Is Most Colitis Viral?
No, most colitis is not viral—infectious colitis accounts for only 38% of acute colitis cases, and within infectious causes, bacterial pathogens (particularly Clostridioides difficile, Salmonella, Campylobacter, and Shigella) far outnumber viral etiologies. 1
Epidemiology of Colitis Etiologies
The distribution of colitis causes depends heavily on clinical context:
Acute Colitis Presentations
In patients presenting with acute hemorrhagic colitis symptoms, infectious colitis represents 38% of cases, with the remaining 62% attributed to inflammatory bowel disease (IBD), ischemic colitis, medication-induced colitis, and other non-infectious causes 1
Among infectious colitis cases, C. difficile accounts for approximately 22%, making it the single most common infectious pathogen 2
Bacterial pathogens dominate the infectious colitis landscape: Yersinia enterocolitica (46%), Campylobacter (20-28%), Salmonella (11-13%), and Shigella (9-21%) collectively represent the majority of infectious cases 2, 3
Viral Colitis: The Minority
Viral causes of colitis are uncommon and primarily affect immunocompromised patients 4:
Cytomegalovirus (CMV) colitis occurs predominantly in immunocompromised hosts (HIV patients, transplant recipients, those on immunosuppressive therapy) and represents only 4% of endemic infectious colitis cases in immunocompetent populations 5, 3
In acute severe ulcerative colitis (ASUC), viral enteropathogens (adenovirus, rotavirus, norovirus) were detected in only 14.9% of cases, and importantly, these viral infections did not alter disease severity, rescue therapy requirements, or colectomy rates 6
Norovirus and rotavirus cause gastroenteritis rather than true colitis, primarily affecting the small intestine rather than the colon 7
Clinical Implications for Diagnosis
When to Suspect Non-Viral Etiologies
The clinical presentation strongly suggests non-viral causes when:
Insidious onset over >1 week favors IBD (56% of IBD cases) rather than infectious colitis, which typically presents acutely within days 8
Bloody diarrhea with fever within the first week suggests bacterial infection rather than viral gastroenteritis 8
Endoscopic findings of continuous colonic involvement starting at the rectum indicate ulcerative colitis, while patchy/segmental disease suggests Crohn's disease or specific bacterial infections (Yersinia, CMV) 1, 2
Critical Testing Strategy
Stool studies remain essential but have significant limitations:
Stool cultures are positive in only 40-60% of bacterial infectious colitis cases, necessitating endoscopy with biopsy as an adjunct diagnostic tool 1
For patients with IBD flares, testing for C. difficile using a 2-step approach is mandatory: high-sensitivity test (glutamate dehydrogenase EIA or nucleic acid amplification) followed by high-specificity toxin detection 9
CMV testing (tissue immunohistochemistry or PCR) should be performed in hospitalized patients with acute severe colitis or steroid-refractory disease, as CMV superinfection increases colectomy risk and mortality 9
Stool lactoferrin or calprotectin (≈90% sensitivity) helps identify patients requiring endoscopy for suspected immune checkpoint inhibitor colitis 2
Common Pitfalls to Avoid
Do not assume viral etiology based solely on negative bacterial cultures, as culture sensitivity is poor and endoscopic/histologic evaluation may be required 1
Do not overlook CMV in immunocompromised patients or those with IBD on immunosuppression, as untreated CMV disease carries high morbidity and mortality 5
Do not dismiss the possibility of bacterial superinfection in established IBD patients, as co-existent enteric infections account for a significant proportion of disease flares 1
Histology alone cannot reliably exclude bacterial infection, particularly C. difficile, requiring microbiologic correlation 2