Acute colitis is a sudden inflammation of the colon’s inner lining that causes bloody diarrhea, abdominal pain, and an urgent need to use the bathroom. It can be triggered by an infection, a loss of blood flow to the colon, or a flare of an underlying inflammatory bowel disease. The word “acute” distinguishes it from chronic, long-simmering inflammation: acute colitis comes on fast, often over hours to days, and typically demands prompt evaluation to identify the cause and prevent complications.
Main Causes of Acute Colitis
The three broad categories are infectious, ischemic (reduced blood flow), and inflammatory bowel disease (IBD) flares. Each one inflames the colon in a different way and requires different treatment, which is why pinpointing the cause matters so much.
Infectious Colitis
Bacteria like Salmonella, E. coli, and Clostridioides difficile (C. diff) are the most common culprits. Salmonella and E. coli usually come from contaminated food or water. C. diff is different: it already lives in the intestines at low levels, but when antibiotics wipe out competing bacteria, C. diff can overgrow and release toxins that damage the colon lining. CDC surveillance data from 2023 recorded about 117 C. diff cases per 100,000 people at monitored sites, making it one of the most significant hospital-associated infections in the country. Viral and parasitic infections can also trigger acute colitis, though they’re less common.
Ischemic Colitis
When blood flow to part of the colon drops suddenly, the tissue becomes inflamed and can start to die. This happens most often in adults over 60, particularly women, and is linked to conditions that narrow blood vessels: atherosclerosis, blood clots, low blood pressure, or irregular heart rhythms like atrial fibrillation. Pain on the right side of the abdomen is a more serious sign, associated with higher rates of surgery and worse outcomes overall. In younger people, ischemic colitis can signal an underlying clotting disorder or blood vessel inflammation that needs further workup.
IBD Flares
People with ulcerative colitis or Crohn’s disease can experience acute flares where the immune system attacks the colon’s own tissue. In these conditions, the normal balance between immune cells that fight threats and those that keep inflammation in check breaks down. The result is a flood of inflammatory signals that damage the colon lining. Complicating matters, people with IBD are also more vulnerable to infections, especially C. diff, which can trigger or worsen a flare at the same time.
Symptoms and How Severity Is Graded
The hallmark symptoms are bloody stool, frequent loose bowel movements, abdominal cramping (often on the left side), and a feeling of urgency. In mild cases, you might notice some rectal bleeding and have fewer than four bowel movements a day. Moderate colitis means more than four bloody bowel movements daily. Severe colitis adds systemic signs on top of that: fever, rapid heart rate, weight loss, and low protein levels in the blood.
In mild disease, a physical exam can be almost normal aside from slight tenderness in the lower left abdomen. As severity increases, tenderness becomes more widespread, and signs of dehydration, such as dizziness and low urine output, may appear. Sudden, severe abdominal pain that makes it impossible to sit still or find a comfortable position is a red flag that warrants emergency care.
How Acute Colitis Is Diagnosed
Figuring out the cause is as important as confirming the inflammation itself, because the treatment path depends entirely on whether you’re dealing with an infection, lost blood flow, or an autoimmune flare.
Stool tests come first. Samples are checked for bacteria, parasites, and C. diff toxins. One important caveat: stool cultures identify a bacterial cause in only 40 to 60 percent of cases, so a negative culture doesn’t rule out infection. A very high white blood cell count can specifically point toward C. diff. Stool markers that measure inflammation, like calprotectin, help distinguish inflammatory conditions from something like irritable bowel syndrome, though they don’t identify the specific cause on their own.
CT scans provide detailed images that help radiologists distinguish between types of colitis based on recognizable patterns. Infectious colitis tends to involve the colon continuously (without skipping sections) and shows an “empty colon” appearance, with relatively little surrounding fat inflammation. Ischemic colitis, by contrast, often affects segments with gaps between them and shows more fat inflammation around the colon wall. IBD-related colitis has its own signature: enlarged lymph nodes, a “comb sign” (a pattern of engorged blood vessels feeding the inflamed bowel), and sometimes involvement of the small intestine as well.
Colonoscopy or a limited scope of the lower colon may follow, particularly when distinguishing a first IBD flare from an infection. The ulcer patterns look different under direct visualization: Crohn’s disease creates distinctive linear ulcers that infections rarely mimic, while ulcerative colitis typically causes continuous inflammation starting at the rectum.
Treatment by Cause
Because the causes are so different, there is no single treatment for acute colitis. The approach depends entirely on what’s driving the inflammation.
For most infectious colitis, treatment centers on hydration and letting the infection run its course. C. diff is an exception and requires targeted antibiotics. Importantly, broad-spectrum antibiotics are not recommended as routine treatment for severe inflammatory colitis, per current gastroenterology guidelines, because they don’t help and can make C. diff worse.
For severe ulcerative colitis flares requiring hospitalization, high-dose intravenous steroids are the standard first-line treatment. These work by dialing down the overactive immune response damaging the colon. Patients admitted with a severe flare should also be tested for C. diff at the time of arrival, since concurrent infection is common and changes the treatment plan.
Ischemic colitis management focuses on restoring blood flow and supporting the colon while it heals. Mild cases often resolve on their own with intravenous fluids and bowel rest. More severe cases, especially right-sided ones, may need closer monitoring and intervention if the tissue starts to die.
Nutrition During Acute Colitis
During the acute phase, the colon is too inflamed to handle its normal workload. If you can eat, soft, low-fiber foods are generally easier to tolerate. Raw fruits, vegetables, and high-fiber foods can irritate inflamed tissue, particularly if there’s any narrowing of the colon. Cooking and processing produce to a soft consistency makes a meaningful difference in tolerability.
In severe cases where the digestive tract can’t absorb enough nutrition, liquid nutrition formulas delivered through a tube (enteral nutrition) or nutrition given intravenously (parenteral nutrition) may be necessary. The American Gastroenterological Association recommends parenteral nutrition specifically for situations like high-output fistulas, prolonged inability of the gut to move food along, short bowel syndrome, or severe malnutrition when oral and tube feeding have failed.
Complications to Watch For
The most dangerous complication of acute colitis is toxic megacolon, a condition where the colon stops contracting, fills with gas, and dilates beyond 6 centimeters. Diagnosis requires that dilation plus at least three of the following: fever over 100.4°F, heart rate above 120 beats per minute, elevated white blood cell count, or anemia. On top of those, at least one additional sign must be present: dehydration, electrolyte imbalance, altered mental state, or low blood pressure. Toxic megacolon is a medical emergency.
Other life-threatening complications include perforation (a hole in the colon wall) and massive hemorrhage. Both are rare but require immediate surgery. The standard emergency operation is removal of the affected colon. For ulcerative colitis specifically, colonic perforation, uncontrolled bleeding, and toxic megacolon that doesn’t respond to medical therapy are the established triggers for emergency colectomy.
How Outcomes Differ by Type
Most infectious colitis resolves within days to a couple of weeks, though C. diff can recur in some people and require repeated treatment courses. Ischemic colitis in its mild, left-sided form often heals on its own, but right-sided disease carries a significantly higher risk of surgery and death. IBD-related acute colitis is the most likely to become a recurring problem, since the underlying immune dysfunction doesn’t go away once the flare subsides. After an initial severe flare, long-term maintenance therapy is typically needed to reduce the chance of another episode.