Parvovirus

Parvovirus

• Smallest virus.
• Not a cultivable virus
• Nonenveloped icosahedral virus 
• Roughly spherical, with surface protrusions
• Single stranded D.N.A.

Pathogenesis of parvovirus

• Affects erythroid progenitors.
  
• Cellular receptors for parvovirus
• Blood group P antigen
• P antigen is expressed on:
•  mature erythrocytes
• erythroid progenitors
• megakaryocytes
• endothelial cells
• placenta
• fetal liver cells.
• Replicates primarily in erythroid progenitors:
• bone marrow
• fetal liver cells
• Primarily causes diseases of these organs.

Clinical manifestations:

Erythema infectiosum (fifth disease)

 The incubation period:

• between 4 and 21 days.
• Individuals with fifth disease are most infectious before the onset of symptoms.

Fifth of the Childhood Viral Rash:

• Measles
• Rubella
• Chicken Pox
• Roseola
• Parvovirus 
• The main symptomatic manifestation 
• Erythema infectiosum / slapped check disease.

• Child present with facial rash (slapped cheek appearance) 
• Preceded by lowgrade fever.

Polyarthropathy syndrome

• Uncommon among children but occurs in 50% of adults.
• Small joints of the hands and ankles are involved symmetrically.

Aplastic crisis

• Asymptomatic transient reticulocytopenia occurs in most individuals with B19 infection..

In normal individuals:

• Normal erythropoiesis
• Few days of arrest of erythropoiesis 
• Not cause detectable anemia.

But, in cases of chronic hemolytic anemias:

• e.g. sickle cell anemia, hereditary spherocytosis.
• Require continuous erythropoiesis.
• Life span of the R.B.C. is shortened. 
• This leads to aplastic crisis in these patients. 
• These patients can readily transmit B-19 infection to other people. 

Pure red cell aplasia/chronic anemia

• Parvovirus causes established persistent infection in patients with iminunosuppression e.g. (AIDS, CLL).
• These patients have persistent anemia with reticulocytopenia.

Hydrops fetalis

• Hydrops fetalis refers to excessive accumulation of fluid in at least two fetal body cavities.
• Cause nonimmunohydrops fetalis if infection occur in first 20 weeks of pregnancy
• Parvovirus has tropism for, fetal liver and heart(abundant P receptors)

Pathogenesis: 

• Decreased erythrocyte production leads to anemia
• High output cardiac failure
• Leads to fluid accumulation and hydrops fetalis
•  Myocarditis leading to heart failure may also contribute to the development of fetal hydrops.

Characterised on ultrasound by :

• generalised skin oedema (skin thickness > 5mm)
• ascites
• pleural effusion
• large placenta (placental thickness > 4 cm)
• The fetus may be in Buddha position and there may be a halo around the head.

Risk:

• Transplacental infection is 30%
• Fetal loss (early in 2nd trimester) is 9%.

Diagnosis

• Done by the detection of B19 IgM and IgG antibodies.
• IgM is detected at the time of rash in erythema infectiosum
• IgG is detected on the 7th day. 

Treatment

• No antiviral is effective against parvovirus.