their heart rates are extremely high, their respiratory rate and effort may be excessive, and they often have a subnormal temperature, although they may alternately have a fever. These types of symptoms are most often seen in horses that have a large amount of mixed bacteria contaminating the peritoneal space. This horse may deteriorate rapidly and die if it doesn't receive emergency treatment. As a rule of thumb, there are more bacteria, and a more virulent mix of bacteria, as the intestine gets closer to the rectum. Ruptures of the small colon or rectum tend to be far more catastrophic than a pinpoint leak from a gastric ulcer.
Horses with chronic peritonitis usually appear very different. They often have a persistent, low-grade fever, they may have weight loss, diarrhea, or intermittent, milder colic symptoms. They may appear painful when they are walking, and they may resent pressure on the abdomen.
Although the peritoneum functions as part of the abdomen's defense system, this very response to infection can also be responsible for some of the long-term sequelae of peritonitis. When the peritoneal cavity becomes infected, the peritoneum's response is to wall off the infection. It lets a tremendous number of inflammatory cells invade the peritoneal space in order to engulf and contain the infection.
It also produces a substance called fibrinogen, that helps to act as a seal over the area that is leaking. The only problem with this is that the body's response is often greater than is needed – the fibrinogen forms into fibrin, which cannot be easily broken down, and instead of merely plugging the hole, as it were, it acts as glue that sticks portions of the intestines to each other. This is called adhesion formation, and it can cause kinks to form in the intestines, thus keeping food from passing normally. This causes chronic colic.
Any cause of colic, such as impaction, gas, or a twisted gut
Any cause of weight loss, such as malabsorption problems
Any cause of chronic diarrhea, such as sand impactions or chronic colitis (inflammation of the large colon)