The peritoneal cavity is a very large space that encloses the abdominal organs. It is covered by a thin tissue layer called the peritoneum. The peritoneum is so thin that it is actually made of only one layer of cells. The peritoneum may seem delicate, but it is part of a critical defense system that protects the abdominal organs.
In addition to covering the abdominal contents, the peritoneum also governs the motion of fluid, proteins, and other molecules from the peritoneal space to the blood vessels. The peritoneum is also extremely important in lubricating the abdominal organs, so that when the horse moves, the internal organs can easily glide past each other.
Peritonitis is the medical term that refers to inflammation of the peritoneum. Peritonitis may be due to both infectious and non-infectious causes. Infectious causes are most common, particularly bacterial contamination from the intestinal tract. Penetrating wounds, ruptured gastric ulcers, migrating intestinal parasites, and foreign bodies can all contribute to the development of peritonitis.
Peritonitis due to ruptured gastric ulcers is most common in foals, and migrating intestinal parasites most commonly do their damage in horses that are not dewormed or are dewormed infrequently. An outbreak of strangles (S. equi) may result in intra-abdominal abscesses in a small number of horses. The majority of cases of peritonitis, however, are unpredictable.
Horses are very sensitive to endotoxin, a by-product of the gram negative bacteria that are the common culprits in peritonitis. Thus, peritonitis, when untreated, can be life-threatening. Survival depends very much on how severe the individual case is. The severity of the peritonitis often depends on the cause. A catastrophic rupture of the intestine usually causes the death of the horse. Microscopic ruptures from gastric ulcers, or penetrating wounds, however, can often be treated.
Horses are prone to forming adhesions (the intestines stick to each other) after peritonitis, so even horses that survive in the short-term can experience long-term difficulties.
What to Watch For
Your veterinarian will take a careful medical history and perform a complete physical examination. Based on the results of these, she will usually choose to perform more tests:
It is important to follow all of your veterinarian's instructions for giving antibiotics. Horses with peritonitis may require weeks or even months of antibiotics. Antibiotic treatment often alleviates the symptoms of peritonitis, such as colic and weight loss, long before the infection itself is completely eradicated.
It is important to follow all of your veterinarian's feeding instructions. Horse with peritonitis may not be able to tolerate a rich diet; other horses who have adhesions may not be able to tolerate a diet with a high fiber content.
It will be important to monitor your horse's appetite and manure output. A horse's appetite may be the single best monitor of his sense of well-being. If your horse has diminished manure output, he may be developing adhesions that are preventing him from moving food and manure through his system.
Most cases of peritonitis are unanticipated and impossible to prevent. It is important to monitor for signs of colic in any outbreak of strangles – this may be the signal that your horse has developed an abdominal abscess.
Foreign body ingestion can be prevented by making sure that your pastures are free of any objects that your horse might want to nibble on – other than hay.
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.
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:
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.
In the case of an infectious peritonitis, there may also be bacteria in this fluid. The bacteria often consist of a mixed bag of gram negative, gram positive, and anaerobic (meaning existing without the presence of air). Your veterinarian will usually choose to culture this fluid in order to identify the bacteria, and test the resultant growth against a battery of antibiotics in order to determine which treatment will work the best. This test is a bacterial sensitivity test.
In the majority of cases, the cause of peritonitis is a bacterial infection. The most effective weapon against bacterial infection is broad spectrum antibiotics. This is one reason why it is so helpful to get a good culture and bacterial sensitivity on the infected peritoneal fluid – it helps us to have a specifically directed therapy.
After broad spectrum antibiotics, supportive care is of utmost importance. If the horse has become dehydrated due to ileus or has such a high fever that he doesn't want to eat or drink, it is important to give him fluids, either intravenously or using a nasogastric tube, depending on the individual case. It is necessary to keep the stomach decompressed in cases of ileus. Other treatments, such as lavage and draining of the peritoneal cavity in order to decrease the number of bacteria and contaminants, may be helpful. In cases when we can identify a physical cause of the peritonitis – say, a foreign body – it is important to remove the cause.
Adhesions and laminitis are two of the most important complications of peritonitis, and they can cause a horse's death despite curing the original problem. We aren't sure if our treatments to prevent adhesions, such as heparin, are effective. Because adhesions can have such devastating consequences for horses, many veterinarians choose to treat with heparin nonetheless.
Treatment for potential laminitis consists of maintaining soft footing, and good supportive care, and anti-inflammatory medications. There are many, many treatments for laminitis – which tells us that we are still unsure of the best way to tackle this devastating problem. Some very good veterinarians choose to try to prevent laminitis, while other equally experienced practitioners are of the opinion that there is no good preventative other than good supportive care.
Optimal treatment for the horse with peritonitis requires a combination of home and professional veterinary care. Follow-up can be critical, especially if your horse doesn't improve rapidly.
It is always important to make sure that your horse receives antibiotics for the length of time prescribed. It is important to monitor your horse's temperature regularly. A fever spike may indicate that the antibiotic choice must be modified.
If your horse has spent time in a referral hospital, your own veterinarian may need to perform follow-up abdominocentesis examinations and will usually want to perform follow-up blood work. It may be necessary to repeat both ultrasound and abdominal x-rays as well.
Peritonitis often requires a lengthy treatment period. Don't expect your horse to be back to normal in a matter of days. It is important to monitor your horse's body condition while he recuperates. Many horses with peritonitis become thin, even emaciated. It will take many weeks and even months to regain the body mass and bloom that your horse has lost.