O Classification Of Viruses

Viruses are classified on the basis of several features:

• Nucleic acid content (DNA or RNA)

• Viral morphology (helical, icosahedral)

• Site of replication in cell (cytoplasm or nucleus)

• Coating (enveloped or nonenveloped)

• Serological typing (antigenic signatures)

• Cell types infected (B lymphocytes, T lymphocytes, monocytes)

The Baltimore Classification Scheme4 (Table 9.1) gives an alternate means of relating the different virus types.

TABLE 9.1 Baltimore Classification Scheme for Viruses

I) Single-stranded RNA viruses

A) Positive sense (virion RNA-like cellular mRNA)

1) Nonenveloped (a) Icosahedral

(i) Picornaviruses (polio, hepatitis A, rhinovirus)

(ii) Calicivirus

(iii) Plant virus relatives of Picornavirus

(iv) MS2 bacteriophage

2) Enveloped

(a) Icosahedral

(i) Togaviruses (rubella, equine encephalitis, Sindbis)

(ii) Flaviviruses (yellow fever, dengue fever)

(b) Helical

(i) Coronavirus

B) Positive sense but requires RNA to be converted to DNA via a virion-associated enzyme (reverse transcriptase) 1) Enveloped

(a) Retroviruses

(i) Oncornaviruses

(ii) Lentiviruses

C) Negative-sense RNA (opposite polarity to cellular mRNA, requires a virion-associated enzyme to begin the replication cycle) 1) Enveloped

(a) Helical

(i) Mononegaviruses (rabies, vesicular stomatitis virus, paramyxovirus, filovirus)

(ii) Segmented genome (orthomyxovirus-influenza, bunyavirus, arenavirus)

II) Double-stranded RNA viruses A) Nonenveloped

1) Icosahedral

(a) Reovirus

(b) Rotavirus

III) Single-stranded DNA viruses A) Nonenveloped

1) Icosahedral

(a) Parvoviruses (canine distemper, adeno-associated virus)

(b) Bacteriophage $X174

IV) Double-stranded DNA viruses

A) Nuclear replication

1) Nonenveloped (a) Icosahedral

(i) Small circular DNA genome (papoviruses, SV40, polyomaviruses, papillomaviruses)

(ii) "Medium" sized, complex morphology, linear DNA (adenovirus)

2) Enveloped-nuclear replicating (a) Icosahedral

(i) Herpesviruses (linear DNA)

(ii) Hepadnavirus (virion encapsidates RNA that is converted to DNA by reverse transcriptase)

B) Cytoplasmic replication

1) Icosahedral (a) Iridovirus

2) Complex symmetry (a) Poxvirus

C) Bacterial viruses

1) Icosahedral with tail

(a) T-series bacteriophages

(b) Bacteriophage A

The following lists some virus types together with diseases that they cause:

• RNA viruses: picornaviruses (polio, hepatitis A, rhinovirus); togavirus (rubella, equine encephalitis); flavivirus (yellow fever, dengue fever, St. Louis encephalitis); bunyaviruses (encephalitis, hemorrhagic fever); rhabdoviruses (vesicular stomatitis); myxoviruses (mumps, measles); reoviruses or rotaviruses (diarrhea); filovirus (Ebola, Marburg); are-naviruses (lymphocytic choriomeningitis); retroviruses (HIV)

• DNA viruses: herpesviruses (herpes, cold sores); papo-vaviruses (polyoma, warts); adenoviruses (respiratory complaints); poxvirus (smallpox); parvovirus (canine distemper)

It has been estimated that viruses cause more than 60% of the infectious diseases that occur in the developing countries. Bacterial infections account for only 15%. Table 9.2 provides a synopsis of virus types with their possible therapeutic modalities.



Prevention of viral infections by conferring artificially acquired active immunity with vaccines is the main approach for preventing most viral diseases. Safe and highly

TABLE 9.2 Classification of Viruses Causing Disease in Humans

Family Agent




RNA Viruses

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