Parvoviruses

Parvoviruses are the smallest DNA animal viruses. They are icosahedral, nonenveloped particles 18-26 nm in diameter. The genome is about 5 kb linear single-stranded DNA with limited coding capacity.      Because of the small coding capacity of their genome, the replication of parvoviruses is dependent on functions supplied by replicating host cells or by coinfecting helper viruses.   Classification :      The family parvoviridae is composed of two subfamilies the parvovirinae which infect vertebrates, and Densovirinae, which infect insects. Parvovirinae comprise three genera: the genus Parvovirus, the genus Erythrovirus and the genus Dependovirus.      Members of both the genus Parvovirus and the genus Erythrovirus are able to replicate autonomously in rapidly dividing cells while the genus dependovirus contains members that are defective and dependent on helper virus (usually an adenovirus) for replication and commonly refered to as adeno-associated viruses. Human adenoassociated viruses have not been linked with any disease.   Parvovirus Infection In Humans :      The most recognised human pathogen in this group of viruses is the parvovirus B19 which is the sole member of Erythrovirus genus. It was formerly named Parvo-virus-like agent (PVLA) strain B19. B19 is a fairly ubiquitous human virus, by the age of 16 years, one-third of the population is seropositive. The immature cells in the erythroid lineage (red cell progenitors) are targets for human B19 parvovirus. Thus the major site of virus replication is the adult marrow and the fetal liver.   Clinical Disease : 1. Erythema infection (fifth disease) is a very mild, febrile, self limiting disease that affects infants. It is associated with a faint rash that has a reticular appearence on the limbs. The face has a "slapped cheek" appearance. 2. Transient aplastic crisis : Parvovirus B19 is the cause of transient aplastic crisis that may complicate chronic hemolytic anaemias e.g. sickle cell anaemia, thalassemia. 3. Infection in immunodeficient host result in persistent infection and cause chronic suppression of the bone marrow. The disease is called pure red cell aplasia. 4. Infection during pregnancy may result in hydrops fetalis and fetal death in 10% of pregnancy metarnal infection before the 20th week of pregnancy due to severe fetal anaemia. There is no evidence that B19 infection causes congenital physical abnormalities.   Laboratory Diagnosis :      The virus is difficult to grow. Diagnostic tests are currently available only in few laboratories : 1. The most sensitive tests detect viral DNA in serum, tissue extracts or fixed tissue by molecular techniques using either nucleic acid probes or PCR. 2. Detection of B19 IgM antibody is indicative of recent infection it is present for 2-3 months after infection. B19 IgG persist for years. Antibodies may not be found in immunodeficient patients with chronic B19 infections.