Question
Infectious mononucleosis.
Answer
Infectious mononucleosis (“glandular fever”) is an acute, generally self-limiting illness most commonly caused by Epstein-Barr virus (EBV), a member of the human herpesvirus family, transmitted primarily via saliva (hence its colloquial name “kissing disease”) and infecting oropharyngeal epithelium and B lymphocytes.
Clinical features: the classic triad of fever, pharyngitis (often exudative), and lymphadenopathy (typically posterior cervical), frequently accompanied by splenomegaly (with an associated risk of splenic rupture, particularly relevant to advice on avoiding contact sports during the acute illness) and, in some patients, hepatomegaly/mild hepatitis; a characteristic maculopapular rash may develop if ampicillin/amoxicillin is administered (mistakenly, for presumed streptococcal pharyngitis) — a well-recognized diagnostic pitfall/clue.
Pathogenesis: EBV infects B lymphocytes (via the CD21 receptor) and oropharyngeal epithelial cells; infected B cells proliferate, and the resulting robust cytotoxic T-cell response against these infected B cells produces the characteristic atypical lymphocytes (reactive, activated CD8+ T cells, not the infected B cells themselves) seen on peripheral blood smear.
Laboratory diagnosis:
- Peripheral blood smear — atypical (reactive) lymphocytosis, showing large lymphocytes with abundant basophilic cytoplasm.
- Heterophile antibody test (Paul-Bunnell / Monospot test) — detects non-specific heterophile antibodies that agglutinate sheep/horse red blood cells; a rapid, widely used screening test, though it can be falsely negative early in illness or in young children.
- EBV-specific serology — IgM against viral capsid antigen (VCA-IgM) indicates acute/recent infection; IgG against VCA and EBNA (Epstein-Barr nuclear antigen) indicates past infection/immunity, useful when heterophile antibody testing is negative or equivocal.
Complications: splenic rupture, upper airway obstruction (from marked tonsillar enlargement), and, rarely, EBV’s association with certain malignancies (Burkitt lymphoma, nasopharyngeal carcinoma, some Hodgkin lymphomas) in the longer term, particularly relevant in immunocompromised hosts or specific endemic regions.