CMV colitis is an infection of the colon caused by cytomegalovirus, a common herpes-family virus that most people carry without symptoms. The infection becomes dangerous when the virus reactivates in the colon’s lining, triggering inflammation, ulceration, and symptoms like bloody diarrhea and abdominal pain. It primarily affects people with weakened immune systems, though it can occasionally strike otherwise healthy individuals.
How CMV Damages the Colon
Cytomegalovirus is carried by a large portion of the adult population. In most people, the immune system keeps it in a dormant state indefinitely. Problems begin when immune defenses drop low enough for the virus to reactivate and begin replicating in tissue.
In the colon, the virus targets immune cells called macrophages that normally patrol the gut lining. These macrophages are usually kept in a calm, non-inflammatory state by signals from surrounding tissue. CMV overrides that calming process. Infected cells retain the ability to pump out inflammatory molecules even in the presence of signals that would normally shut them down. The result is localized, persistent inflammation in the colonic wall, leading to tissue damage, ulceration, and bleeding.
Who Is at Risk
CMV colitis occurs overwhelmingly in people whose immune systems are compromised. The highest-risk groups include:
- HIV/AIDS patients with severely depleted immune cell counts
- Organ and bone marrow transplant recipients on immunosuppressive medications
- Cancer patients undergoing chemotherapy
- People on long-term corticosteroids or other immune-suppressing drugs
- People with inflammatory bowel disease (IBD), particularly ulcerative colitis
The connection to ulcerative colitis deserves special attention. In patients with moderate to severe ulcerative colitis, CMV colitis is found in 25 to 30% of cases, especially among those whose flares don’t respond to steroids. Pre-existing bowel inflammation damages the colon’s protective lining, creating conditions that favor viral reactivation. High cumulative steroid doses (above 400 mg over four weeks) and disease that extends across the entire colon both increase the risk further. This makes CMV a key suspect when a UC flare worsens despite aggressive treatment.
Rarely, CMV colitis develops in people with no known immune deficiency. These cases tend to occur in older adults or people with other serious illnesses like kidney failure.
Symptoms
CMV colitis causes both digestive and whole-body symptoms. The most common include diarrhea (often bloody), abdominal pain, an urgent or constant need to use the bathroom, and a sensation of incomplete evacuation. Fever, fatigue, and weight loss frequently accompany the gastrointestinal symptoms. Because these overlap heavily with an ulcerative colitis flare or other forms of infectious colitis, the diagnosis is easy to miss without specific testing.
How CMV Colitis Is Diagnosed
Diagnosis requires looking directly at colon tissue, typically obtained during a colonoscopy. During the procedure, the colon’s lining often shows characteristic damage: deep ulcers, “punched-out” ulcers that look like small craters stamped into the tissue, geographic ulcers with irregular borders, and longitudinal ulcers running along the bowel wall. In patients with ulcerative colitis, punched-out ulcers are particularly telling, with one study finding they were roughly 12 times more likely to appear in CMV-positive patients.
Biopsy samples from these ulcers are examined under a microscope for the hallmark sign of CMV: “owl’s eye” inclusion bodies. These are large, dark-staining structures inside the nuclei of infected cells, surrounded by a clear halo that gives them a distinctive owl-like appearance. When present, they are considered definitive proof of CMV infection.
However, these inclusion bodies don’t always show up. The European Crohn’s and Colitis Organisation recommends two more sensitive methods: immunohistochemistry (IHC), which uses antibodies to stain CMV proteins in tissue, and tissue PCR, which detects viral DNA. Tissue PCR is more sensitive than IHC and produces faster results. In one study, IHC came back negative in all seven patients tested, while PCR detected viral DNA in several of those same biopsies. A blood test for CMV DNA can support the diagnosis but isn’t reliable on its own. In one analysis, blood PCR detected the virus in only 55% of patients with confirmed tissue infection.
Complications
Left untreated, CMV colitis can progress to life-threatening complications. The colon can bleed massively from deep ulcers eroding into blood vessels. Toxic megacolon, where the colon dilates dangerously and loses its ability to contract, occurs in roughly 13% of CMV colitis cases. This is one of the most feared outcomes because it can lead to perforation, where the colon wall ruptures and spills its contents into the abdominal cavity, requiring emergency surgery.
Treatment and Resistance
The primary treatment is antiviral medication targeting the virus’s ability to replicate. For most patients, this means an intravenous antiviral given in a hospital setting, followed by oral medication as symptoms improve. Treatment courses typically last several weeks.
Some patients develop drug-resistant CMV, most commonly through mutations in a gene called UL97. Resistance is more likely after prolonged treatment. When first-line therapy fails, alternative antivirals are available, though they carry a higher risk of kidney damage (reported in 14 to 78% of patients depending on the drug). A newer agent, maribavir, has emerged as a preferred option for resistant cases in transplant recipients because of its better safety profile. However, cross-resistance can develop: a specific mutation has been identified that causes high-level resistance to maribavir while also reducing sensitivity to other antivirals.
For patients with ulcerative colitis, treatment decisions are more complex. Antiviral therapy needs to be balanced against the ongoing management of the underlying bowel disease. Steroids, which are a mainstay of UC treatment, can worsen CMV reactivation. Identifying CMV early in a steroid-refractory flare can change the treatment approach entirely, shifting the focus from escalating immune suppression to fighting the viral infection.