Parvovirus B19 is the smallest known DNA animal virus, coming in at an itty bitty 18 to 28 nanometers in diameter.
In comparison, the average size of a single human red blood cell is a whopping 7200 nanometers!
While it’s mostly known for causing fifth disease, or “slapped cheek syndrome,” in children, parvovirus B19 can also affect adults and it can cause serious illness in individuals with pre-existing conditions like sickle-cell anemia and HIV.
Parvovirus B19 is part of the parvoviridae family.
It’s a single-stranded DNA virus surrounded by an icosahedral capsid, which is a spherical protein shell made up of 20 equilateral triangular faces.
And it’s “naked” because the capsid isn’t covered by a lipid membrane.
Parvovirus B19 is primarily transmitted by respiratory droplets when someone coughs or sneezes.
You can also catch it via an infected blood transfusion and a pregnant female can also transmit it through the placenta to her unborn child.
Now, although the virus first enters cells of the respiratory tract by binding to receptors on host cells, it doesn’t replicate in them.
Instead it keeps travelling through cells and into the circulatory system until it reaches bone marrow, where red blood cells are made, a process called erythropoiesis.
Once there, parvovirus B19 uses receptor-mediated endocytosis to enter erythroid progenitor cells, also called proerythroblasts, the early cells that eventually become red blood cells.
It then uses these cells’ DNA replication machinery in the nucleus to replicate its DNA and assemble new copies of the virus.
Why not simply replicate in cells of the respiratory system?
Well it turns out that Parvovirus B19 needs two things: it prefers to bind to a specific receptor, the P antigen, which is found in large numbers on proerythroblasts’ cell membrane and it needs cells that pass through the S phase of the cell cycle, which is the phase where cell DNA is replicated.
Since the body is constantly producing new red blood cells, there are always proerythroblasts going through the S phase at any given time.