What Causes Oral Thrush?
Oral thrush is caused by overgrowth of Candida species, predominantly Candida albicans, which normally exists as a commensal organism in the oral cavity but becomes pathogenic when host defenses are compromised or the oral microflora balance is disrupted. 1, 2
Primary Causative Organisms
- Candida albicans accounts for the vast majority of oral thrush cases, existing as a commensal in up to two-thirds of healthy individuals' oral cavities 2, 3
- Non-albicans species including C. glabrata, C. dubliniensis, C. tropicalis, and C. krusei cause infection particularly in refractory cases or after repeated azole exposure 1, 2, 3
Key Risk Factors by Population
Immunocompromised Patients
- HIV/AIDS with CD4+ counts <200 cells/μL represents the most significant risk, with oropharyngeal candidiasis recognized as an indicator of immune suppression 4, 3
- Approximately 30% of HIV-positive patients develop oral candidiasis, with the highest risk occurring when CD4 counts fall below 50-200 cells/μL 3
- Malignancies, particularly hematologic cancers like leukemia, substantially increase susceptibility 2
Medication-Related Causes
- Corticosteroid use (both systemic and inhaled) disrupts local immune defenses and promotes fungal overgrowth 2, 3
- Broad-spectrum antibiotic therapy eliminates competing bacterial flora that normally inhibit Candida proliferation 2, 3
- Proton pump inhibitor use has been identified as a risk factor for candida esophagitis 4
- Immunosuppressive medications significantly increase susceptibility to Candida overgrowth 3
Elderly Population-Specific Factors
- In elderly long-term care residents, 84% are colonized with yeast, making them particularly vulnerable when predisposing factors are present 2
- Denture use creates an environment conducive to Candida growth and requires both antifungal therapy and proper denture disinfection 2, 5
- Impaired salivary gland function and poor oral health contribute to increased risk 5
Diabetic Patients
- Diabetes mellitus increases risk through impaired glycemic control, local immunosuppression, and altered host defenses 3, 6
Infants
- Oral thrush commonly occurs in newborns due to immature immune systems 7
Additional Predisposing Factors
- Recent antibiotic use disrupts normal oral flora balance 4
- Chronic alcohol use increases susceptibility 4
- Chronic kidney disease represents a systemic risk factor 4
- Motility disorders leading to esophageal stasis can predispose to candida esophagitis 4
- Radiation therapy to the head and neck increases risk 1
- Hyposalivation from any cause promotes candidal overgrowth 6
Clinical Pitfalls to Avoid
- Failure to identify underlying predisposing factors leads to treatment failure in 20-30% of cases 3
- In immunocompromised patients with oral thrush who report dysphagia, odynophagia, or retrosternal pain, suspect esophageal involvement (occurs in 10-20% of cases) 3
- Resistance development occurs in 10-20% of immunocompromised patients with prolonged or repeated fluconazole exposure, particularly with CD4 counts <50 cells/μL 3
- Non-albicans species should be considered in refractory disease, as these organisms may have intrinsic azole resistance 3
- The diagnosis is frequently missed due to similarity with other white lesions, requiring careful clinical examination 8