Why does influenza cause backaches, nausea, diarrhea, sore throat, and oral thrush?

Influenza Symptom Pathophysiology: Understanding Diverse Manifestations

Influenza causes backaches, gastrointestinal symptoms, sore throat, and white patches in the mouth through systemic viral inflammation, cytokine release, and potential secondary infections.

Core Influenza Symptoms and Mechanisms

Influenza is characterized by abrupt onset of both constitutional and respiratory symptoms that result from the body's immune response to viral infection 1.

Primary Influenza Symptoms:

• Fever: Present in 68% of cases, typically 38-40°C, peaking within 24 hours and lasting 1-5 days 1, 2
• Cough: Present in approximately 85% of cases, usually dry 2
• Myalgia/Backaches: Affects about 53% of cases, primarily in back and limbs 1, 2
• Headache: Present in about 65% of cases 2
• Malaise: Present in about 80% of cases 2
• Sore throat: Present in about 50% of cases 2

Mechanism of Backaches

Backaches during influenza result from:

• Systemic inflammatory response with cytokine release
• Direct viral effects on muscle tissue
• In some cases, progression to myositis (muscle inflammation) 1

Gastrointestinal Symptoms in Influenza

Contrary to common perception, gastrointestinal symptoms can occur with influenza:

• Nausea and diarrhea: Uncommon in adults (<10%) but more frequent in children 1, 2
• Mechanism: Likely due to:
  1. Systemic inflammatory response affecting the GI tract
  2. Possible direct viral invasion of intestinal epithelium
  3. Cytokine-mediated effects on the gastrointestinal system

Sore Throat Pathophysiology

Sore throat is a common symptom of influenza, occurring in approximately 50% of cases 2. It results from:

1. Direct viral invasion of pharyngeal mucosa
2. Local inflammatory response
3. Increased mucus production
4. Possible secondary bacterial infection

White Patches in the Mouth (Oral Thrush)

Important note: White patches in the mouth (oral thrush) are not a typical feature of uncomplicated influenza infection 1, 2.

When white patches appear in the context of influenza, they likely represent:

1. Secondary opportunistic infection: Candida albicans (thrush) can develop as a secondary infection, particularly in:

   • Immunocompromised patients
   • Patients using inhaled corticosteroids for respiratory symptoms
   • Patients receiving antibiotics for secondary bacterial infections

2. Differential diagnosis: White patches could also represent:

   • Exudative pharyngitis (bacterial co-infection)
   • Dehydration effects on oral mucosa
   • Other viral co-infections

Clinical Implications and Management

Assessment Algorithm:

1. Evaluate for classic influenza symptoms (fever, cough, myalgia)
2. If white patches are present, distinguish between:
   • Exudative pharyngitis (localized to throat)
   • True oral thrush (can appear on tongue, palate, inside cheeks)

Management Considerations:

• For influenza symptoms: Consider antiviral therapy if within 48 hours of symptom onset 3
• For oral thrush: Evaluate for underlying causes (immunosuppression, medication effects) and treat with appropriate antifungal therapy 4

Special Considerations

• Timing: Influenza symptoms typically resolve within 7 days, though cough and malaise may persist for >2 weeks 1
• Complications: Influenza can lead to various complications including secondary infections, exacerbation of underlying conditions, and rarely, neurological or cardiac complications 1
• High-risk groups: Elderly, very young children, and those with underlying medical conditions are at higher risk for complications 1

Common Pitfalls

1. Misattribution of symptoms: Not recognizing that GI symptoms can be part of influenza presentation
2. Overlooking secondary infections: Failing to recognize that white patches may indicate a secondary fungal infection requiring specific treatment
3. Delayed treatment: Not initiating antiviral therapy within the 48-hour window when it's most effective 3, 5

Remember that when influenza is circulating in the community, patients with both cough and fever within 48 hours of symptom onset have approximately 79% likelihood of having influenza 5.