What causes thrush in the oral cavity?

Causes of Oral Thrush (Oral Candidiasis)

Oral thrush is primarily caused by an overgrowth of Candida species, most commonly Candida albicans, which is a normal commensal organism found in the oral cavity of up to two-thirds of healthy individuals. 1

Primary Causative Organisms

• Candida albicans is the most common pathogen responsible for oral thrush, accounting for the majority of cases 1, 2
• Non-albicans Candida species such as C. glabrata, C. dubliniensis, and C. krusei can also cause oral thrush, particularly in cases of fluconazole resistance 1
• C. dubliniensis was first identified in association with oral candidiasis in HIV-infected patients 1

Predisposing Factors

Local Factors

• Poor oral hygiene contributes significantly to the development of oral thrush 3, 4
• Denture use, particularly ill-fitting dentures or inadequate denture hygiene 1, 4
• Impaired salivary gland function (hyposalivation) 3, 4
• Local tissue damage or mucosal barrier disruption 2

Systemic Factors

• Immunosuppression is a major risk factor, particularly:
  • HIV infection with CD4+ counts <200 cells/μL 1
  • Diabetes mellitus 3, 1
  • Malignancies and leukemia 1
  • Use of immunosuppressive medications 3, 4
• Prolonged use of broad-spectrum antibiotics that disrupt normal oral flora 3, 1
• Corticosteroid use, including inhaled steroids 1
• Radiation therapy to the head and neck 1
• Advanced age, particularly in combination with other risk factors 4
• Malnutrition 4

Pathophysiology

• Candida species normally exist as commensals in the oral cavity but can become pathogenic when the balance of oral microflora is disrupted 1, 5
• Transition from commensal to pathogen occurs when:
  • Host defense mechanisms are compromised 1
  • Normal bacterial flora is altered (e.g., by antibiotics) 3
  • Local or systemic factors favor fungal overgrowth 4
• No particular strains of C. albicans have a preponderance to cause mucosal candidiasis 1

Antifungal Resistance

• Acquired fluconazole (or pan-triazole) resistance is related to previous exposure to these medications, particularly if repeated and prolonged in the context of profound immunosuppression 1
• Patients failing fluconazole therapy have typically received larger cumulative dosages (mean value, 8.7 g) 1
• The emergence of non-albicans Candida species with intrinsic reduced azole susceptibility (particularly C. krusei and C. glabrata) often follows repeated antifungal exposure 1
• The introduction of effective antiretroviral therapy in HIV patients has led to a dramatic decline in the incidence of refractory disease and infections caused by resistant Candida isolates 1

Clinical Relevance

• Oral thrush can present in three main clinical patterns: pseudomembranous candidiasis (white patches), erythematous candidiasis (red patches), and angular cheilitis 1, 2
• The presence of erythematous and pseudomembranous forms in HIV-infected patients is predictive of progressive immunodeficiency 1
• Oral thrush can significantly impact quality of life through pain, altered taste, and difficulty eating, which may contribute to weight loss and poor nutrition 1, 2

Prevention Strategies

• Maintaining good oral hygiene and proper denture care is essential for prevention 4
• For denture wearers, disinfection of dentures and removing them at night is recommended 1, 4
• In HIV-infected patients, effective antiretroviral therapy is the best prophylaxis against recurrent oral thrush 1
• While fluconazole prophylaxis can reduce the risk of mucosal candidiasis in high-risk patients, routine primary prophylaxis is not recommended due to concerns about drug resistance development 1