Peritonitis in horses: understanding the dangers

Colic in horses

Inflammation of the normally smooth stomach lining, a condition called peritonitis, can be life-threatening – as Karen Coumbe MRCVS explains

LINING the abdominal cavity is the peritoneum, a thin, membranous layer that acts as a natural, biological wrapper to protect the abdominal organs. When the normally smooth peritoneal surfaces become inflamed, the area becomes sore and irritated – a condition called peritonitis. Colic in horses and peritonitis in horses are closely linked; one may lead to the other and both need to be taken seriously.

In normal circumstances the peritoneum secretes a fluid (about 60ml per hour), which helps to lubricate and clean the abdomen with its anti-bacterial properties. This produces a healthy surface for the gut, so that the various structures glide smoothly and do not become entangled or stick together.

Unfortunately, the equine abdomen is fragile and somewhat an accident waiting to happen. The average-sized horse has a stomach capacity of between five and 15 litres, which is relatively small – especially for a horse who is stabled and eating irregularly, rather than grazing continuously as nature intended.

Unsurprisingly, the 22 metres or so of small intestine and four metres of large bowel (colon) that are crammed into the abdominal cavity have the potential to develop twists, blockages or motility problems that can cause colic. Further possibilities include damage from parasitic worm infestations, or perforation of the bowel by ingested matter with sharp points, such as thorns.

What causes peritonitis in horses?

PERITONITIS is frequently thought of as some kind of abdominal infection. In many cases, bacteria are involved.

There has to be some reason why bacteria multiply in the wrong place inside the abdomen, which is normally a clean, securely closed space. Possible underlying causes include trauma, such as a penetrating wound, or a foaling or breeding accident that leads to perforation of the uterus.

Most commonly, the cause is an area of diseased gut, which becomes leaky (often due to colic) when natural defences are impaired and allows bacteria to seep out. Peritonitis can also develop as a result of a tumour, or an internal abscess following a severe bout of strangles – so-called bastard strangles.

An underlying cause cannot always be established. In a recent study in Sweden, which looked at 130 such cases, the most commonly recovered bacteria (where microbial cultures were performed) were species of Actinobacillus. All of the horses received antimicrobial therapy, with more than 90% of them responding to treatment and surviving more than 12 months after discharge from equine hospital – which is encouraging, even when the precise cause can be a mystery.

Signs of peritonitis in horses

The early signs of peritonitis in horses can be subtle, including a general reluctance to move, loss of appetite and lethargy. Another indication is a raised temperature, but fever may be disguised if phenylbutazone (bute) is administered. It is never a good idea to treat a horse without a clear diagnosis, since pain and fever are always there for a reason.

Additional indications range from weight loss, sometimes with a tucked-up appearance, to a raised heart rate, a fast pulse or even diarrhoea. If less obvious signs are missed, then colic is a strong indication that something is wrong. A raised temperature with colic would make peritonitis a more probable diagnosis.

Signs may vary, however, depending on the nature and severity of the condition. A localised peritonitis, possibly involving a small abscess deep within the abdomen, may grumble on and can be hard to identify. On the other hand, a sudden (peracute) episode due to a ruptured bowel can present with terminal signs of shock, sweating and a painful abdomen. The horse may be unwilling to move and may collapse; in the worst-case scenario he will be found dead.

Diagnosis of peritonitis in horses

IN the majority of cases, diagnosis of peritonitis in horses is possible by examining a peritoneal fluid sample.

A relatively straightforward and generally low-risk clinical procedure, commonly known as a belly tap (or more properly as abdominal paracentesis), involves collecting a sample of peritoneal fluid from the horse’s abdomen. If fluid is not readily forthcoming, an ultrasound scanner can be used to find a pocket of fluid to sample. A few drops can be sufficient to provide answers.

Lab tests are used to check the type and numbers of cells present and to measure protein content, but just looking at the fluid is informative. The normal colour is similar to white wine; if it resembles red wine or, even worse, has the consistency of custard, then it is time to worry. Red signals bleeding, inflammation and possible bowel damage. Cloudy and custard-like fluid means there are pus cells involved.

If the bowel is ruptured, the peritoneal fluid will be a murky green/brown colour due to the gut contents floating free within the abdomen. Sadly, this is a catastrophic and irreparable situation.

Ways to treat peritonitis

Treatment will depend on the severity of the condition and the underlying cause, if known.

Medical treatment is not the only option and some horses will require surgical intervention. Opening the abdomen may permit contamination to be identified and enable diseased bowel to be removed. This a major procedure, however, so surgery is usually reserved for cases that fail to improve with medical therapy or for horses with worsening abdominal pain.

Laparoscopy, using small telescope-like instruments, can be an alternative approach to investigating the abdomen in the conscious, standing horse, but it does not permit the whole abdomen to be examined.

Another therapy option is abdominal drainage and lavage, where a tube is inserted into the abdomen and fluid is flushed through. This is not always effective, for a number of reasons, including the size of the horse and the risk of causing further complications.

Recently, a detailed knowledge summary reviewed published scientific evidence to guide vets on the optimal treatment for such challenging cases. Frustratingly, this showed that the quality of evidence is insufficient to direct clinical practice – beyond confirming that the use of antiseptic solution to lavage the abdomen can cause inflammation and is detrimental, and that treatment with appropriate antibiotics may be sufficient for peritonitis caused by certain bacteria.

What we do know is that the sooner a vet can identify that peritonitis is the cause and instigate proper, effective treatment, the better the chance of recovery.

Peritonitis: equine case studies

SOME high-profile horses have lost their lives to peritonitis this year, including grand prix gelding Talented Mr Ripley.

“One morning, Ripley didn’t want his breakfast and he showed mild colic symptoms,” says Georgia Stokes, who’d produced the 15-year-old from a yearling. “The vet did a tap and diagnosed peritonitis instantly. Ripley was treated at home, but we couldn’t stabilise him. We never did identify a cause.”

Mary King’s eventing mare Kings Temptress was put down in April when she developed peritonitis after foaling. The foal’s foot ruptured her uterus, allowing amniotic fluid to leak into her peritoneum.

“She allowed the foal to suckle, but she looked dull and just stood there sipping her water,” recalls Mary of “Tess”, her Kentucky 2011 winner and a mother to eight foals, five of them by embryo transfer.

“The fluid that came out of the tummy tap was horrible,” adds Mary, whose Badminton and Burghley winner Star Appeal had died from the condition some years ago. “We knew Tess could not be saved.”

Yet Igor VIII, who developed peritonitis three years ago, survived after being given a less than 10% chance of survival.

“We think he ate a twig, which tore his peritoneum,” says owner Tyler Bradshaw of Igor, who is now winning at prix st georges. “It was scary to see him undergo surgery, but he made a full recovery.”

H&H 24 September 2020

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