CLASSIFICATION PETER H. RUSSELL, BVSc, PhD, FRCPath, MRCVS Department of Pathology and Infectious Diseases, The Royal Veterinary College, Royal College Street, London NW1 OTU. Web siteWeb site

Objectives Students should be able to: describe in outline how viruses may be classified into families using a number of different criteria including structure and genome. compare and contrast conserved and variable antigens on virus proteins

Viruses are classified into families based on appearance and into species based on the host and disease.

Within a species there are different isolates e.g. the Newmarket 1976 isolate of equine influenza virus.

Classification of viral families  presence or absence of an envelope (tested by ether sensitivity),  capsid symmetry  size and shape of particle,  RNA or DNA, and now their sequence,  The number and polarity (sense) of strands of nucleic acid.  enzymes

Classical terms in classification refer to Virus morphology, Location, Lesion; Virus enzyme:

Antigens

Large numbers of different monoclonal antibodies (mAb) have been made to these different epitopes e.g for a particular virus.

Antigenic variation

Genetic variation is more common in RNA compared to DNA viruses.

Conserved antigens are on many isolates.

The closer isolates are related the more antigens they will share and this is the basis for antigen fingerprinting with monoclonal antibodies

Antigenic fingerprinting is useful to epidemiologists and MAFF eg for tracing the spread of influenza viruses and choosing which vaccines to use

Conserved epitopes involved in neutralisation make for a successful vaccine e.g. for canine distemper.

Summary Families are classified by their structure and genomic sequence. Isolates of a species are classified by their antigens and sequence Conserved antigens are shared between isolates. Variable antigens are on some but not other isolates.