SN5 SHORT NOTE 5M ★ High-yield

Describe parvovirus B19 with its clinical syndromes and their basis.

Microbiology · 2nd Professional — Para-Clinical

The Virus

Parvovirus B19 — the smallest DNA virus, a naked icosahedral virus with single-stranded DNA; it is the only medically important single-stranded DNA virus and the only human pathogen in its family
Being naked it is highly resistant to heat, detergents and lipid solvents, and survives in the environment and in blood products — it resists the usual viral inactivation steps in plasma manufacture
Its receptor is the P antigen (globoside) on erythroid precursors in the bone marrow — and this single fact explains the entire disease. People rare individuals lacking the P antigen are naturally resistant
It replicates only in dividing cells, and destroys erythroid progenitors, arresting red cell production
Transmission — respiratory droplets, transplacental, and blood products

CLINICAL PEARL

One receptor explains every manifestation of parvovirus B19. It binds the P antigen on red cell precursors, so it stops erythropoiesis for about a week. In a healthy person, whose red cells live 120 days, that is imperceptible. In chronic haemolysis, where red cells live days, it causes an aplastic crisis. In the fetus, with rapid erythropoiesis and a short red cell lifespan, it causes hydrops fetalis. Same virus, same week of arrest, three entirely different consequences.

Clinical Syndromes

Syndrome	Host	Features
Erythema infectiosum (fifth disease)	Healthy children	A "slapped cheek" facial rash followed by a lacy reticular rash on the limbs. By the time the rash appears the child is NO longer infectious, since it is immune-complex mediated — so exclusion from school is pointless
Arthropathy	Adults, especially women	Symmetrical polyarthritis of hands, wrists and knees, closely mimicking rheumatoid arthritis; usually self-limiting over weeks
Aplastic crisis	Chronic haemolytic anaemia — sickle cell disease, thalassaemia, hereditary spherocytosis, G6PD deficiency	Abrupt severe anaemia with a REticulocyte count of essentially zero; may require urgent transfusion. Often without a rash
Pure red cell aplasia	Immunosuppressed — HIV, transplant, chemotherapy	Chronic anaemia from persistent infection, since antibody cannot be made to clear it; treated with intravenous immunoglobulin
Hydrops fetalis and fetal loss	Fetus, especially in the second trimester	Severe fetal anaemia causing high-output cardiac failure and hydrops; may resolve spontaneously or require intrauterine transfusion. It does not cause congenital malformation
Papular-purpuric "gloves and socks" syndrome	Young adults	A distinctive acral rash with fever

Diagnosis and Management

Serology — B19 IgM indicates recent infection and is the usual test in an immunocompetent patient; IgG indicates past infection and immunity
PCR for B19 DNA — required in the immunosuppressed, who may not make antibody, and in aplastic crisis and fetal infection, where viral load is very high
Bone marrow shows giant pronormoblasts with intranuclear inclusions and an absence of later erythroid forms
Management — supportive in the healthy; transfusion in aplastic crisis; intravenous immunoglobulin in chronic infection in the immunosuppressed, since it supplies the antibody they cannot make; and fetal monitoring with middle cerebral artery Doppler and intrauterine transfusion where indicated
There is NO vaccine and no specific antiviral

Applied Aspects

Consider parvovirus in any sudden severe anaemia in a patient with a chronic haemolytic disorder; a reticulocyte count of near zero in the face of profound anaemia is the diagnostic clue, and is the opposite of what haemolysis alone would produce
This matters greatly in India, where sickle cell disease and thalassaemia are common; an aplastic crisis may be the first serious event in an undiagnosed haemoglobinopathy
Infectivity precedes the rash, so a child with erythema infectiosum need not be excluded from school — the transmission has already happened during the preceding non-specific illness
Advise susceptible pregnant women to avoid known outbreaks, though this is of limited practical value given asymptomatic transmission; and investigate maternal exposure with serology
Consider parvovirus before diagnosing rheumatoid arthritis in a woman with an acute symmetrical polyarthritis, particularly with a history of contact with children; the illness resolves and the diagnosis would otherwise be lifelong
Immunosuppressed patients with unexplained chronic anaemia should be tested by PCR rather than serology, and respond well to immunoglobulin

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