What are the causes and risk factors for thrush?

Causes of Thrush (Oral Candidiasis)

Thrush results from overgrowth of Candida species—predominantly Candida albicans—when normal protective mechanisms are disrupted, most commonly by immunosuppression, antibiotic use, corticosteroid therapy, or conditions that alter the oral microbiome. 1

Primary Causative Organism

• Candida albicans is the predominant pathogen responsible for the majority of thrush cases, though non-albicans species including C. tropicalis, C. glabrata, C. parapsilosis, and C. krusei are increasingly isolated. 1, 2
• Candida exists as part of normal flora in healthy individuals but becomes pathogenic when host defenses are compromised or the microbial balance is disrupted. 3, 4

Major Risk Factors and Predisposing Conditions

Immunosuppression

• HIV/AIDS represents the highest risk, with oropharyngeal candidiasis affecting up to 20% of patients with advanced HIV disease, often accompanied by esophageal candidiasis. 1
• Patients with cellular immune deficiency from Hodgkin lymphoma, non-Hodgkin lymphoma, or recipients of blood, marrow, or solid organ transplants are at substantially increased risk. 1
• Neutropenic patients and those receiving immunosuppressive agents face elevated susceptibility to mucocutaneous fungal infections. 1

Medication-Related Causes

• Corticosteroid use—both systemic and inhaled—is a major predisposing factor by suppressing local and systemic immune responses. 1
• Broad-spectrum antibiotic therapy disrupts normal protective bacterial flora, allowing Candida overgrowth by eliminating competing microorganisms. 1, 2
• Use of IL-17 inhibitors and other targeted biologics predisposes patients to Candida infections. 1

Metabolic and Systemic Conditions

• Uncontrolled diabetes mellitus creates an environment favorable for fungal proliferation through elevated glucose levels and impaired immune function. 1
• Chronic liver disease, particularly cirrhosis, represents an independent risk factor for invasive candidiasis. 1

Local Oral Factors

• Poor oral hygiene allows accumulation of debris and creates conditions favorable for Candida colonization. 1, 4
• Denture use, particularly ill-fitting dentures, can cause denture stomatitis and create microenvironments conducive to fungal growth. 1
• Xerostomia (dry mouth) from any cause reduces the protective cleansing action of saliva. 4

Age-Related Factors

• Newborns and infants are susceptible due to immature immune systems and potential maternal transmission during delivery. 2, 5
• Elderly residents of long-term care facilities show high colonization rates (84% in one study) due to multiple risk factors including polypharmacy, comorbidities, and functional decline. 1

Mechanism of Infection

• The loss of protective H₂O₂-producing Lactobacillus species and other normal flora allows opportunistic Candida overgrowth. 6
• Candida transitions from commensal colonizer to pathogen when host defenses are compromised or when competing microorganisms are eliminated. 3, 4
• In long-term care settings, mucocutaneous candidiasis results from overgrowth of endogenous resident fungi in moist macerated tissues, particularly in association with antimicrobial and corticosteroid use. 1

Clinical Pitfalls

• Not all white oral patches represent Candida infection—one study found no C. albicans in infants clinically diagnosed with thrush, suggesting alternative etiologies should be considered when diagnosis is uncertain. 5
• Azole-resistant species, particularly C. glabrata, are increasingly common in patients with prior azole exposure, making species identification important in treatment failures. 1
• Maternal mastitis and antibiotic use are significantly associated with neonatal thrush presentation. 5