Systematic Veterinary Virology

INTRODUCTION

• Parvoviruses are among the smallest and simplest eukaryotic viruses. They are widespread in nature. Parvoviruses are divided into two groups, defective viruses, which are dependent on helper virus for replication and autonomous or replication-competent viruses.
• Morphology: Virions are not enveloped and hence are not ether sensitive. The capsid/nucleocapsid is round and exhibits icosahedral symmetry. The virions measure 18-26nm diameter and consist only of protein (50%) + DNA (50%). The capsid is round and consists of 60 capsomers. The surface projections are small and surface appears rough with distinct spikes. There are three capsid proteins, VP1-3. VP2 is essential for virulence. The capsids can be penetrated by stain and some appear dark in the centre. The genome is not segmented and consists of a single molecule of linear negative-sense, or negative-sense and positive-sense single-stranded DNA. The complete genome is 5000 nucleotides long. Palindromic sequences are found at the 3’ and 5’ end of the genome.
• Classification: As per the VIII report of ICTV, the family Parvoviridae is divided into two subfamilies Parvovirinae and Densovirinae. These two subfamilies have been further subdivided into 5 and four genera respectively.
• Replication: Replication of parvoviruses occurs at nucleus and they depend entirely on the host cell mechanisms. The replication strategies of parvovirus genome are poorly understood. The virus enters into the cell through endocytosis. Upon uncoating, the DNA is translocated to the nucleus, where is transcribed into mRNA and also serve as template for the synthesis of daughter strands. Host cell DNA polymerase is necesary for genome replication. The mRNA moves to the ribosomes and gets translated into proteins. Capsid assembly takes place in the cytoplasm and DNA is packed into proformed capsid. The virus is released through cell lysis.