an introduction to parvoviridae

2/19/2016 Dr. Kaveh Haratian, Dept. of Microbiology and Immunology, ABZUMS 1

An Introduction to

Parvoviridae

The most dependent viruses

• Latin Parvus = small• The smallest of all human viruses(only 20-25 nm in diameter).

• It might be thought that viruses as a class represent the ultimate inparasitism.

• Reliant as they are on their host cells to provide most of the machinery orreplication.

• The Parvoviruses, show a still further degree of dependence .

• As they can replicate only in the presence of active DNA synthesis inrapidly dividing host cells, or under the influence of another virusinfection(helper virus).

• The reason lies in their minute size(smallest of all human viruses).2/19/2016

Dr. Kaveh Haratian, Dept. of Microbiology and Immunology, ABZUMS

2

Little about it`s 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:• 5kb for human parvovirus B19

• 4.7kb for human dependoviruses

2/19/2016Dr. Kaveh Haratian, Dept. of Microbiology and Immunology,

ABZUMS3

More about it`s small genome• The ssDNA genome is of negative polarity!

• But some virions package a positive strand instead(up to 50% of them in the case of the genus Dependovirus).

• All genomes display long terminal palindromic sequences enabling each of ends of the molecule to fold back on itself to form a hairpin structure.


ABZUMS4

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


ABZUMS5

• Parvoviruses of cats, dogs, and mink cause panleukopenia and enteritis.

• Rat parvovirus causes congenital malformation of the fetus.


ABZUMS6

Human parvovirus B19

• Discovered in the serum of asymptomatic blood donors.

• Associated with:• A very common exanthematous disease of children (Fifth disease)

• Aplastic crisis (in patients with chronic hemolytic anemia)

• Hydrops fetalis


ABZUMS7

5th.


ABZUMS8

More infections

Another human parvovirus cause

gastroenteritis2/19/2016

Dr. Kaveh Haratian, Dept. of Microbiology and Immunology, ABZUMS

9

The capsid

• Composed:• One major polypeptide

• Two minor polypeptides

• Arranged to form 60 protein subunits.


ABZUMS10

Some physicochemical properties

• The virus is very stable, resisting:• 60˚C for some hours.

• Variation from pH 3 to 9


ABZUMS11

Simple Classification

Parvoviridae

Parvovirinae

Parvovirus Erythrovirus

H.Parvovirus B19

Dependovirus Bocavirus Amdovirus

Densovirinae

Include 4 genera


ABZUMS12

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


ABZUMS13


ABZUMS14

Parvovirus receptor & Co-receptors


ABZUMS15

Parvovirus replication

• No enzyme in the virion.


ABZUMS16

Viral ssDNA

dsDNA

mRNA

Several different mRNA

Several different proteins

Cellular DNA polymerase

Cellular DdRp II

Alternative splicing

Nonstructural and structural proteins are encoded by the left and right side of the genome respectively

Transactivation vs. down regulation


ABZUMS17

Genome replicationPalindromic 3` terminal sequence

Ds-re

plicative

inte

rme

diate fo

rm

In infected bone marrow cells (concatemers)

Parvovirus:Clinical aspect

• Parvovirus clinical syndromes:• Erythema infectiosum(fifth disease)

• Arthritis (especially in young women)

• Aplastic crisis in chronic hemolytic anemia

• Chronic anemia in immunodeficiency syndromes

• Hydrops fetalis


ABZUMS18

Clinical aspects

Intranasal inoculation

Viremia

• Short lived

• High level

Shedding from throat


ABZUMS19

Over a weekfew days

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.


ABZUMS20

Erythema infectiosum

• Fifth disease

• Innocuous contagious exanthema of childhood.

• Erythematous rubella-like rash on face flushed cheeks

• Though fleeting: rash may reappear in weeks /months.

• Arthralgia:• Children: occasionally• Adult: regular

• Especially in women: peripheral joints: hands, wrists, knees, ankles.• Polyarthritis is often the dominant feature

• 20-25% of infections are asymptomatic.


ABZUMS21

LimbsTrunk

Fades rapidly in 1-2 day, a fine lace


ABZUMS22

The pathogenesis of erythema infectiosum is probably aresult of antibody-antigen immune complex depositionsin skin, blood vessels and synovia. The rash typicallyappears on the cheeks followed by a lace-likemaculopapular rash on the upper part of the body. Jointsymptoms are more common in adults than in children.In addition to deposition of immune complexes, theinflammatory response in synovial tissue may be a resultof the secreted phospholipase A2 motif in the uniqueregion of the B19 minor capsid protein .

Transient Aplastic Crisis(anemia)• Temporary, potentially life-threatening complication in:

• Chronic hemolytic anemia:• Sickle cell anemia

• Thalassemia

• Hereditary spherocytosis

• Characteristics of severe anemia:• Pallor, weakness, lethargy

• Recovery occurs spontaneously in a week.

• Blood transfusion is sometimes lifesaving.


ABZUMS23

Infected with Parvovirus B19

Usually no rash

Sudden drop in hemoglobin

Total disappearance of erythrocytes precursors

from bone marrowReticulocytes from the

blood


ABZUMS24

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 cytotoxicityof the NS1 protein .

Parvovirus B19: other complications

• Chronic anemia in immunodeficient patients in:• Acute leukemia on chemotherapy

• AIDS patients

• Bone marrow recipient transplant

• Children with congenital immune deficiency state.

• Hydrops fetalis: general edema :• Severe anemia

• Congestive cardiac failure


ABZUMS25

Dying of fetuses

Parvovirus in pregnancy


ABZUMS26

Vertical transmission of B19 from a primary infected mothermay cause fetal infection. Pathogenic mechanisms includedevelopment of acute anemia upon infection of fetalhematopoietic cells. In early pregnancy hematopoiesis is seenin the liver and in later pregnancy this shifts to the bonemarrow. The anemia may resolve spontaneously or proceed bycausing cardiac failure and development of hydrops fetalis andin rare cases fetal death. The virus may also cause myocarditisand heart arrest by direct infection of myocardialtissue. Modified from Anderson and Young .

Clinical complications associated with B19


ABZUMS27

Epidemiology• Ubiquitous, common, highly contagious.

• Year-round infection, spring epidemics among school-children 4-10 years.

• Readily transmitted by:• Respiratory secretions & close contacts

• During the incubation period of erythema infectiosum• Chron.hemo. Anem. Patients are infectious up to 1 week after aplastic crisis.• B19 in immunocompromised patients: excrete virus for months/years.

• Transplacental transmission:• Less than 10% leads to fetal death.• Rarely congenital malformations

• Blood transfusion:• Factor VIII is a great problem


ABZUMS28

Attack rate:25 – 50%

Rash appearance: no more infectious

30% maternal infections transmit into fetus

Usually no harm!

The virus is heat stable: survive in clotting factor

concentrate

Treatment and control

• 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


ABZUMS29

B19 in different risk groups


ABZUMS30

DependovirusesAdeno-associated viruses


ABZUMS31


ABZUMS32

Laboratory diagnosis• Cell culture :

• bone marrow, fetal lever (with erythropoietin & IL-3).• MB-02(H.megakaryocytic leukemia cell line)(with GM-CSF).

• Serology :• EIA/RIA: IgM or significant rise of IgG.

• Molecular detection:• Detection of virus DNA by:

• Nucleic acid hybridization• PCR

• EIA: for viral antigens• EIA

• Electron microscopy:• Nuclear inclusion bodies: crystalline array of virions in nucleus.


ABZUMS33

Antigen produced by molecular cloning

In hydrops fetalis: in situ hybridization


ABZUMS34

an introduction to parvoviridae

Health & Medicine