Causes of Oral Thrush
Oral thrush is primarily caused by overgrowth of Candida albicans, a fungus that normally exists harmlessly 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 majority of oral thrush cases and is found in the oral cavity of up to two-thirds of healthy individuals as part of normal flora 2
- Non-albicans Candida species can also cause oral thrush, particularly C. glabrata, C. dubliniensis, C. tropicalis, and C. krusei, especially in fluconazole-resistant or refractory cases 1, 2
Systemic Risk Factors
Immunosuppression
- HIV infection is a major risk factor, particularly when CD4+ counts fall below 200 cells/μL 1, 3
- Malignancies, especially leukemia and other hematologic cancers 2, 3
- Diabetes mellitus increases susceptibility to oral candidiasis 3, 4
Medication-Related Causes
- Corticosteroid use, including both systemic and inhaled steroids, significantly increases risk 1, 2
- Broad-spectrum antibiotic use disrupts normal bacterial flora that typically compete with and inhibit Candida overgrowth 2, 3
- Immunosuppressive therapy following organ or bone marrow transplantation 5
Other Systemic Factors
Local Oral Factors
- Denture use is a significant local risk factor, with denture-related candidiasis requiring both antifungal therapy and proper denture disinfection 2, 3
- Impaired salivary gland function and reduced saliva production 6
- Poor oral hygiene 6
Pathophysiology
- Candida species transition from harmless commensals to pathogens when the balance of oral microflora is disrupted or when host defense mechanisms become compromised 1, 7
- In elderly long-term care residents, 84% are colonized with yeast, making them particularly vulnerable when predisposing factors are present 8
Important Clinical Caveat
While most cases occur in the context of identifiable risk factors, rare cases of persistent and refractory thrush can occur without obvious immunodeficiency, such as in patients with chromosomal abnormalities like Klinefelter syndrome 5. When thrush fails to respond to standard therapy despite no apparent immunosuppression, consider investigating for underlying genetic or unrecognized systemic conditions.