The Epidemiology Of Colic
Epidemiology is the study of disease incidence. When we think of epidemiology, we most often think of infectious diseases or tracking down the cause of an epidemic. The same science that has been used to study human diseases has been applied to numerous animal diseases, and recently to the study of equine colic. Because there are so many causes for colic, epidemiology can be helpful in determining those things that are associated with an increased risk of colic. In some cases, it also can help identify the cause.
Mild episodes of colic often are described as gas colic, spasmodic colic, or ileus, depending on the consistency of feces and the character of bowel sounds heard on auscultation. Most of these mild cases of colic have no known cause, and frequently the veterinarian cannot determine which part of the intestine caused the pain. Other types of colic-such as colon impactions or intestinal obstruction due to displacement or strangulation-can be diagnosed by a rectal examination or surgery, but the actual cause or causes of these intestinal disorders often are not known.
Causes Of Colic
Infection with intestinal parasites is known to cause colic. Infection with Strongylus vulgaris once was reported as causing 90% of all colic in horses.10 Although this report never was substantiated by scientific studies, several reports have shown that Strongylus vulgaris larvae disturb intestinal movement, initiate inflammation, and stimulate blood clots in the arteries (thromboembolism) to the intestine.
With the availability of new medications to control intestinal parasites, colic due to Strongylus vulgaris now is considered to be relatively rare.
Adult small strongyles, which are found in the horse's large colon, also are suspected of causing colic. When small strongyle infections are controlled by regular anthelmintic (de-worming medication) administration programs, colic incidence in groups of horses is reduced.9 Roundworms, tapeworms, and bot larvae-although much less of a problem-also have been reported to cause intestinal diseases that cause colic.
Several drugs are known to cause bowel injury or dysfunction, which can lead to colic. When non-steroidal anti-inflammatory drugs such as phenylbutazone (Bute) or flunixin meglumine (Banamine) are administered in excessive amounts or for extended periods of time, they are known to cause gastrointestinal ulcers or kidney damage. Average adult horses (1,000-1,200 pounds) receiving four grams of phenylbutazone per day for four to seven days often develop gastric ulcers. Some horses appear to be more sensitive than others with toxicity occurring at lower doses.
Other drugs that cause intestinal stasis (a stoppage or diminishing of fluid flow) resulting in colic include Amitraz, an arachnicide used to kill ticks and mites, and drugs that affect the intestinal nervous system such as atropine or scopolamine. Common analgesics used to treat colic (such as xylazine, detomidine, and butophanol) also cause portions of the intestine to stop moving for short periods of time, but these drugs, used appropriately, do not cause colic.
Lipomas (fat tumors) cause obstruction or strangulation of the small intestines.
Although lipomas are benign and don't metastasize like cancer, they can cause damage by physical obstruction. Other tumors rarely are responsible for colic. Some tumors reported to cause colic include lymphosarcoma of the spleen and intestine, squamous cell carcinomas of the stomach, intestinal adenocarcinoma, and leiomyoma in the wall of the intestine.
Toxic substances rarely cause colic unless they inadvertently are ingested. Toxic compounds that have caused colic include organophosphates (usually in the form of an insecticide), monensin (which is a coccidiostat added to cattle and chicken feeds), and cantharidin (a caustic chemical found in the blister beetle).
Gastric ulcers can cause colic in foals and adult horses. Unlike the ulcers caused by non-steroidal anti-inflammatory drugs such as phenylbutazone, spontaneous ulcers are most common in horses used for strenuous athletic events such as racing.
Horses with a history of recurrent mild colic episodes should be considered candidates for gastric ulcers. Colic can result directly from the ulcers, or ulcers can be diagnosed concurrent with colon stasis, colon displacements, or feed impactions.4 However, the relationship between ulcers and other bowel diseases is unknown.
Infectious diseases including salmonellosis, Potomac horse fever, and clostridiosis (caused by clostridial bacteria) are associated with colitis or enteritis and can cause the horse to show signs of colic. In these diseases, colic usually is not the primary sign. The pain comes from the inflammation in the intestinal wall and presumably from endotoxin, which is absorbed during infection.
An infectious agent also is suspected of causing duodenitis-proximal jejunitis, an inflammation of the small intestines. The agent and the disease mechanism are unknown.
Grain overload usually occurs as an accident or in group feeding situations and can cause colic. Horses with reduced water intake due to cold or frozen water sources are reported to be more susceptible to impaction colic. Chronic ingestion of sand or gravel causes large colon obstruction.
This is seen most frequently in horses fed on the ground where sandy soil is exposed, in horses which lack proper nutrition, or in horses which are bored.
Poor dentition frequently is blamed for colic and should be considered a possible problem, particularly in older horses which might have problems with tooth loss and poor mastication of food.
Strangulation of the intestine has several known causes, including congenital defects, lipomas, mesenteric rents, and enlargement of the inguinal rings in stallions.
The cause of other types of displacements, such as small intestinal volvulus and large colon torsion/volvulus, is not known. These diseases result from a twisting of the intestines, which occurs due to intestinal movement within the abdomen. This does not occur when a horse rolls to scratch its back. It is thought to occur from abnormal intestinal movement. There is no known reason for this abnormal twisting. Other reasons for intestinal strangulation include inguinal hernia, internal hernias in the mesentery, epiploic foramen, or in defects or openings in other abdominal organs.
Horses can have colic from diseases involving other organ systems.10 Liver disease causes brain dysfunction and dementia, which can look like colic. Horses with acute pleuritis can have chest pain that causes behavior similar to that seen with abdominal pain. Horses with acute laminitis might stand with a stretched stance and appear to have abdominal discomfort. Horses might exhibit signs similar to colic because of muscle pain during acute rhabdomyolysis (tying-up). Just before foaling, some mares show mild colic apparently related to uterine contractions. Pain from the urogenital system can cause colic, but colic from kidney disease (sometimes called kidney colic) is very rare.
Colic Diagnosis, Incidence, And Mortality
Approximately 10% of all horses are affected by colic. Most cases are simple colic, which makes up approximately 80% of cases in a normal population of horses.7 Studies of colic cases diagnosed in veterinary practices also have shown a predominance of simple obstruction or spasmodic colic cases. Often no diagnosis is made in that type of case where signs are easily treated or are mild and transient. Impaction of the colon is the second-most-frequent diagnosis in horse populations or veterinary practices.
Colic caused by disease and requiring surgery is rare, only 1%-3% of all horses with colic.6,9
In the normal farm population, horse mortality from all types of colic was from seven deaths per 1,000 horses over a one-year time period.7 This is nearly twice as high as other diseases, including injuries or other illnesses. Of all horses with colic, 6.7% died. Most of the fatal cases were due to stomach rupture, strangulated intestine, or enteritis.
The frequency of diseases, including colic, is recorded at university hospitals and some practices. Simple colic and impaction colic are reported as the most common diseases. When the segment of bowel can be determined, the large colon is the most commonly affected part of the GI tract involved, followed by the small intestine, cecum, and small colon.10 Diseases that cause strangulation obstruction have the highest case fatality rate. Of those causing fatality, the large colon torsion is the most common, with strangulation of the small intestine the next-highest killer.
Case fatality (the number of horses which die from a specific disease) varies from no deaths in cases of simple colic to 75% for cases of strangulated intestine. Case fatality of diseases requiring surgery is related to disease duration prior to surgical correction. Therefore, fatality rates vary among hospitals depending on the time inherent in recognizing the problem and the time required to travel to a surgical facility.
Simple obstructions of the large colon, such as impaction, have a low case fatality rate (less than 10%), whereas simple obstructions or impactions of the small intestine are somewhat higher (30%, although lower than for strangulating lesions). Impactions of the cecum have a higher case fatality rate than impactions of the large colon, but overall, the survival is relatively high (85%). Small intestinal volvulus, incarcerations in mesenteric rents and the epiploic foramen have the highest case fatality rate (75%-95%). Large colon volvulus also has a high case fatality rate, although in some locales where this is recognized and treated early, the case fatality rate is less than 50%.
In recent years, these fatality rates appear to be decreasing. Subjectively this is due to both increased surgical expertise and early owner response in seeking veterinary care. It is not unusual to have 80%-90% of horses having abdominal surgery discharged from the hospital. Long-term survival of horses having abdominal surgery indicates a fatality rate varying from 45.5% to 66%, with evidence of an overall increase in long-term survival in the last 10-15 years. The case fatality rate for horses after discharge from the hospital (25%) appears to be improving as well. Of those horses surviving surgery for the long term, most (90%) are able to return to their original function.
Risk Factors For Colic
A risk factor can be a cause of a disease, or it might be an event or marker that can be used to predict the likelihood of a disease. An example of a risk factor in humans is high cholesterol levels, which indicate an increased risk of coronary artery disease and heart attacks. Similar to risk factors for human disease, risk factors for horses only predict a higher incidence of disease in a population rather than in individuals.
Several risk factors for colic have been accepted by virtue of common recognition or have been based on numerous anecdotal reports. Breed can be a risk factor. For example, Standardbred stallions are known to be at higher risk for inguinal hernias. In that case, the Standardbred breed is a marker of the real cause of the problem, which is an increased size of the inguinal ring. Although this is an accepted risk factor, there is no information to tell just how much of an increased risk for inguinal hernia there is for a Standardbred stallion compared to a Thoroughbred stallion.
Geographic environment also can be associated with increased risk of certain diseases. As an example, horses have a higher risk of enteroliths when they live in certain parts of California. Impactions in the ileum (the last segment of the small intestine) are much more common in the southeastern and Gulf states compared to other parts of the United States.
Other commonly accepted risk factors include young age for meconium impaction in foals (one to five days of age); large colon torsion in aged pregnant mares; gas colic associated with ingesting rapidly growing grass; strangulating lipoma in horses older than 12 years; inguinal hernia in Standardbred, Saddlebred, Tennessee Walking Horse, and Warmblood breeds; and small colon obstruction in ponies.
Several epidemiologic studies on colic have helped identify certain risk factors for colic. These projects included case control studies from Northeastern Universities,6 veterinary practices in Texas,1 a prospective study of farms in Virginia and Maryland,7,8 a study of a horse farms by Michigan State University3, and colic in veterinary practices in the United Kingdom.5 Numerous factors were examined and all supported several common risk factors.
All studies found that horses having a history of colic are at higher risk for more colic episodes. Also, horses which had previous abdominal surgery are at higher risk of repeat colic, which in most cases was felt to be due to adhesions or bowel scarring with stricture.
Arabian horses are reported to have a higher risk of colic than other horses in the Northeast University study and in the Texas study. This also has been reported in other hospital studies. It has not been determined whether this is directly inherent in the breed or due to differences in management or use.
Parasite infection is assumed to increase the risk of colic; however, experimental infection of horses does not always cause colic. A study in the United Kingdom showed that tapeworm infection markedly increases the risk of colic due to diseases in the ileum and cecum. Small strongyles also increase the risk of colic.
Feeds or feeding activity has been blamed for colic. Moldy grain or hay has been suggested as a cause of colic, and while this makes sense, it is rare to document cases due to moldy feed. Anecdotal reports incriminate certain types of hay and lush pasture as risk factors. Practitioners associate a change in diet and exposure to lush grass with an increased frequency of gas colic. Coastal Bermuda hay has been identified as a risk factor for ileal impactions in a recent study at Texas A&M University. Other types of hay have not been found to be risk factors for colic, but changes in diet-both grain and hay-were related to increased risk of colic in both the Texas and the Virginia-Maryland studies. Gradual change to new types or amounts of feed has been recommended for many years and is supported by those findings.
There still is little information about specific types of feed, or the amounts of specific nutrients in the feed (such as minerals or fiber) in relation to colic. Equine nutrition requires more research, as the factors with the greatest risk are associated with feeding, including type of diet, changes in diet, and the amount of grain.
Overfeeding of grain has been reported to cause colic. Both the Virginia-Maryland study and the Texas studies found increased colic risk in horses when the type of grain or hay was changed. A recent study at Texas A&M found a very high risk after acutely changing the hay diet.
In the Virginia-Maryland report, feeding horses grain (specifically sweet feed mixes and pelleted feeds) increases the odds of their having colic. Colic risk increases when the amount of grain fed daily is higher. Feeding from 5.5 to 11 pounds of grain/day and greater than 11 pounds of grain/day to adult horses increased the risk of colic 4.8 and 6.3 times, respectively, compared to feeding horses no grain. Although this information helps identify risk associated with the horse's diet, the relationship of colic risk to the type of grain fed needs further investigation prior to making conclusions about a causal relationship. A horse should have 60% or more of its diet from forage to help reduce the reliance on grains.
Feeding small amounts of grain at frequent intervals has been reported to reduce the fluid shifts in the large colon. These occur with twice daily grain feedings. Although no relationship was found between feeding frequency and colic in the Northeast University study, the Virginia-Maryland study found that feeding more than twice daily increases the risk of colic.8 This is suspected to be due to an increased daily intake of grain rather than the frequency of feeding. As with other risk factors involving the diet, more research is needed before specific recommendations for a feeding schedule can be made.
Housing and confinement on the farms, according to the Virginia-Maryland study, are not risk factors for colic. However, other reports have suggested there is an increased risk of cecal and large colon impaction in horses which have acute decreases in activity, such as curtailing regular exercise or changing from turn-out activity to strict stall confinement due to an injury.2 This was supported by studies at Texas A&M University, which found that changes in activity are a risk factor for colic.1 Depression or reduce fecal production can be early signs in those cases.
The Northeast University study found that horses on pasture are significantly more likely to have colic compared to stall confined horses, although the risk of colic in pastured horses decreases if horses have recent access to two or three different pastures. This risk of pasture was not confirmed by the other studies. The Northeast University study also reported that turnout in a paddock without water even for a short period of time increases the risk of colic. It is difficult to prove that decreased water intake is related to colic incidence, but common sense would suggest that horses not ingesting adequate water on a daily basis will have increased risk of intestinal or systemic abnormalities.
To date, studies have not fully appraised the risks associated with exercise or activity level. Although horses used for racing or eventing had the highest incidence of colic in the Virginia-Maryland study, those activities did not pose an increased risk when compared to other factors, such as diet.
Management factors are difficult to compare among farms, and changes in management are even more difficult to detect accurately. The increased risk associated with care by trainers and managers compared to owners in the Northeast University study suggests that differences in management are important in the cause of colic.
Several factors appear to reduce the risk of colic. Horses used for lessons in riding stables have a lower incidence of colic than horses used for other activities. Studies in Texas suggest that a lower horse density on pasture and regular worming are associated with a lower risk of colic. Generally, horses turned out on pasture 24 hours a day and with no grain have the lowest risk of colic in the Virginia-Maryland study.
Event-Associated Colic Risk
Pregnancy-The mare has been reported to have a higher risk for colon displacement or volvulus during late pregnancy or lactation. However, all studies have been based on selected populations of mares or in regions with high numbers of broodmares.
When compared by random time for exposure for events in the Virginia-Maryland study, mares had an increased risk of colic from 60-150 days after foaling.7 The physiologic events that predispose to this increased risk are not known, but changing calcium levels during lactation and alterations in diet-including increases in energy in the diet-might be related to this increased risk.
Horse transport-Shipping increases colic risk in a study by Uhlinger9 and in the Virginia-Maryland study.7 This has been suspected by practitioners, who commonly administer laxatives prior to shipping to prevent impaction colic. The mechanism or cause of the increased incidence is unknown. Combining another risk with transport, such as feeding grain or cribbing, markedly increases colic risk.
Fever-It is logical that horses with an infection could have alteration of the intestine, predisposing them to colic. Fever within 14 days of a colic episode was highly associated with increased risk of colic in the Virginia-Maryland study.8 Since the reported causes of fever in those cases were varied, no specific relationship or cause for each colic was established.
Vaccination-When examining horses for the entire year of the Virginia-Maryland study, there was an increased risk of colic due to Potomac horse fever vaccination.8 When cases and controls were examined during a period of 14 days after any vaccination, there was a significant increase in risk of colic after all types of vaccination. Vaccination previously has not been identified as a risk factor and was not found to increase colic risk in the Texas study. Vaccination as a possible risk factor requires further study, including the possibility that a systemic reaction from a vaccination is sufficient to increase the risk of a colic episode. This increase in risk was small and much less than the risk of infection if a horse were not vaccinated.
Weather-Owners and veterinarians frequently associate weather changes with increased frequency of colic, but previous reports have been unable to find statistical proof of an association. When examined as a direct exposure in the Virginia-Maryland study, weather did not appear to be related to colic. When events were investigated by looking at a 14-day window preceding colic episodes in both horses with colic and controls, low humidity and snow marginally increased colic risk. There is no explanation for that relationship unless weather is a confounder (a false risk factor) for management changes, such as owners altering activity during weather extremes. Further work is needed to look at a combination of factors involving weather in a large number of horses.
Nutrition-The mechanism by which grain excess causes intestinal problems is not known, but is speculated to be the result of feeding high levels of soluble carbohydrate in grain. The increased carbohydrate can alter the microbial population, hydration, or pH of the feed material in the cecum or colon.
Miscellaneous-Several events that have been reported or suspected to cause colic, but which were not found to be significant risks in any of the studies, include recent anthelmintic administration and vices such as cribbing. Enough horses received anthelmintics during these studies to confirm that there is no increased risk in horses receiving regularly scheduled medication for parasite control. Cribbing, on the other hand, is considered to predispose to flatulent colic, but the incidence of cribbing was not high enough in the Virginia-Maryland population to make it an increased risk when compared to the other factors.
When risk was examined in the Virginia-Maryland study by combining event factors, feeding grain combined with vices and feeding grain combined with transport increased colic risk approximately eight and nine times, respectively, compared to not exposing horses to those combinations. Only a few of those factors were combined and examined by analysis in this study. Other factors that cause colic might be important and deserve further investigation.
Author's Note: This article has summarized information from a chapter entitled "Risk Factors Associated with Colic" by N.A. White in Current Therapy in Equine Medicine, Editor N.E. Robinson, published by W.B. Saunders, Philadelphia, 1997; pp. 174-179.
Cohen, N.D., Matejka, P.L., Honnas, C.M., Hooper, R.N., and Texas Equine Colic Study Group. Case control study of the association of between various management factors and development of colic is horses. J. Am. Vet. Med. Assoc., 206:667-673, 1995.
Dabareiner, R.M., White, N.A. Large colon impaction in horses: 147 cases (1985-1991). J. Am. Vet. Med. Assoc., 206:679-685, 1995.
Kananee J.B., Miller R.A., Ross W.A., Gallagher K., Marteniuk J., Rook J. Risk Factors for Colic in Michigan Equine Population. Preventive Vet Med 1997; 30:23-26.
Murray, M.J. Gastric Ulcers in Adult Horses. Compend. Cont. Ed. Pract. Vet., 16:792-794, 1994.
Proudman, C.J. A two year, prospective survey of equine colic in general practice. Equine Veterinary Journal 1992;24:90-93.
Reeves, M.J., Salman, M., Smith, G. 1996 Risk factors for equine acute abdominal disease (colic): Results from a multi-center case-control study. Preventive Vet. Med. 26:285-301.
Tinker, M.K., White, N.A., Lessard, P., Thatcher, C.D., Pelzer, K.D., Davis, B., Carmel, D.K. Prospective Study of Equine Colic Incidence and Mortality. Equine Vet J. 1997; 29:448-453.
Tinker, M.K., White, N.A., Lessard, P., Thatcher, C.D., Pelzer, K.D., Davis, B., Carmel, D.K. Prospective Study of Equine Colic Risk Factors. Equine Vet J. 1997; 29:454-458.
Uhlinger, C. Investigations into the incidence of field colic. Equine Vet. J. (Suppl 13):16-18, 1992.
White, N.A. Epidemiology and etiology of colic. In: White, N.A.(ed). The Equine Acute Abdomen, Philadelphia, Lea and Febiger, 1990, pp. 49-64.
About the Author
Nat White, DVM, DVM, MS, Dipl. ACVS, is the Jean Ellen Shehan Professor and Director at Virginia Tech's Marion duPont Scott Equine Medical Center. After receiving a doctor of veterinary medicine at Cornell University in 1971, he completed an internship and residency in surgery at the University of California-Davis from 1971 to 1973, and earned a master of science in pathology at Kansas State University in 1976. He is a Diplomate of the American College of Veterinary Surgeons (ACVS). Dr. White, who has served on the faculties of both Kansas State University and the University of Georgia, joined the Marion duPont Scott Equine Medical Center in 1985, and held the position of Theodora Ayer Randolph Professor of Surgery at Virginia Tech from 1987 to 2003. A world-renowned expert in colic, Dr. White has authored several books on the topic including Equine Acute Abdomen and Handbook of Equine Colic as well as the surgical texts Current Techniques in Equine Surgery and Lameness and Current Practice of Equine Surgery. He has been a director for the ACVS Veterinary Symposium since 1997, and is a past president of the ACVS and of the ACVS Research and Education Foundation. Dr. White is a former director-at-large for the American Association of Equine Practitioners (AAEP) and is currently AAEP President. Dr. White's research interests include pathophysiology of ischemia-reperfusion, epidemiology of colic, abdominal and orthopedic surgery, and treatment of orthopedic diseases. He is a member of the AAEP, the ACVS and the American Veterinary Medical Association.
POLL: Picky Eaters