Paper II
Question
Classify Herpes viruses. Discuss the clinical features and laboratory diagnosis of Epstein Barr virus infection.
Answer
Classification of Herpes viruses (Herpesviridae family):
Alphaherpesvirinae (short replication cycle, latency in sensory neurons):
- Herpes simplex virus 1 (HSV-1).
- Herpes simplex virus 2 (HSV-2).
- Varicella-zoster virus (VZV).
Betaherpesvirinae (longer replication cycle, latency in various tissues): 4. Cytomegalovirus (CMV). 5. Human Herpesvirus 6 (HHV-6). 6. Human Herpesvirus 7 (HHV-7).
Gammaherpesvirinae (latency in lymphoid tissue): 7. Epstein-Barr virus (EBV, HHV-4). 8. Human Herpesvirus 8 (HHV-8, Kaposi Sarcoma-associated Herpesvirus).
Epstein-Barr virus (EBV) infection
Clinical features:
- Infectious mononucleosis — the classical clinical presentation of primary EBV infection in adolescents/young adults, characterized by fever, exudative pharyngitis/tonsillitis, marked lymphadenopathy, and hepatosplenomegaly; primary infection in young children is often asymptomatic or mild.
- Malignant associations: EBV is implicated in Burkitt’s lymphoma, nasopharyngeal carcinoma, and certain B-cell lymphomas, particularly in immunocompromised hosts (e.g., post-transplant lymphoproliferative disease).
- Oral hairy leukoplakia — seen in HIV/AIDS patients.
Laboratory diagnosis:
- Peripheral blood smear — shows characteristic atypical lymphocytes (large, reactive lymphocytes with abundant cytoplasm — Downey cells) and a relative/absolute lymphocytosis.
- Heterophile antibody test (Monospot/Paul-Bunnell test) — detects heterophile antibodies that agglutinate sheep/horse red blood cells; a rapid, classical screening test, though it can be falsely negative early in illness or in young children.
- EBV-specific serology — IgM antibody to Viral Capsid Antigen (VCA-IgM) indicates acute/recent infection; IgG to VCA persists lifelong (past infection/immunity); antibody to EBV Nuclear Antigen (EBNA) appears later (weeks to months after acute infection) and its presence helps exclude very recent primary infection.
- PCR — detection/quantification of EBV DNA, particularly useful in immunocompromised patients and for monitoring EBV-associated malignancy/post-transplant lymphoproliferative disease.