Equine Colic: II. Causes and Risks for Colic

1. Prevalence

Colic is one of the most difficult diseases to study with epidemiologic methods due to the large number of diseases that cause colic (abdominal pain) as a clinical sign. Therefore, epidemiologic data related to colic is only meaningful if an accurate diagnosis of the primary disease process can be determined. Nevertheless, epidemiology has provided important information about incidence, mortality, and risk factors for colic, all of which may help the clinician make decisions about individual cases as well as herd problems.

Determining the incidence of colic can help determine if the rate of colic on farms or in stables is excessive. Out of 100 horses in the general population, 4 - 10 cases of colic is expected during 1 yr (Fig. 1) [1,2]. The annual number of colic cases, however, may vary greatly between farms, ranging from 0 to 25 or 30 cases per 100 horses [3-5]. Approximately 10 - 15% of colic cases occur in horses that have experienced previous episodes of abdominal pain, with 2 - 4 colic episodes per year for some horses [1]. Eighty to eighty-five percent of colic cases can be designated as simple colic or ileus because no specific diagnosis is identified, and most horses respond to medical treatment or resolve spontaneously. In one cohort study, approximately 30% of horses with colic were identified by owners but never examined by a veterinarian because the colic was transient or resolved with treatment by the owner [1]. Studies of horses with colic that present to veterinary practices have also reported to have a predominance of simple obstruction or spasmodic colic, with impactions diagnosed in approximately 10% of affected horses [6]. Obstructing or strangulating diseases which require surgery represent only 2 - 4% of colic cases, though some risk factors in certain populations may increase this rate [7].

Colic is responsible for more deaths in horses than any other disease. In the normal farm population, horse mortality from all types of colic was 0.7 deaths per 100 horse-years, with a colic case fatality rate of 6.7% [1]. The predominant reasons for death were stomach rupture, strangulating lesions, or enteritis [8].

The true incidence of specific intestinal diseases causing colic in the general equine population is not known. Simple and impaction colic were found to be the most common reason for abdominal pain in horses presented to veterinary teaching hospitals or practices. When the segment of bowel involved could be determined, the large colon was the most commonly affected, followed by the small intestine, cecum, and small colon [7]. Large colon torsion was the most common cause of strangulation obstruction, with strangulating diseases of the small intestine causing the highest case fatality rate [7,9,10].

In a study of colic in a population of 28,000 horses, loss of use due to colic averaged 2 - 3 days, less than that resulting from trauma, lameness, or neurological disease [3]. The value of horses lost due to colic in the United States in 1998 - 1999 was estimated at $70 million, while the total cost of colic to the industry was estimated at $144 million [3]. Based on smaller studies and anecdotal information from veterinary hospitals, the number of abdominal surgeries performed on horses with colic in the United States is estimated to be 12,000 - 24,000 annually, or possibly as many as 2.7 colic surgeries every hour.

2. Risk Factors for Colic

Since only natural disease has been studied, determining the causes of different diseases that cause colic is problematic. In some cases, such as grain overload or enterolithiasis, the proximate cause may be evident, but the underlying mechanism or physiologic problems often remain unknown. Determination of risk factors for specific types of colic may help identify the cause and may help to decrease disease incidence by decreasing exposure to an incriminated risk.

Figure 1. The incidence, mortality, and surgical risk for colic in normal horse populations. The numbers reflect the rate in 100 horses examined over 1 yr. Surgical risk was not assessed in the United Kingdom study [4], United States study [3], Virginia-Maryland study [1], Michigan study [2].

Risk of colic is defined as the odds that colic incidence will increase in a group of horses exposed to a particular factor compared to the colic incidence in a group that is not exposed to that factor. Horses that have had a previous episode of colic are 3 X more likely to have a second colic episode compared to a horse that has never had colic [11]. Said another way, if the incidence of colic in a normal population of horses with no previous history of colic is 10 out of 100 horses in a year, the rate of colic in a population of horses with a history of previous colic would be 30 out of 100 horses per year. Colic risk may also be categorized into internal and external risks. Breed or enlarged inguinal rings are examples of an internal risk, while diet and housing are considered external risks.

Signalment

While colic may affect horses of any breed, several studies suggest an increased incidence of disease in Arabian [12] or Thoroughbred [3] horses. Standardbreds, gaited horses, and Warmblood stallions have an apparent increase in incidence of inguinal hernias due to the increased size of their inguinal rings [13]. Though rare, the recessive and lethal trait of aganglionosis, which occurs in American Paint Horse foals born to overo mares mated with overo stallions, is the only cause of colic that has been proven to date to have a genetic basis.

Younger (<2 years) and older horses (>10 years) appear to be at less risk for simple colic [11]. Middle-aged horses are at higher risk of colic than older horses; however older horses with colic are more likely to require surgery [6]. Weanling and yearling horses are more likely to have ileocecal intussusceptions, and older horses (>12 years) are at increased risk of strangulating lipoma [6].

Gender is an apparent risk for diseases such as inguinal hernia in stallions and large colon displacement/volvulus in periparturient mares. Male horses (geldings and stallions) and older horses appear to be at slightly higher risk of entrapment of the small intestine in the epiploic foramen [7]. For the most part, male and female horses appear to be equally affected by simple colic, which is probably related to management or activity.

Diet

Feeds or feeding activity have long been associated with the incidence of colic, though information is still largely anecdotal. Course roughage with low digestibility or particularly course fiber is associated with impaction colic [14]. Poor dentition has been proposed to predispose horses to colic due to poor mastication of food, although this has not been confirmed [15]. Grain overload increases the risk of colic and laminitis. Feeds such as lush clover and lush pasture have been implicated as causes of tympany. Horses fed poor quality Bermuda grass hay have an increased risk of ileal impaction, [16] and anecdotal reports suggest some horses have more colic when fed alfalfa hay. Feeding from hay in round bales is also associated with an increased risk of colic [17].

Figure 2. Odds of colic in horses fed concentrate (in kilograms) as part of their daily diet compared to horses fed no grain. The stars indicate a significant difference (P <0.05) for odds of having colic compared to horses not fed grain.

Case control and cohort studies indicate that increased amounts of grain or changes in the type of hay and grain fed increase the odds of colic compared to horses without grain or changes in feed [11,18]. Daily feeding of concentrate at 2.5 - 5 kg/day and >5 kg/day to adult horses increased the risk of colic 4.8 and 6.3 X, respectively, compared to horses fed no grain (Fig. 2) [11]. Horses fed grain in the form of pellets or sweet feeds had an increased risk of colic compared to horses that were not fed grain or were fed single grain diets. Grain diets decrease the water content of ingesta in the colon due to a decrease in fiber, which binds to water [19]. Grain in the diet also increases gas production and is more likely to create an intraluminal environment that favors gas production or altered motility leading to displacements.

Feeding small amounts of grain at frequent intervals reduces fluid shifts in the large colon, compared to fluid shifts that occur with twice-daily feeding of larger quantities of concentrate [20]. Although no relationship was found between feeding frequency in one study, [21] feeding more than twice daily increased the risk of colic in a Virginia-Maryland study [11]. This increased risk was thought to be due to an increased daily intake of grain rather than the frequency of feeding.

3. Environment/Management

Housing and confinement on the farms in a Virginia-Maryland study were not risk factors for colic [4,11].

However, other reports suggest there is an increased risk of cecal and large colon impaction in horses that have acute decreases in activity, such as curtailing regular exercise or changing from turn out activity to strict stall confinement due to an injury or after surgery [22]. A case-control study in Texas found decreased colic risk with lower horse density on pasture and with access to a pond; these observations are supported by studies from the United Kingdom [15,18].

Turn out in paddocks without water is associated with an increased risk of colic [21]. The type of activity is often related to the type of housing, possibly confounding interpretation of the results in some studies, and further investigations into the relationship between housing and type and frequency of exercise and their effect on the incidence of colic is needed.

Management factors are difficult to compare between farms, and changes in management are difficult to detect. The increased risk of colic associated with care by trainers and managers, compared to care by owners, is supported by two studies [15,21]. These findings suggest either a difference in the quality or frequency of observation between these two groups or better management by owners, compared to trainers, of horses with more intensive exercise.

It seems logical that housing, diet, and feeding routine are associated with a risk of colic. Anecdotal information from large breeding farms suggest that the routine of feeding horses grain after being brought in from pasture and then confining them in stalls for part of the day increases the risk of colic, and specifically colon tympany and displacement of the large colon. By altering this daily routine, including keeping horses turned out after grain feeding, colic incidence is decreased. Similarly, when hay is available to horses on lush pasture, the hay will be consumed as part of the diet, and incidence of colic is decreased in horses turned out 24 hours per day.

Specific causes of colic occur at differing frequencies in different regions of the world [7]. Grass sickness occurs in the United Kingdom, Europe, and South America, but not in North America. Ileal impactions occur predominantly in horses in the southeastern United States and Europe. Enteroliths are seen observed more frequently in horses in California, the Midwest, and Florida [7] Enterolith formation appears to be related to diet and, potentially, mineral intake. Horses consuming hard water, alfalfa hay, and which have a higher pH of mineral concentrations in colonic ingesta, as well as horses living in California, are at higher risk for this problem [23-25]. Sand colic and impactions are more common where horses graze on pastures with sandy soils or where horses are forced to eat off of a ground consisting predominantly of sand or fine gravel. These diseases are not common causes of colic, and their incidence may not be impacted by the same management, diet, and environmental changes previously described [11,15,18].

4. Event Associated Colic Risk

Previous Colic

Horses with a history of previous colic are at higher risk for future colic episodes [1,26]. Horses with a prior history of abdominal surgery are at higher risk of repeat colic episodes, which is often due to adhesions or bowel scarring with stricture [12]. Horses have a higher rate of colic (one or more episodes) within the first 2 - 3 months after abdominal surgery; after that time, the incidence of colic decreases to that of the normal population.10 Horses with colon impactions have a high rate of repeat colic [22]. The reason for this increased risk is not known. Decreased numbers of neurons in the myenteric plexus of the pelvic flexure and right dorsal colon in horses with chronic colon obstruction may create alterations in bowel motility predisposing to future obstruction [27].

Parasites

Parasites (ascarids, tapeworms, strongyles) are associated with an increased risk of colic in several studies. Obstructions due to ascarids in foals and tapeworm-related and strongyle infections have all been reported as causes of colic, usually based on small groups of cases. Uhlinger [5] observed a decrease in colic after controlling small strongyle infection on several farms with a high colic incidence. Tapeworm infection has been associated with an increased frequency of colic. Specifically, tapeworm infection is a risk for colic associated with ileocecal intussusception or cecocecal intussusception [28]. Although there are no studies describing the incidence or prevalence of colic associated with thrombosis of the cranial mesenteric artery due to Strongylus vulgaris larva, the decrease in mesenteric artery thrombus formation observed in horses at surgery and necropsy appears to parallel the increased use of ivermectin in horses over the last 25 years.

Cribbing

Recently, cribbing, long associated with an increased risk of colic, was demonstrated to be associated with an increased risk of simple large colon obstruction and entrapment of the small intestine in the epiploic foramen [29]. The act of aerophagia likely creates negative pressure in the abdomen, leading to movement of bowel into the potential space within the lesser omental sac. The author speculates a similar event predisposes horses to inguinal herniation due to pressure changes in the inguinal canal when testicles descend after being retracted during breeding.

Figure 3. Graph of mean total serum calcium concentrations (g/dl) in 15 periparturient mares plotted at 2-week intervals before and after the foaling date. Serum calcium concentrations were significantly decreased at 4, 6, and 8 weeks prior to foaling compared to the weeks before and after, although concentrations were never abnormal.

Pregnancy

Mares have an increased risk for colon displacement or volvulus during late pregnancy and lactation [30,31]. However, these studies were based on select populations of mares or mares in regions with high numbers of broodmares. In a Virginia-Maryland study, mares had an increased risk of colic from 60 - 150 days after foaling [11]. The physiologic events that predispose horses to this increased risk are not known, but serum calcium concentrations and alterations in diet, including increases in energy due to more concentrates in the diet to support lactation, may be related to the increased risk (Fig. 3).

Exercise/Performance

To date, studies have not fully assessed the relationship between incidence of colic and exercise or activity level. Racehorses, event horses, and horses used on endurance rides have an increased risk of gastric ulceration, which may be associated with some colic episodes [32-35]. Although horses used for racing or eventing had the highest incidence of colic in a Virginia-Maryland study, these activities did not pose a significantly increased risk when compared to other factors such as diet [11]. Recent studies found a decreased risk of colic in horses on premises where horses were trained for eventing versus those housed in stables and trained for racing on the flat track [15].

Horse Transport

Horse transport increased the risk of colic in several studies [5,11,15]. This has been suspected by practitioners, who commonly administer a laxative prior to shipping to prevent colic from impactions. The mechanism or cause of the increased incidence is unknown. There was a marked increase in colic risk with the concomitant presence of transport, grain feeding, and cribbing [8].

Fever

Horses with a systemic inflammatory reaction may have an alteration of the gastrointestinal tract, predisposing them to colic [11]. In a Virginia-Maryland study, horses with fever within 14 days of a colic episode were 2 X as likely to have colic [11]. Because the reported cause of fever in these cases varied, no specific relationship or cause for each colic was established.

Weather

Veterinarians and owners frequently associate weather changes with increased frequency of colic, but many studies have been unable to find statistical evidence of increased risk. A recent study in Texas found an increased risk of colic associated with weather changes as recalled by owners of horses with colic [18]. Cold weather, which affects water intake, has been linked to increased impaction colic. When examined as a direct exposure factor in a Virginia-Maryland study, weather did not appear to be related to colic [11]. When events were investigated by looking at a 14-day window preceding colic episodes, low humidity and snow marginally increased colic risk [11]. In a study in Virginia, seven cases of colic occurred during a heavy snowstorm over a 3-day period [8]. This was unusual since there were only 104 cases of colic in approximately 1000 horses monitored for a full year. What became apparent from records kept by the farms was the change in management due to the snowstorm. Horses were kept in stalls, rather than being turned out, and the diet was not altered, even though horses had no turnout or exercise. The focal increase in colic episodes in this study was most likely not directly related to the weather, but rather due to management changes caused by the weather.