Colic Epidemiology in Horses

Epidemiology can be very useful and informative in understanding equine colic. It can provide information regarding the causes, risk factors and survival rates of different types of colic. The incidence of colic can be estimated and monitored. Epidemiology can provide an approach to investigating an outbreak of colic. Ultimately, epidemiology can consolidate all of information and variables to inform veterinarians about methods to prevent colic.

Incidence

Colic occurs relatively frequently in horses. Studies have revealed an incidence estimated at 0.1-0.2 episodes per horse-year, in a normal managed population of horses. In context, this would mean an average holding of 100 horses could reasonably expect to see 10-20 cases every year. Values exceeding this would be considered abnormally high and would warrant further investigation.

Classification and Survival Rates

Approximately 90% of colic episodes can be successfully managed using medical treatments, with the remaining 5-10% requiring surgery. Assuming surgical and medical cases of colic are accurately distinguished, survival rates of 95% and 80% are considered normal for medical and surgical colic, respectively. The average cost of colic surgery is £2500. Horses that have an episode of colic are at a greater risk of experiencing another episode in the future.

Post-operative Survival

As previously mentioned, the short-term survival for surgical colic patients is encouraging (80%). Long-term survival is currently unknown. The first few days post-surgery have the highest mortality, hence the careful monitoring and care required for these patients. The mortality then reduces over the following 100 days. Different types of surgical colic patients will have different post-operative mortality rates. For example, when assessing the survival of small-intestinal cases, studies have shown that patients with epiploic foramen entrapment have a higher post-operative mortality rate than horses with ileal impactions, pedunculated lipoma obstructions and jejunal strangulations. Studies have shown that there is an increased risk of death with certain factors:

• Abnormal Packed Cell Volume (PCV) on presentation
• Increased length of intestine resected during surgery
• Increased duration of surgery
• Elevated peripheral lactate
• Elevated peritoneal fluid lactate

Increasing age is not associated with reduced survival. These parameters can be useful in providing owners with the most accurate long-term prognosis.

Risk Factors for Medical Colic

Studies have shown that there is an increased risk of medical colic with certain factors:

• Changes in management
• Increased concentrate content of feed
• Carbohydrate overload
• Water restriction
• Poor parasite control

Diet and feeding are extremely important areas for owners to be knowledgeable about. Improper feeding can put horses at an increased risk of colic. The domestication of horses has result in a reduction in their grazing time, forage and water intakes. Horses in the wild normally graze a low energy, high forage diet for an average of 16 hours a day. By contrast, the competition is fed 3 times a day on a high energy concentrate ration and experiences a reduced forage intake. Good anthelmintic control and pasture management will reduce the incidence of colic. The incidence of pelvic flexure impactions has been associated with a recent change in management. This includes horses being taken off the pasture and stabled or on box-rest for a prolonged period. Horses that eat their straw bedding are also at an increased risk of developing a pelvic flexure impaction. There is an increased incidence of this form of colic in autumn.

Risk Factors for Surgical Colic

Studies have shown that there is an increased risk of surgical colic with certain factors:

• Poor parasite control
• Increasing age
• Previous colic surgery
• Geldings

Horses that have had previous colic surgery are thought to be at an increased risk due to the formation of adhesions within the abdomen. Geldings are also thought to be at an increased risk due to a mild peritonitis developing despite a clean castration procedure. There are many different types of surgical colic and they have a range of different risk factors. Obstructions due to pedunculated lipomas more commonly occur in older animals over the age of 15 years. Ponies and geldings are also at an increased risk of developing this condition. Large colon torsions are associated with high mortality rates and must be treated surgically within a few hours of the onset of clinical signs to enhance the horse’s chance of survival. Whilst this condition is not fully understood, post-parturient mares are at an increased risk of developing this condition. This is due to an increased food intake during lactation, more space in the abdomen once the foal is delivered and a loss of tone of the abdominal muscles during pregnancy.

Tapeworms

Heavy tapeworm burdens have been associated with spasmodic colic; a mild form of colic which can be easily treated medically. Horses with an increased Tapeworm antibody ELISA (i.e. a higher level of infection) are at an increased risk of developing spasmodic colic. There is an increased risk of horses with tapeworm infestations developing colic due to an ileal impaction. This is a rare but serious condition which requires surgery for survival. Both these forms of colic are easily prevented with good parasite control.

Grass Sickness

Whilst the cause of grass sickness remains unknown, there have been several studies into the risk factors for the development of the disease. These include:

• Recent pasture changes
• Heavy breed horses
• Access to grass pasture
• Young horses aged 2-7 years
• Previous outbreaks certain pastures
• Season (May – July)

There are several ways to minimize the risk of horses developing grass sickness. These mainly involve the management of young horses and heavy breed horses as they are at an increased risk but can be applied to all horses. The veterinarian should advise owners to avoid grazing on pastures known to have been associated with cases of grass sickness and to move young horses off these pastures immediately, especially if a case arises. These pastures should particularly be avoided in May to July. For further information on grass sickness in horses, click here.

References

• Proudman C (2009), Epidemiology of Equine Colic, Equine Gastroenterology courtesy of the University of Liverpool, pp 32 - 39