What are the differential diagnoses for a systemic viral infection?

Differential Diagnoses for Systemic Viral Infections

The differential diagnosis for systemic viral disease should be systematically approached based on the primary organ system involved, with HSV-1, HSV-2, VZV, and enteroviruses accounting for 90% of identifiable viral encephalitis cases in immunocompetent individuals, while considering HIV testing universally regardless of perceived risk factors 1, 2.

Algorithmic Approach by Clinical Presentation

CNS Manifestations (Encephalitis/Meningitis)

Most Common Viral Causes:

• Herpes Simplex Virus (HSV-1 and HSV-2) - Most critical to identify due to treatment implications
• Varicella-Zoster Virus (VZV)
• Enteroviruses - Together with HSV and VZV, these represent 90% of diagnosed viral encephalitis cases 2
• Epstein-Barr Virus (EBV) - Especially in immunocompromised patients 1, 2

Additional Considerations:

• Cytomegalovirus (CMV) - Particularly in immunocompromised states 1, 2
• Mumps virus - If parotid involvement present 1
• Arboviruses - Based on travel history and insect exposure 1, 2
• HIV - Can present as acute meningoencephalitis during seroconversion 1

Critical Pitfall: HIV testing should be performed on ALL patients with encephalitis or suspected encephalitis, regardless of apparent risk factors, as it is both a direct cause of acute meningoencephalitis and predisposes to opportunistic CNS infections 1.

Respiratory System Involvement

When systemic viral infection presents with respiratory features, consider:

• Influenza virus
• Parainfluenza virus
• Respiratory Syncytial Virus (RSV)
• SARS-CoV-2 - Now endemic and causes significant morbidity in immunocompromised patients 3
• Measles virus - Can cause subsequent encephalitis 4

Diagnostic Approach: Obtain nasopharyngeal swabs, throat swabs, or bronchoalveolar lavage for viral culture or PCR 1. MxA levels above 50 ng/mL demonstrate 77.8% sensitivity and 80.2% specificity for distinguishing viral from bacterial/fungal respiratory infections 5.

Gastrointestinal Manifestations

Enteroviruses are the primary consideration when systemic viral disease includes GI symptoms:

• Throat and rectal swabs should be obtained 1
• Key distinction: Vesicular fluid is most diagnostically useful, as fecal shedding can persist long-term and may not indicate acute infection 1
• If vesicles present, consider enterovirus or VZV 1

Cardiac Involvement

Systemic viral infections with cardiac manifestations include:

• Coxsackievirus (enterovirus family)
• Cytomegalovirus (CMV)
• Epstein-Barr Virus (EBV)
• Hepatitis B and C viruses (HBV/HCV) - Associated with endothelial dysfunction and myocarditis 6
• Human T-Lymphotropic Virus (HTLV) 6

Ocular Manifestations

Congenital infections:

• Cytomegalovirus (CMV) - Most common cause of congenital viral retinitis 4
• Rubella virus 4

Acquired infections:

• CMV retinitis - Most common in AIDS patients 4
• HSV and VZV - In both immunocompetent and immunocompromised individuals 4
• Measles, influenza, EBV, Rift Valley fever virus - Typically following acute systemic illness 4

Sexually Transmitted Systemic Viral Infections

• HIV - Universal testing recommended 1
• HSV-2 - Can cause chronic encephalitis, especially in HIV patients 1
• Hepatitis B virus (HBV)
• Human Papillomavirus (HPV) - Oncogenic with potential cardiovascular involvement 6

Essential Diagnostic Workup

CSF Analysis (When Encephalitis Suspected)

Mandatory PCR testing for:

• HSV-1 and HSV-2
• VZV
• Enteroviruses
• Consider EBV 2

CSF characteristics:

• Lymphocytic pleocytosis typical for viral infections
• Elevated red cell count in ~50% of HSV encephalitis cases (hemorrhagic) 2
• Acellular CSF can occur with VZV, EBV, CMV, particularly in immunocompromised patients 2
• CSF lactate <2 mmol/L effectively rules out bacterial disease 2

Blood Testing

Serological testing based on epidemiological clues:

• EBV serology
• Arbovirus serology (based on travel/exposure)
• Borrelia burgdorferi (Lyme disease)
• Brucellosis, rickettsioses, ehrlichioses
• Mycoplasma 1

Acute and convalescent samples should be obtained for appropriate serological comparisons 1.

Site-Specific Sampling

• Vesicles (if present): Most diagnostically useful - indicates acute systemic infection 1
• Parotid duct swabs: After 30-second gland massage for mumps (within 9 days of symptom onset) 1
• Respiratory samples: Throat/nasal swabs, nasopharyngeal aspirate, or BAL if respiratory symptoms present 1

Distinguishing Viral from Bacterial Infections

Laboratory markers for differentiation:

• C-reactive protein (CRP) - Lower in viral infections
• CSF glucose - Normal in viral, low in bacterial
• CSF protein - Mildly elevated in viral, markedly elevated in bacterial
• CSF lactate - <2 mmol/L rules out bacterial meningitis 2
• Complement receptor expression (CR1/CD35) on neutrophils - Can distinguish bacterial from viral with 98% sensitivity and 97% specificity when combined with clinical data 7

Common Pitfall: Partially treated bacterial meningitis can mimic viral meningitis with lymphocytic pleocytosis - clinical context and CSF parameters (glucose ratio, protein, lactate) are critical 2.

Special Populations

Immunocompromised Patients

Expanded differential includes:

• Toxoplasma
• CMV (more aggressive)
• Pneumocystis
• Cryptococcus
• JC virus (progressive multifocal leukoencephalopathy)
• Chronic HSV-2 encephalitis 1

Note: Streptococcus pneumoniae has increased incidence and unusual presentations in HIV patients 1.

Pediatric Considerations

The same viral pathogens apply, but bacterial meningitis occurs at much younger ages than viral meningitis 8. A bacterial meningitis score incorporating CRP, CSF glucose, and CSF protein can avoid unnecessary antibiotics in viral cases while maintaining 100% sensitivity for bacterial disease 8.