Group II, The single stranded DNA viruses, contains only one family of viruses that are pathogenic in humans: Class Parvoviridae. Class Parvoviridae contains environmentally resistant viruses that infect a wide range of animals. Parvoviruses are the smallest viruses currently known. Parvovirus B19 is the agent that causes fifth disease, so named because the diseases was the fifth type of common childhood exanthem among 6 rashes listed in most textbooks. Until the 1980s, no cause of this fifth listed exanthem was known; so it came to be known as “fifth disease”. Other names for fifth disease are erythema infectiosum and slapped face disease. Human herpesviruses 6 and 7 (Chapter 39) cause the sixth childhood rash, “sixth disease”.
Keywords
Hydrops fetalis, Fifth disease, Erythema infectiosum, Slapped face disease, Transient hemolytic anemia, Aplastic anemia
“Everything should be made as simple as possible, but not simpler.”
Albert Einstein
Group I, dsDNA (Chapter 39)
- Group II, ssDNA (Chapter 40)
- Parvoviridae
- *Bocavirus
- *Human parvovirus B19
Group III, dsRNA (Chapter 41)
Group IV (+)ssRNA (Chapter 42)
Group V (−)ssRNA (Chapter 43)
Group VI, ssRNA-RT (Chapter 44)
Group VII, dsDNA-RT (Chapter 45)
Prions (Chapter 46)
Group II, the single-stranded DNA viruses, contains only one family of viruses that are pathogenic in humans: Class Parvoviridae. Class Parvoviridae contains environmentally resistant viruses that infect a wide range of animals. Parvoviruses are the smallest viruses currently known.
- Group II, ssDNA
- Parvoviridae
- *Bocavirus
- *Human parvovirus B19
Parvovirus B19 is the agent that causes fifth disease, so named because the diseases was the fifth type of common childhood exanthem among six rashes listed in textbooks. Until the 1980s, the cause of this fifth listed exanthem was unknown; so it came to be known as “fifth disease”. Other names for fifth disease are erythema infectiosum and slapped face disease. Human herpesviruses 6 and 7 (Chapter 39) cause the sixth childhood rash, “sixth disease”. The rash of fifth disease results from an immune response of the host to the virus particles. Essentially, fifth disease is an allergic phenomenon, and not the direct, cytopathic effect of the virus.
Infection by parvovirus occurs from contact (usually via respiratory droplets) with actively infected hosts. The virus is known to infect humans and dogs. Serologic evidence indicates that at least half of the human population has been infected with parvovirus B19.
Members of Class Parvoviridae characteristically infect rapidly dividing host cells, using host processes to support their own replication. The target cells for parvovirus B19 are the dividing precursor erythroid cells. Another name for parvovirus B19 is erythrovirus B19, indicating the target cell for the virus. In the active stage of infection, huge amounts of virus are produced. Death or dysfunction of the target hematopoietic (blood precursor) cells can lead to a transient pancytopenia (i.e. anemia of all blood cell lineages). In rare cases, aplastic anemia may occur, in which most of the precursor erythroid cells are destroyed, leading to a massive decline in circulating mature forms. When aplastic anemia occurs, it usually occurs in individuals who have a concurrent condition that requires an excessive production of blood cells to maintain the normal blood profile of mature cells. These conditions include: autoimmune hemolytic anemia, sickle cell anemia, and inherited blood dyscrasias that increase the fragility of red blood cells or that decrease the life-span of red blood cells. Basically, a co-infection with parvovirus B19 is the last straw for bone marrows that are barely keeping pace with the body’s demand for erythrocytes.
The intense viremia that occurs in parvovirus B19 infection, and the small size of parvovirus particles, may predispose to cross-placental transmission occurring in some cases of infection in pregnant women. Though rare, parvovirus may cause miscarriage or hydrops fetalis (fluid accumulation in the fetus) with anemia.
Bocavirus has been associated with some cases of respiratory disease and diarrhea in young children. Though it is rarely detected in healthy persons, there is indication that it can occur in up to 9% of pediatric patients hospitalized with lower respiratory infections [131]. Bocavirus should not be confused with Bocas virus, a type of Coronavirus (Group IV, Chapter 42).
SEN virus (SEN-V) is a newly discovered single-stranded non-enveloped DNA virus that has been found in the blood of donors and recipients of transfusion blood [132]. In addition, another Group II virus, TT virus, also known as transfusion transmitted virus or Torque teno virus, has been isolated from transfusion blood. TT virus is currently a suspected hepatitis virus. At this time, the pathogenicity of both SEN-V and TT viruses are in doubt; hence neither virus is included in the list of Group II virus pathogens.
Infectious species:
Bocavirus (respiratory disease and diarrhea in children)
Human parvovirus B19, alternately known as erythrovirus B19 (fifth disease, erythema infectiosum, slapped face disease, transient hemolytic anemia, aplastic anemia)