What causes oral candidiasis (thrush)?

What Causes Oral Thrush (Yeast Infection of the Throat)

Oral thrush is caused by overgrowth of Candida species, predominantly Candida albicans, which normally exists as part of the oral flora but becomes pathogenic when host defenses are compromised or the oral microflora balance is disrupted. 1, 2

Primary Causative Organism

• Candida albicans is responsible for the majority of oral thrush cases and is found in the oral cavity of up to two-thirds of healthy individuals as normal flora 1, 2
• Non-albicans Candida species including C. glabrata, C. tropicalis, C. dubliniensis, and C. krusei can also cause oral thrush, particularly in refractory or fluconazole-resistant cases 1, 2

Major Risk Factors Leading to Candida Overgrowth

Immunosuppression (Most Important Systemic Factor)

• HIV infection is the most significant risk factor, especially when CD4+ counts fall below 200 cells/μL 3, 2
• Malignancies, particularly leukemia and other hematologic cancers, substantially increase risk 1, 3
• Corticosteroid use, both systemic and inhaled formulations, predisposes to oral thrush 1, 2

Antibiotic Use

• Broad-spectrum antibiotics disrupt the normal bacterial flora that typically compete with and inhibit Candida overgrowth 1
• This disruption removes the competitive inhibition that normally keeps Candida in check 4

Local Oral Factors

• Denture use is a significant local risk factor, with 84% of elderly long-term care residents colonized with yeast 1, 5
• Poor oral hygiene and impaired salivary gland function facilitate colonization 5
• Radiation therapy to the head and neck increases susceptibility 2

Other Contributing Factors

• Diabetes mellitus creates a favorable environment for Candida growth 3, 5
• Malnutrition compromises immune defenses 5

Pathophysiology of Transition from Colonization to Infection

• Candida species normally exist as commensals in the oral cavity but become pathogenic when the balance of oral microflora is disrupted or host defense mechanisms are compromised 2, 4
• The yeast adheres to oral surfaces through specific interactions with complement receptors, extracellular matrix proteins, and sugar residues, enabling it to resist host clearance mechanisms 4
• Tissue invasion occurs through secreted hydrolytic enzymes, hyphal formation, and contact sensing when physical and immunological defenses are undermined 4

Common Clinical Pitfalls

• In elderly populations, 84% are colonized with yeast, making them particularly vulnerable when any predisposing factor is present 1
• The presence of oral thrush in HIV-infected patients serves as an indicator of immunosuppression and progressive immunodeficiency 3, 2
• Repeated and prolonged exposure to fluconazole is the predominant cause of azole resistance, leading to emergence of non-albicans species with reduced susceptibility 3, 2