Chapter 28: Infections – Virology & HIV

🧬 Use of the virology laboratory · HIV infection in children · Diagnosis · Antiretroviral therapy · Prevention of mother-to-child transmission (PMTCT) · Opportunistic infections
Forfar & Arneil's Textbook of Pediatrics – Comprehensive coverage: PCR, serology, antigen testing, viral loads, resistance testing, pediatric HIV management, WHO staging, and ART.

📖 Virology laboratory & HIV infection in children

🔬 Virology lab – key tests
PCR (qualitative/quantitative) for viral nucleic acids; serology (IgM/IgG) for antibodies; antigen detection (e.g., RSV, influenza, HIV p24); viral culture (enterovirus, HSV); genotyping/resistance testing (HIV, CMV).
🧪 Choosing the right test
Acute infection: PCR (HIV, CMV, EBV, enterovirus). Immune status: serology (rubella, VZV, measles). Viral load monitoring: HIV, CMV, EBV. Resistance: HIV, CMV, HBV.
🦠 HIV infection in children
Vertical transmission (most common). Diagnosis: HIV DNA/RNA PCR (in infants <18 months – antibody may reflect maternal IgG). Clinical staging (WHO). ART: two NRTIs + NNRTI or protease inhibitor (PI).
🛡️ PMTCT (prevention of mother-to-child transmission)
Antepartum ART (maternal), intrapartum prophylaxis, infant post-exposure prophylaxis (zidovudine/nevirapine). Avoid breastfeeding where safe replacement available (otherwise exclusive breastfeeding + ART).
⚠️ Critical notes: HIV serology in infants <18 months is not diagnostic (maternal antibodies). Use HIV DNA or RNA PCR. Viral load <1000 copies/ml is good control; ART adherence critical. Opportunistic infection prophylaxis (PCP: TMP-SMX) for CD4 <15-20%.

🔬 Use of the virology laboratory – a clinician's guide

🧫
Polymerase chain reaction (PCR) – Gold standard for HIV diagnosis in infants, CMV, EBV, enterovirus, HSV (CNS). Quantitative PCR (viral load) for HIV, CMV, HBV.
🩸
Serology (IgM, IgG) – Useful for acute infection (IgM) and immunity (IgG). Examples: measles, rubella, VZV, EBV (VCA, EBNA). Not diagnostic for HIV in infants <18 months.
🎯
Antigen detection – RSV, influenza (rapid tests). HIV p24 antigen used in fourth-generation assays.
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Resistance testing (genotype) – For HIV (before ART or after virologic failure), CMV (ganciclovir resistance), HBV (lamivudine resistance).
📌 Practical tip: For suspected congenital CMV: urine CMV PCR within 2-3 weeks of life (confirm congenital). For HIV-exposed infant: PCR at birth, 6-8 weeks, 4-6 months.

🩺 Stepwise management of pediatric HIV infection

1
Diagnosis & staging – HIV PCR (DNA/RNA) for infants; ELISA + confirmatory Western blot/RNA for older children. WHO clinical stage (1-4). Baseline CD4 count/percentage, viral load.
2
Initiation of ART (regardless of CD4 for children <5 years) – Two NRTIs (e.g., abacavir/lamivudine or tenofovir/emtricitabine) + NNRTI (efavirenz if ≥3y, nevirapine) or boosted PI (lopinavir/ritonavir).
3
Monitoring – Viral load every 3-6 months (target <1000 copies/ml, ideally undetectable). CD4 count/percentage every 3-6 months. Adherence support.
4
Opportunistic infection prophylaxis – TMP-SMX for PCP (if CD4% <15-20% or age-specific thresholds). Isoniazid prophylaxis for TB if high risk. Immunizations (inactivated vaccines safe; avoid live vaccines if severely immunocompromised).
5
Treatment failure – Resistance testing, switch to second-line (usually boosted PI + two NRTIs, or integrase inhibitor).
📌 PMTCT: Maternal ART during pregnancy, infant prophylaxis (zidovudine or nevirapine), and avoidance of breastfeeding in resource-rich settings dramatically reduce transmission to <1%.

🧠 Reflex prompts – virology & HIV

🧪 6-week-old HIV-exposed infant. Test of choice?
HIV DNA or RNA PCR. Serology not diagnostic (maternal antibody).
🦠 Child with fever, lymphadenopathy, oral thrush, chronic diarrhea. HIV suspected. Confirmatory test?
HIV antibody test (ELISA + Western blot) if >18 months; PCR if younger.
💊 First-line ART for a 2-year-old with HIV (PI-based).
Lopinavir/ritonavir + two NRTIs (e.g., abacavir/lamivudine).
🛡️ HIV-exposed newborn: what prophylaxis reduces PCP risk?
TMP-SMX starting at 4-6 weeks of age until HIV infection excluded.
📈 HIV viral load is undetectable on ART. What does this mean?
Excellent adherence and viral suppression, but virus persists in reservoirs.
🔬 Which test differentiates acute EBV from past infection?
Viral capsid antigen (VCA) IgM for acute; EBNA IgG for past infection.
🧬 HIV drug resistance testing is indicated when?
At baseline before ART initiation and after virologic failure.
🩺 What is the WHO clinical stage 4 indicator for HIV in children?
Severe opportunistic infections (PCP, disseminated TB, toxoplasmosis, cryptococcosis), HIV wasting syndrome, Kaposi sarcoma.
🧫 Most sensitive test for congenital CMV?
Urine or saliva CMV PCR within 2-3 weeks of birth.
🧪 What is the role of virology lab in influenza?
Rapid antigen or PCR for diagnosis; antiviral resistance testing if treatment failure.