Parvovirus B19
Definitions & Key takeaways
Parvovirus B19 is a single-strand DNA virus of the parvoviridae family, which is commonly known for causing diseases in the pediatric population, though it can also affect adults.
Parvovirus B19 is primarily spread by infected respiratory droplets and causes the Fifth disease or "slapped cheek syndrome" in children, characterized by a distinctive red rash on the cheeks. It can also cause redness and joint pain (arthritis) in adults. Parvovirus B19 can also affect the bone marrow, resulting in anemia secondary to decreased erythropoiesis. Anemia can even be worse in patients with pre-existing bone marrow stress, for example, sickle cell anemia or hereditary spherocytosis which can lead to an aplastic crisis.
Treatment for parvovirus B19 varies according to the symptoms, and can involve blood transfusion for transient aplastic crisis and hydrops fetalis and immune globulin intravenous therapy for chronic infections.
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.As the virus replicates and matures, it produces a protein called non-structural protein 1 or NS1, which is toxic to human cells and causes apoptosis, or cell death.
This means that erythropoiesis breaks down, and fewer new red blood cells go into circulation as a result of parvovirus B19 infection.
But thankfully this is only temporary. When the cell dies, it bursts open, releasing copies of the virus into the blood, also called viremia.
Our immune system detects the virus and starts producing specific immunoglobulin M and immunoglobulin G antibodies to fight the infection by forming immune complexes with the parvovirus B19 antigen.
For individuals with a functioning immune system, this typically happens between 10 and 14 days after first becoming infected with the virus.Parvovirus B19 is most common in young children and those who live or work with them, like parents, siblings, and daycare workers.
Fetuses are at risk of parvovirus B19 if their pregnant mother has never had the virus in the past. Immunocompromised individuals are also particularly at risk of chronic parvovirus B19 infection, since their immune system cannot mount an appropriate response to the virus.The incubation period for parvovirus B19 - basically the period before viremia starts - is between 4 and 14 days, after which symptoms develop.
Flu-like symptoms - like a mild fever, headache, and aching muscles - are most common during viremia. Once the immune response begins and the viremia ends, these symptoms go away and some individuals will then develop a rash and/or joint pain.
The rash appears as uniform redness of the cheeks, but not the area around the mouth, giving the classic fifth disease “slapped cheek” appearance.
A lace-like rash might also appear on the trunk and the limbs. Joint pain and inflammation, or arthralgia and arthritis, linked to parvovirus B19 infection usually affects the small joints of the hands, wrists, feet, and knees, and are often symmetrical, meaning that the same joints on both sides of the body will be affected.
Children tend to get the rash whereas adults are more likely to develop joint pain, but it’s not exclusive to either group.There are a few complications caused by parvovirus B19 infection.
The decreased red blood cell production can cause transient aplastic crisis in individuals who have underlying conditions like sickle cell anemia, hereditary spherocytosis, and thalassemia.
Because they already have fewer red blood cells, the breakdown of erythropoiesis results in severe anemia, with symptoms like pale skin, fatigue, and weakness.
Parvovirus B19 in a pregnant female can cause anemia in her fetus.Because there are fewer red blood cells to carry oxygen, the heart will pump a larger volume of blood to give the growing fetus all the oxygen it needs.
This raises the pressure inside blood vessels and fluid can start to leak out of capillaries as a result. This can result in hydrops fetalis, or the abnormal accumulation of fluid in soft tissues.
Fetal anemia is also linked to fetal loss, particularly if the parvovirus B19 infection is in the first half of the pregnancy.
The good news is that there no fetal defects associated with parvovirus B19 for those fetuses that survive the infection.
Lastly, immunocompromised individuals, such as organ transplant recipients and people with HIV, can develop a serious complication from parvovirus B19 called pure red blood cell aplasia.
This is a form of chronic, severe anemia where there are very few immature red blood cells in circulation in blood vessels or erythroid progenitor cells in the bone marrow.
Symptoms of pure red blood cell aplasia are similar to other forms of anemia, such as lethargy and malaise.Parvovirus B19 infection is usually diagnosed by clinical examination.
Blood tests looking for antibodies to the virus - specifically immunoglobulin M and immunoglobulin G - are reserved for atypical presentations of the virus and for pregnant individuals.
Another option is polymerase chain reaction, or PCR, which looks for viral DNA. It’s the preferred diagnostic method of parvovirus B19 infection in immunocompromised individuals, who typically do not have high IgM or IgG levels in response to an infection.
PCR can also be used on amniotic fluid to diagnose infection in a fetus. When it comes to treatment for parvovirus B19 infection, it varies based on the symptoms.
Fifth disease is usually mild and gets better on its own, while a transient aplastic crisis often requires transfusion of blood products.
Arthralgia and arthritis are treated with NSAIDs and usually resolve on their own. Chronic infections in immunocompromised individuals are treated with immune globulin intravenous therapy or IVIG, which involves giving antibodies - mostly immunoglobulin G - taken from donor blood plasma.
Severe anemia in a fetus between 18 and 35 weeks of gestation can be treated with an intrauterine blood transfusion. Before 18 weeks, the procedure is too difficult technically.
After 35 weeks, risks of the transfusion are high compared to simply delivering the baby.Finally, there is no vaccine for parvovirus B19.
Preventive measures include proper hand washing with antiseptic soap and water and sanitizing surfaces that would’ve come into contact with respiratory droplets, since the virus can survive on surfaces.##SummaryAll right, as a quick recap… Parvovirus B19 is a single-strand DNA virus of the parvoviridae family.It infects and replicates in erythroid progenitor cells in the bone marrow.
Viral replication results in apoptosis of infected cells. It’s transmitted by respiratory droplets.
Flu-like symptoms are common during the viremia phase of the infection, followed by a “slapped cheek” rash and/or joint pain.
Treatment for parvovirus B19 varies according to the symptoms, and can involve blood transfusion for transient aplastic crisis and hydrops fetalis and immune globulin intravenous therapy for chronic infections.
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