Viral Infections
Core concepts
Viral syndromes are diagnosed mainly by nucleic acid amplification tests (PCR) of the relevant site: nasopharyngeal swab for respiratory viruses, CSF for enterovirus, parechovirus, and HSV, vesicle fluid for HSV and VZV, and blood or throat for EBV and CMV. Serology is best for immunity or retrospective diagnosis, not acute exclusion. Bronchiolitis (RSV, rhinovirus) is managed with supportive care: no routine bronchodilators, corticosteroids, or antibiotics. Influenza antiviral therapy (oseltamivir) is indicated for hospitalized, severe, or high-risk children and is most effective within 48 hours, but still give later in severe disease. Neonatal HSV needs intravenous acyclovir 20 mg/kg every 8 hours: 14 days for skin-eye-mouth disease and 21 days for CNS or disseminated disease. Varicella in high-risk children warrants acyclovir; avoid aspirin because of Reye syndrome.
Traps
- IgM serology is prone to false positives and cross-reactions (EBV, CMV, parvovirus).
- PCR can remain positive for weeks after clinical recovery; do not treat the test alone.
- Early CSF HSV PCR can be negative; repeat and treat if clinical suspicion is high.
- Measles requires airborne isolation and vitamin A in severe cases.
Worked example
A 10-day-old presents with vesicles on the scalp, lethargy, and poor feeding. Do not wait for PCR: obtain surface swabs, CSF PCR, and ALT, start empiric IV acyclovir, and continue until HSV is excluded.
Rule of thumb: for acute viral disease, test the site where the virus is replicating with PCR; use serology for exposure and immunity questions.