Colic in horses is defined as abdominal pain, but it is a clinical symptom rather than a diagnosis. The term colic can encompass all forms of gastrointestinal conditions which cause pain as well as other causes of abdominal pain not involving the gastrointestinal tract. What makes it tricky is that different causes can manifest with similar signs of distress in the animal. Recognizing and understanding these signs is pivotal, as timely action can spell the difference between a brief moment of discomfort and a life-threatening situation. The most common forms of colic are gastrointestinal in nature and are most often related to colonic disturbance. There are a variety of different causes of colic, some of which can prove fatal without surgical intervention. Colic surgery is usually an expensive procedure as it is major abdominal surgery, often with intensive aftercare. Among domesticated horses, colic is the leading cause of premature death. percent over the course of the average lifespan. Clinical signs of colic generally require veterinary evaluation and treatment. The conditions that cause colic can become life-threatening in a short period of time.
Pathophysiology
Colic can be divided broadly into several categories:
- excessive gas accumulation in the intestine (gas colic)
- simple obstruction
- strangulating obstruction
- non-strangulating infarction
- inflammation of the gastrointestinal tract (enteritis, colitis) or the peritoneum (peritonitis)
- ulceration of the gastrointestinal mucosa
These categories can be further differentiated based on location of the lesion and underlying cause (See Types of colic).
Simple obstruction
This is characterised by a physical obstruction of the intestine, which can be due to impacted food material, stricture formation, or foreign bodies. The primary pathophysiological abnormality caused by this obstruction is related to the trapping of fluid within the intestine oral to the obstruction. This is due to the large amount of fluid produced in the upper gastrointestinal tract, and the fact that this is primarily re-absorbed in parts of the intestine downstream from the obstruction. The first problem with this degree of fluid loss from circulation is one of decreased plasma volume, leading to reduced cardiac output, and acid-base disturbances.
The intestine becomes distended due to the trapped fluid and gas production from bacteria. It is this distension, and subsequent activation of stretch receptors within the intestinal wall, that leads to the associated pain. With progressive distension of the intestinal wall, there is occlusion of blood vessels, firstly the less rigid veins, then arteries. This impairment of blood supply leads to hyperemia and congestion, and ultimately to ischaemic necrosis and cellular death. The poor blood supply also has effects on the vascular endothelium, leading to an increased permeability which first leaks plasma and eventually blood into the intestinal lumen. In the opposite fashion, gram-negative bacteria and endotoxins can enter the bloodstream, leading to further systemic effects. Other causes of strangulating obstruction are intussusceptions, torsion or volvulus, and displacement of intestine through a hole, such as a hernia, a mesenteric rent, or the epiploic foramen. Foals, however, may show signs of gas within the small intestines with severe abdominal distention. This allows the impaction to be treated without actually cutting into the ileum. Prognosis for survival is good. This impaction may be missed since decreased manure production can be attributed secondarily to surgery, and often rupture occurs before severe signs of pain. Surgery has a good prognosis, although rupture can occur during surgical manipulation. The cause of cecal impactions are not known. and occur when food is not cleared at the appropriate rate. It is most commonly associated with ingestion of foods that swell after eating or feeds that are coarse (bedding or poor quality roughage), poor dental care, poor mastication, inadequate drinking, ingestion of a foreign object, and alterations in the normal function of the stomach. Persimmons, which form a sticky gel in the stomach, and haylage, have both been associated with it, Horses usually show signs of mild colic that is chronic, unresponsive to analgesics, and may include signs such as dysphagia, ptyalism, bruxism, fever, and lethargy, Other gastric impactions are often resolves with enteral fluids.
Small colon impaction
Small colon impactions represent a small number of colics in the horse, and are usually caused by obstruction from fecaliths, enteroliths, and meconium. Horses usually present with standard colic signs (pawing, flank watching, rolling) in 82% of horses, and occasionally with diarrhea (31%), anorexia (30%), straining (12%), and depression (11%), and rectal examination will reveal firm loops of small colon or actually palpable obstruction in the rectum.
- Right dorsal displacement: the colon moves between the cecum and body wall.
- The pelvic flexure retroflexes towards the diaphragm
- The colon develops a volvulus of 90 to 180 degrees, minimally occluding the vasculature of the organ.
The cause of displacement is not definitively known, but one explanation is that the bowel becomes abnormally distended with gas (from excessive fermentation of grain, a change in the microbiota secondary to antibiotic use, or a buildup of gas secondary to impaction) which results in a shift in the bowel to an abnormal position. Because much of the bowel is not anchored to the body wall, it is free to move out of position. Displacement is usually diagnosed using a combination of findings from the rectal exam and ultrasonography.
Many displacements (~96% of LDD, 64% of RDD) NSAIDs can cause slower-onset of colitis, usually in the right dorsal colon (see Right dorsal colitis).
Treatment involves administration of large volumes of intravenous fluids, which can become very costly. Antibiotics are often given if deemed appropriate based on the presumed underlying cause and the horse's CBC results. Therapy to help prevent endotoxemia and improve blood protein levels (plasma or synthetic colloid administration) may also be used if budgetary constraints allow. Other therapies include probiotics and anti-inflammatory medication. Horses that are not eating well may also require parenteral nutrition. Horses usually require 3–6 days of treatment before clinical signs improve.
Due to the risk of endotoxemia, laminitis is a potential complication for horses suffering from colitis, and may become the primary cause for euthanasia. Horses are also at increased risk of thrombophlebitis.
Gastric ulceration
Horses form ulcers in the stomach fairly commonly, a disease called equine gastric ulcer syndrome. Risk factors include confinement, infrequent feedings, a high proportion of concentrate feeds, such as grains, excessive non-steroidal anti-inflammatory drug use, Horses develop immunity to parascarids between 6 months age and one year and so this condition is rare in adult horses. Prognosis is fair unless the foal experiences hypovolemia and septic shock, with a survival rate of 33%.
Cyathostomes
Acute diarrhea can be caused by cyathostomes or "small Strongylus-type" worms that are encysted as larvae in the bowel wall, particularly if large numbers emerge simultaneously. The disease most frequently occurs in winter time. Pathological changes of the bowel reveal a typical "pepper and salt" color of the large intestines. Animals suffering from cyathostominosis usually have a poor deworming history. There is now a lot of resistance to fenbendazole in the UK.
Large strongyles
Large strongyle worms, most commonly Strongylus vulgaris, are implicated in colic secondary to non-strangulating infarction of the cranial mesenteric artery supplying the intestines, most likely due to vasospasm. It is normally passed within the first 24 hours of the foal's life, but may become impacted in the distal colon or rectum.
Congenital abnormalities
Atresia coli and atresia ani can also present as meconium impaction. The foal is missing the lumen of its distal colon or anus, respectively, and usually show signs of colic within 12–24 hours. Atresia coli is usually diagnosed with barium contrast studies, in which foals are given barium, and then radiographed to see if and where the barium is trapped. Atresia ani is simply diagnosed with digital examination by a veterinarian. Both situations requires emergency surgery to prevent death, and often still has a poor prognosis for survival with surgical correction.
Infectious organisms
Clostridial enterocolitis due to infection by Clostridium perfringens is most commonly seen in foals under 3 months of age. Clostridial toxins damage the intestine, leading to dehydration and toxemia. Foals usually present with signs of colic, decreased nursing, abdominal distention, and diarrhea which may contain blood. Diagnosis is made with fecal culture, and while some foals do not require serious intervention, others need IV fluids, antibiotics, and aggressive treatment, and may still die. Other bacterial infections that may lead to enterocolitis include Salmonella, Klebsiella, Rhodococcus equi, and Bacteroides fragilis.
Parasitic infection, especially with threadworms (Strongyloides westeri) and ascarids (Parascaris equorum) can produce signs of colic in foals (See Ascarids). Other conditions that may lead to signs of colic in foals include congenital abnormalities, gastric ulcers (see Gastric ulceration), which may lead to gastric perforation and peritonitis, small intestine volvulus, and uroabdomen secondary to urinary bladder rupture. Pink or cyanotic (blue) membrane colors are associated with a greater chance of survival (55%). Both injected mucous membranes and the presence of a toxic line correlate to a decreased likelihood of survival, at 44%.
{| class="wikitable"
|-
! Percent Dehydration !! Heart rate !! Mucous membrane quality !! CRT !! Time skin tent holds !! Other
|-
| 5% || Normal || Moist to slightly tacky || < 2 seconds || 1–3 seconds || Decrease in urine production
|-
| 8% || 40-60 bpm || Tacky || Usually 2–3 seconds || 3–5 seconds || Decrease in blood pressure
|-
| 10-12% || 60+ bpm || Dry || Usually > 4 seconds || 5+ seconds || Decrease in jugular fill and quality of peripheral pulses; sunken eyes present
|}
Rectal temperature can help ascertain if an infectious or inflammatory cause is to blame for the colic, which is suspected if the temperature if >103F. Temperature should be taken prior to rectal examination, as the introduction of air will falsely lower rectal temperature. Coolness of extremities can indicate decreased perfusion secondary to endotoxemia. Elevated respiratory rate can indicate pain as well as acid-base disturbances. Approximately 40% of the gastrointestinal tract can be examined by rectal palpation, although this can vary based on the size of the horse and the length of the examiner's arm.
Abdominocentesis allows for the evaluation of red and white blood cells, hemoglobin concentration, protein levels, and lactate levels. A high lactate in abdominal fluid suggests intestinal death and necrosis, usually due to strangulating lesion, and often indicates the need for surgical intervention. A strangulating lesion may produce high levels of red blood cells, and a serosanguinous fluid containing blood and serum.
Clinical analysis is not necessarily required to analyze the fluid. Simple observation of color and turbidity can be useful in the field.
- Sanguinous fluid indicates an excess of red blood cells or hemoglobin, and may be due to leakage of the cells through a damaged intestinal wall, splenic puncture during abdominocentesis, laceration of abdominal viscera, or contamination from a skin capillary.
- Colorless (dilute) peritoneal fluid, especially in large quantities, can indicate ascites or uroperitoneum (urine in the abdomen).
Clinical signs
Clinical signs of colic are usually referable to pain, although the horse may appear depressed rather than painful in cases of necrosis (tissue death) of the gastrointestinal tract, inflammation of the intestines, endotoxemia, or significant dehydration. Heart rate is often used as a measure of the animal's pain level and a heart rate >60 bpm is more likely to require surgery. However, this measure can be deceiving in the early stages of a severe colic, when the horse may still retain a relatively low rate.
- Increased attention toward the abdomen, including flank watching (turning of the head to look at the abdomen and/or hind quarters), nipping, biting, or kicking
- Poor coat or weight loss (chronic colic) can be successfully managed medically.
Analgesia and sedation
The intensity of medical management is dependent on the severity of the colic, its cause, and the financial capabilities of the owner. At the most basic level, analgesia and sedation is administered to the horse. The most commonly used analgesics for colic pain in horses are NSAIDs, such as flunixin meglumine, although opioids such as butorphanol may be used if the pain is more severe. Endotoxemia occurs when there is an overgrowth and secondary die-off of gram negative bacteria, releasing mass quantities of endotoxin. This is especially common when the mucosal barrier is damaged, as with ischemia of the GI tract secondary to a strangulating lesion or displacement. However, surgery may be required to save the life of the horse, and 1–2% of all colics require surgical intervention. This injury causes neutrophils to move into the serosa and mesothelium to be lost, which the body then attempts to repair using fibrin and collagen, leading to adhesion formation between adjacent tissues with either fibrinous or fibrous material. and requiring further surgery and risk of adhesion. Generally, adhesions form within the first two months following surgery. It is recommended that a horse receive ideally 18 hours of grazing time each day,