Parvoviruses

1. Parvoviruses
By- Sanju Sah
St. Xavier’s College, Maitighar, Kathmandu
Department of Microbiology

2. Background
• The Parvoviruses are significant pathogens in the veterinary sciences.
• They are particularly associated with reproductive failure.
• The family Parvoviridae encompasses 3genera which infect a variety of host
species.
• They are small naked icosahedral viruses with a single-stranded DNA.
• This dependoviruses requires a helper virus e.g. Adenovirus or HSV for
replication.
• They have been described in a number of mammalian species including man
but have yet to be linked with any disease.
• The third genus consists of autonomously replicating parvoviruses which are
frequently associated with disease.

3. INTRODUCTION
• Parvoviruses are the smallest of the DNA viruses belonging to the family
Parvoviridae. (Latin Parvus = small) (only 20-25 nm in diameter).
• They are icosahedral, nonenveloped viruses containing a single-stranded
DNA.
• Human parvovirus B 19 (B 19) is the only known parvovirus that is
pathogenic for humans, and it shows tropism for erythroid progenitor cells.
• The megakaryocyte–erythroid progenitor cell (or MEP, or hMEP to specify
human) is a cell that gives rise to megakaryocytes and erythrocytes

4. • The Parvovirus B19 virus, generally referred to as Parvovirus B19 or
sometimes Erythrovirus B19.
• Was the first(and until 2005 the only) known human virus in the
family of Parvoviruses, genus Erythrovirus.
• Parvovirus B19 virus is most known for causing disease in the
paediatric population; however, it can also affect adults.
• It is the classic cause of the childhood rash called fifth disease or
erythema infectiosum, or ‘slapped cheek syndrome.’

6. History:
• The virus was discovered by chance in 1975 by Australian virologist Yvonne
Cossart.
• In 1974, Cossart et al. first identified B19 while evaluating tests for
hepatitis B virus surface antigen.
• The name originates from the coding of a serum sample, number 19 in
panel B, that gave anomalous results when tested by counter immune
electrophoresis and radioimmunoassay.
• It might be thought that viruses as a class represent the ultimate in
parasitism.

7. • Reliant as they are on their host cells to provide most of the machinery
or replication.
• The Parvoviruses, show a still further degree of dependence .
• As they can replicate only in the presence of active DNA synthesis in
rapidly dividing host cells, or under the influence of another virus
infection(helper virus).
• The reason lies in their minute size(smallest of all human viruses).

8. Simple Classification
Parvoviridae
Parvovirinae
Parvovirus Erythrovirus
H.Parvovirus
B19
Dependovirus Bocavirus Amdovirus
Densovirinae
Include4
genera

9. Family: Parvoviridae
Subfamily: Parvovirinae
Genus: Erythrovirus
Species: Human Parvovirus B19

10. Classification
• The family Parvoviridae consists of three genera:
• Dependo-virus, Parvovirus, and Erythrovirus.
• The genus Dependovirus consists of poxviruses which are defective and
replicate only in association with a second helper virus.
• They neither cause illness by themselves nor alter the infection caused
by helper viruses.

11. • These are usually found in association with an adenovirus, hence are
known as adeno-associated viruses.
• These viruses do not cause any disease in humans.
• The genus Parvovirus includes the viruses which include animal
viruses of veterinary importance.
• The genus Erythrovirus consists of B 19, the only member of the
Parvoviridae family known to cause disease in humans.

12. • Parvoviruses of cats, dogs, and mink cause panleukopenia and enteritis.
• Rat parvovirus causes congenital malformation of the fetus.
12

13. Canine Parvovirus Infection in Dogs
• The canine parvovirus (CPV) infection is a highly contagious viral illness that
affects dogs.
• The virus manifests itself in two different forms.
• The more common form is the intestinal form, which is characterized by
vomiting, diarrhea, weight loss, and lack of appetite (anorexia).
• The less common form is the cardiac form, which attacks the heart muscles
of very young puppies, often leading to death.
• The majority of cases are seen in puppies that are between six weeks and
six months old.
• The incidence of canine parvovirus infections has been reduced radically by
early vaccination in young puppies.

14. Major fact about their replication
• They are able to replicate only in dividing cells,
• Or, In the case of members of the Dependovirus genus, in the presence of a
helper virus.
• This requirement for dividing cells accounts for their predilection for:
• Bone marrow
• Gut
• Developing fetus
14

15. Structure and composition
• Icosahedral, non- enveloped, 32 capsomers
• Diameter: 18-26 nm
• Resistant: stable pH= 3-9
• Heat= 56 °C (60 minutes)
• Inactivated: formalin, Beta propiolactone, Oxidizing agents
• Protein coat: VP1, VP2 (90% protein coat)
• DNA: 20%, 5kb, single strandes, linear,
• protein: 80%

16. The capsid
• Composed:
• One major polypeptide
• Two minor
polypeptides
• Arranged to form 60
protein subunits.

17. 17

18. PARVOVIRUS B19

19. PARVOVIRUS B19
• Parvovirus B19, or B19 virus, is the
causative agent of erythema infectiosum
• ('fifth disease'-it was fifth of the six
classified exanthematous diseases of
childhood),
• a mild viral illness of children, and
• polyarthralgia-arthritis syndrome in
immunocompetent adults.

20. • Parvovirus B19 infects only humans.
• We can have a range of symptoms depending on your age and overall
health.
• About two out of 10 people who get infected with this virus will have no
symptoms.
• Others may have only mild, rash illness.

22. Fifth disease (Erythema infectiosum)
• In the pre-vaccination era, it was frequently the "fifth disease" that a child
would develop.
• It is caused by a virus known as parvovirus B 19.
• Symptoms include low-grade fever, fatigue, a "slapped cheeks rash," and a
rash over the whole body.
• While the illness is not serious in children, 80% of adults have joint aches
and pains (arthritis) which may become long-term with stiffness in the
morning, redness and swelling of the same joints on both sides of the body
(a "symmetrical" arthritis), most commonly involving the knees, fingers, and
wrists.

24. polyarthralgia-arthritis

25. • Pregnant women (who have not previously had the illness)
should avoid contact with patients who have fifth disease.
• The fifth disease virus can infect the fetus prior to birth.
• And, while no birth defects have been reported as a result of
fifth disease, it can cause the death of the unborn fetus.
• The risk of fetal death is 5-10% if the mother becomes
infected.

26. Human infections caused by B19virus

27. Properties:
• It belongs to the Parvoviridae family
• It is small DNA viruses.
• It is a non-enveloped
• It has icosahedral in symmetry.
• It contains a single-stranded linear DNA genome.
• Approximately equal proportions of DNA of positive and negative
sense are found in separate particles.

28. Morphology
• B 19 viruses are extremely small viruses,
measuring 18-26 nm in diameter.
• They possess a nonenveloped,
icosahedral capsid.
• The viral genome contains a single-
stranded DNA measuring 4000-6000
bases in length.
• The genome is negative-strand DNA.

29. • Molecular biologists call a single strand of
DNA, sense (or positive (+)) if an RNA version of the same
sequence is translated or translatable into protein.
• Its complementary strand is called antisense (or negative
(-) sense)
• The genome encodes for many proteins which include
three structural, one major nonstructural, and several
smaller proteins.

30. Viral replication
• B 19 virus shows tropism for (a) bone marrow cells, (b) erythroid cells from
fetal liver, and (c) erythroid leukemia cells.
• Replication of virus occurs in the nucleus.
• The single- stranded DNA genome has hairpin loops at both of its ends which
facilitate double-stranded areas for the cellular DNA polymerase to start
synthesis of the progeny genomes.
• The cellular RNA polymerase synthesizes viral mRNA from the double-
stranded DNA intermediate.
• This is followed by assembly of virions in the nucleus.
• Viral replication results in the cell death.

32. • The life cycle of Parvovirus B19, like those of other nonenveloped
DNA viruses, includes
• binding of the virus to host cell receptors,
• internalization,
• translocation of the genome to the host nucleus,
• DNA replication,
• RNA transcription,
• assembly of capsids and packaging of the genome, and
• finally cell lysis with release of the mature virions.

33. • The Parvovirus B19 was initially shown to agglutinate human red cells
• It was hypothesized that the same agglutinin may act as the host cell
receptor on erythroid progenitor cells.
• Thus, the hemagglutinin was identified as the glycolipid globoside, also
known as the blood group P antigen, by using hemagglutination as a
surrogate marker.
• Parvovirus B19 and Parvovirus B19 VP2 VLP both bind directly to P
antigen.
• In tissue culture either excess P antigen or anti-P monoclonal antibody
can protect erythroid progenitors from infection with Parvovirus B19,
• thus demonstrating that P antigen is the Parvovirus B19 receptor.

34. Genomic organization
• A The Parvovirus B19 genome consists of a single stranded linear
molecule of 5.6 kb nucleotides
• Which is composed of an internal coding sequence of 4.8 kb nucleotides
flanked by terminal repeat sequences of 0.3 nucleotide each.
• These terminal repeat sequences are imperfect palindromes and fold
back on themselves to form hairpin Loops as shown in Figure.
• The P6 promoter at the far leftside the genome initiates transcription of
all Parvovirus B19 proteins.
• The nonstructural protein, NS1 is encoded by the left side of the genome
(nucleotides 0.4-2.4kb) and is approximately 77 kDa.

35. Small genome
• Contain so little genetic
information.
• Consist of a single icosahedral
shell, surrounding a linear single-
stranded DNA molecule of very
limited coding potential:
• 5.6kb for human parvovirus
B19
• 4.7kb for human
dependoviruses

36. Small genome
• The ssDNA genome is of negative polarity
• All genomes display long terminal palindromic sequences enabling
each of ends of the molecule to fold back on itself to form a hairpin
structure.
36

38. • Parvovirus non-structural proteins are fairly homologous between
species, consistent with their role in virus propagation and
• Parvovirus B19 NS1 contains two phosphorylation sites, an amidation
site and a nucleotide binding site.
• NS1 is localized to the nucleus of infected cells is found covalently
bound to mature virions.
• Parvovirus B19 particles are icosahedral and made up of 60 copies of
the capsid proteins:
• 96% VP1 and 4% VP2 a ratio resulting from the relative inefficiency
of VP1 translation.

39. Palindromic 3` terminal sequence
Genome replication
Ds-replicative
intermediate
form
In infected bone marrow cells
(concatemers)

41. Antigenic properties
• Only one serotype of B 19 virus is known to occur.
Other properties
• B 19 virus is highly resistant to inactivation but can be inactivated
by formalin, beta propiolactone, and oxidizing agents.
• The viruses withstand heating at 56°C for 30 minutes and are
stable between pH 3 and 9.

42. Parvovirus replication
• Unlike the double-stranded DNA
viruses
• ssDNA parviruses can replicate only in
dividing cells.
• Parvoviruses replicate in the nucleus:
• Transcription and replication of the
genome
• Accumulation of nonstructural
proteins
• Assembly of virions

43. Parvovirus replication
• No enzyme in the virion.
mRNA
Several
different
mRNA
Several
different
proteins
dsDNA
Cellular DNA polymerase
Viral
ssDNA
Cellular DdRp II
Alternative splicing
Nonstructural and structural proteins are
encoded by the left and right side of thegenome
respectively
Transactivation vs. down regulation

44. Pathogenesis and Immunity
• B 19 virus shows a tropism for two types of cells:
• (a) red blood cell (RBC) precursors and
• (b) endothelial cells in the blood vessels.
• The virus infects rapidly dividing erythrocyte precursors such as
bone marrow cells, erythroid cells from fetal liver, and erythroid
leukemia cells and destroys these cells after infection, thereby
causing aplastic anemia.
• Infection of the endothelial cells in the blood vessels leads to
erythema infectiosum.

46. • It has been demonstrated that the B19 virus first enters through the
nasopharynx or upper respiratory tract and then spreads to the blood,
causing viremia.
• The virus then infects mitotically active erythroid precursor cells in
bone marrow and establishes the infection.
• Aplastic anemia, also spelled aplastic anaemia, is a rare disease in
which the bone marrow, and the hematopoietic stem cells that reside
there, are damaged.
• This causes a deficiency of all three blood cell types (pancytopenia): red
blood cells (anemia), white blood cells (leukopenia),
and platelets (thrombocytopenia).
• Aplastic refers to inability of the stem cells to generate mature blood
cells.

48. Clinical aspects
• Patients displayed a biphasic illness:
• (day 8-11) Fever, malaise, myalgia, chills, peak level of virus in blood, destruction of
erythroblasts in bone marrow.
• (day 17-24) rash and arthralgia occurred, viremia had disappeared, IgM had peaked, IgG
had begun to rise.
48

49. Pathogenesis and Immunity
• The virus enters susceptible cells through the P blood antigen receptors
on the erythrocyte precursors.
• Inside the red cells the virus enters the nucleus, starts replicating,
followed by killing of the red cells.
• The production of RBCs is stopped for approximately 1 week due to
killing of the erythroid precursor cells by the viruses.
• The initial stage is associated with flu-like illness caused by large viremia.
• The viruses are shed in the oral and respiratory secretions and even cross
the placenta.
• Subsequently, viremia is controlled by the production of specific
antibodies against B 19 virus.

50. • Toenter host cells, parvoviruses bind to a cell surface receptor.
• Penetration into the cytoplasm is mediated by a phospholipase A2 activity
carried on the amino-terminal peptide of the capsid VP1 polypeptide.
• Once in the cytoplasm, the intact virus is translocated to the nucleus prior to
uncoating.
• Transcription only initiates when the host cell enters S-phase under its own
cell cycle control, at which time the cell's replication machinery converts the
incoming single strand into a duplex transcription template, allowing
synthesis of mRNAs encoding the non-structural proteins, NS1 and NS2.
• The mRNAs are transported out of the nucleus into the cytoplasm where the
host ribosomes translate them into viral proteins.
• Viral DNA replication proceeds through a series of monomeric and
concatemeric duplex intermediates by a unidirectional strand-displacement
mechanism that is mediated by components of the host replication fork,
aided and orchestrated by the viral NS1 polypeptide.

51. • NS1 also transactivates an internal transcriptional promoter that directs synthesis of
the structural VP polypeptides.
• Once assembled capsids are available, replication shifts from synthesizing duplex
DNA to displacement of progeny single strands, which are typically negative-sense
and are packaged in a 3'-to-5' direction into preformed particles within the nucleus.
• Mature virions may be released from infected cells prior to cell lysis, which
promotes rapid transmission of the virus, but if this fails then the virus is released at
cell lysis.
• Unlike most other DNA viruses, parvoviruses are unable to activate DNA synthesis in
host cells.
• Thus, in order for viral replication to take place the infected cells must be non-
quiescent (i.e. must be actively mitotic).
• Their inability to force host cells into S-phase means that parvoviruses are non-
tumorigenic.
• Indeed, they are commonly oncolytic, showing a strong tendency to replicate
preferentially in cells with transformed phenotypes.

52. • The rash and arthralgia present the second stage of the disease, and is
believed to be immunologically mediated.
• This stage coincides with the disappearance of B 19 virus from the circulation,
appearance of B19 virus-specific IgM and IgG antibodies, and finally the
formation of immune complexes.
Host immunity
• “The disease exhibits two stages: initial stage is flu-like illness and second
stage is appearance of rash and arthralgia.
• Host immunity to B 19 virus infection is primarily antibody mediated.
• The circulating antibodies stop the viremia and are important for resolution of
the disease.
• The role of cell-mediated immunity in conferring immunity to B 19 virus is
unknown.

53. Clinical Syndromes (Symptoms)
• Parvovirus B19 most commonly causes fifth disease, a mild rash illness
that usually affects children.
• Adults can get infected with parvovirus B19 resulting in fifth disease too.
• Less common symptoms of parvovirus B19 infection include
• painful or swollen joints (polyarthropathy syndrome), which is more
common in adults, and
• severe anemia (a condition in which the body doesn’t have enough
healthy red blood cells).
• In rare cases, some of these symptoms can persist for a long time.

54. • B19 virus causes following clinical syndromes:
• (a) Flu like illness, (b) erythema infectiosum or fifth disease, (c) infection in pregnant
women, and (d) chronic B 19 infection (Chronic anemia in immunodeficiency
syndromes),(e)Arthritis (especially in young women)
Flu-like illness
• B 19 virus most commonly causes a flu-like illness.
• Malaise, headache, myalgia, and rhinorrhea are the common symptoms.
Erythema infectiosum or fifth disease
• B 19 virus is an additional causative agent of erythema infectiosum or fifth
disease, the condition seen most commonly in children.
• The infection begins with nonspecific symptoms, followed by appearance of a
distinctive rash on 5th day of infection.

55. Clinical Syndromes
• A bright red rash develops
on both cheeks that appear
as they have been slapped.
• The rash then appears on
the trunk, which spreads
gradually toward the arms
and legs.
• The condition usually
subsides within 1-2 weeks.

56. Clinical Syndromes
Aplastic anemia
• Transient but severe aplastic anemia can
occur in children with chronic anemia such
as sickle cell anemia, thalassemia, and
spherocytosis (aplastic crisis) after infection
with B 19 virus.
• Gloves and sock syndrome is another serious
complication caused by B19 virus.
• In this syndrome, erythematous exanthema
appears on the hands and feet, with a well-
defined margin on the wrist and ankle joints.

57. Gloves and sock syndrome

58. • B19 binds to immature
erythroblasts thereby arresting
production of mature
erythropoietic cells.
• Following acute infection, the
reticulocyte count in peripheral
blood is zero and if the patients
have an underlying disorder with
pathologic red cell survival, the
number of erythrocytes may fall
dramatically in peripheral blood.
• The pathogenesis of
thrombocytopenia is thought to
be explained by the cytotoxicity
of the NS1 protein .

59. • The pathogenesis of erythema infectiosum is probably a result of
antibody-antigen immune complex depositions in skin, blood vessels
and synovia.
• The rash typically appears on the cheeks followed by a lace-like
maculopapular rash on the upper part of the body.
• Joint symptoms are more common in adults than in children. In
addition to deposition of immune complexes, the
• inflammatory response in synovial tissue may be a result of the
secreted phospholipase A2 motif in the unique region of the B19
minor capsid protein .

60. Infection in pregnant women
• If the B 19 virus causes reinfection in a pregnant mother who is infected
earlier by the same virus, and is already immune to the virus (showing
positive B19 antibodies), then no adverse effects are seen in the fetus.
• In non immune seronegative pregnant mothers, B19 virus infection is
increasingly associated with risk for fetal death.
• The infection may cause severe anemia in the fetus, and subsequently, the
fetus may develop signs of high-output cardiac failure (hydrops fetalis).
• B19 virus, however, does not cause any congenital anomalies in the fetus.
• Vertical transmission of B19 from a primary infected mother may cause fetal
infection.
• Pathogenic mechanisms include development of acute anemia upon
infection of fetal hematopoietic cells.

61. • In early pregnancy hematopoiesis is seen in the liver and in later pregnancy
this shifts to the bone marrow.
• The anemia may resolve spontaneously or proceed by causing cardiac
failure and development of hydrops fetalis and in rare cases fetal death.
• The virus may also cause myocarditis and heart arrest by direct infection of
myocardial tissue.

64. Clinical Syndromes
Chronic B19 infection
• This infection occurs in immunocompromised patients such
as patients with HIV, those receiving immunosuppressive
therapy, and transplant patients.
• Chronic anemia, leukopenia, and thrombocytopenia are the
common manifestations.

65. Epidemiology
• Geographical distribution
• B 19 virus is distributed worldwide.
• The exact data on sero-positivity in population world over is not known.
• Approximately, 90% of adults older than 60 years are seropositive in the
United States.
• Similar data from other parts of the world are lacking.
• Reservoir, source, and transmission of infection
• B 19 virus infection is strictly a human disease.
• Humans are the only reservoir of infections.
• Viruses are excreted in respiratory samples which are the primary source of
infection.

66. • Parvovirus B19 infections are seasonal, with peak occurrence in late
winter, spring, and summer.
• Between 10% and 60% of susceptibly schoolchildren develop fifth
disease in school outbreaks; and 20% to 30% of susceptible adult school
and daycare personnel.
• The infected patient is contagious from 24 to 48 hours before developing
prodromal syndrome till the appearance of rash.
• The infection is transmitted by
• (a) vertical transmission during birth,
• (b) respiratory route through respiratory secretions,
• (c) percutaneous exposure to blood and transfusion of blood and blood
products (pooled RBCs, platelets, intravenous immunoglobulins, etc.).

67. B19 in different risk groups
Intravenous immunoglobulin (IVIG)

68. Laboratory Diagnosis
• Demonstration of specific IgG and IgM antibodies in the serum is useful for diagnosis of
erythema infectiosum caused by B19 virus.
• ELISA (enzyme-linked immunosorbent assay), RIA (radioimmunoassay), and IFA(indirect
fluorescent antibody) for demonstration of IgG and IgM antibodies are available.
• In pregnant women, the serum positive for IgG and IgM antibodies indicates B19 virus
infection within 7 days to 4 months, and a possible risk to fetus.
• Positive IgG but negative IgM result indicates only past infection, hence no risk to fetus.
• Furthermore, polymerase chain reation (PCR) is available to demonstrate B19 virus
genome in the blood. The positive result suggests viremia or infection.
• Nucleic acid hybridization
• Cell culture
• Electron microscopy
• Nuclear inclusion bodies: crystalline array of virions in nucleus

69. Treatment
• No specific antiviral therapy is available for treatment of B 19 virus
infection.
• Human Immunoglobulin (HNIG):
• Intravenous immunoglobulin (IVIG) therapy has been shown to be
effective for parvovirus B19-associated pure RBC aplasia in
immunosuppressed persons and also for several cases of other clinical
manifestations in association with persistent parvovirus B19 infection
Prevention and Control
• No specific measures are available for prevention of the infection.
• Development of a vaccine against B 19 virus is undergoing.

70. NOTE:
• Erythema infectiosum requires no treatment.
• Aplastic crisis requires supportive care and blood transfusion.
• Severe persistent anemia(Immunocomp. Patients) requires IVIG.
• Persons of potential risk:
• Pregnant non-immune women
• Immunocompromised individuals
• Chronic hemolytic anemia

75. Bocavirus

76. Bocavirus
• Bocavirus are members of the Parvoviridae virus family that are small
(20 nm), non-enveloped viruses with single-stranded DNA.
• Bocavirus is found usually in infants and children who are hospitalized
with pneumonia or diarrheal symptoms.
• Bocavirus is often detected in patients who are infected with other
viruses.
• Although some investigators suspect bocavirus to cause infection and
disease, there is no definitive proof that bocavirus causes infection or
disease, either alone or with other viruses.

77. What is bocavirus?
• Bocavirus (also termed HBoV or human bocavirus) is a small (20 nm in size)
non-enveloped virus with a single strand of DNA that comprises its
genome.
• The bocavirus genus is a member of the Parvoviridae family, and to date,
three strains have been identified: HBoV, HBoV-2, and HBoV-3.
• Bocavirus is a new viral genus that was discovered in 2005 in upper
respiratory secretions from acutely ill children.
• The name bocavirus was derived by combining the names bovine
parvovirus with canine minute virus with which bocavirus shares some
genetic and structural characteristics.

78. • The ICTVdB (International Committee on Taxonomy of Viruses
database) has detailed its genome and structure.
• Because bocavirus is usually only found in individuals (usually infants,
children, and infrequently in young adults) with lower respiratory
infections or diarrhea,
• There are no commercially available tests for any bocavirus strains.
• Researchers, however, use a polymerase chain reaction(PCR) to
detect virus in nasopharyngeal aspirates (NPA)
• Although this virus has been found worldwide in humans and
animals, there is ongoing research and discussion about this virus as
being a pathogen that causes infection, either alone or in conjunction
with other virus types.

79. • Many investigators consider this newly discovered virus genus as an
"emerging viral pathogen" because it is only proven to be associated
with infections but not yet proven to be a cause of them, either alone
or in conjunction with other viruses.
• However, another member of the Parvoviridae family, a parvovirus
termed B19, causes erythema infectiosum (fifth disease or "slapped
cheek" syndrome), hydrops fetalis (severe anemia in pregnant
women), and aplastic crisis (cessation of red blood cell production) in
individuals that have sickle cell disease.
• Bocavirus has not been associated with these conditions caused by
parvovirus B19.

80. • There is no treatment, medical or antiviral, that is known to effectively
target bocavirus strains.
• A few investigators suggest that since there is no definitive evidence that
bocavirus causes infection or disease, either alone or in combination with
other viruses, there should be no treatment directed toward bocavirus.
• Other investigators believe that since bocavirus strains are usually
associated with patients with respiratory or gastrointestinal symptoms,
treatments should be considered.
• Currently, the only available treatments (for example, oxygen, respiratory
support, and hydration) are for the relief of symptoms since no specific
anti-bocavirus treatments are available.