Structure and Classification of Animal Viruses

The structure of viruses was covered in chapter 13 (see figure 13.2); we give a brief review here. The structures of phage, and animal and plant viruses are similar, namely nucleic acid, either DNA or RNA, surrounded by a protein coat, the capsid. The capsid and nucleic acid together are called the nucleocapsid. The capsid, composed of a defined number of units called capsomeres, are held together by non-covalent bonds. If there is no additional covering, the virus is termed naked. Many viruses that infect humans and other animals have a lipid membrane or envelope that surrounds the protein coat. Such a virus, called an enveloped virus, is rarely found among phage or plant viruses. The envelope is usually acquired from the cytoplasmic membrane of the infected cell during viral release from the cell. Thus, the structure of the viral envelope is similar to the membrane of the cell, a lipid bilayer containing various proteins. In certain virus families, a matrix protein is found just inside the lipid envelope. The attachment proteins, or spikes, that bind the virus to the cell project from the envelope or the capsid. Plant viruses, such as tobacco mosaic virus, do not bind to specific sites on the plant cell wall; rather, they enter through wounds and have no protruding attachment proteins.

Another distinguishing feature of some RNA animal and plant viruses is that they have more than one RNA molecule enclosed in their capsids. The RNA molecules may not be identical. For example, the influenza virus has eight RNA molecules, each carrying different genetic information. Such viruses are termed segmented viruses.

The virion can have a number of shapes (figure 14.1). One is isometric in which the protein subunits are arranged in groups of equilateral triangles, the most common arrangement being icosahedral symmetry. These viruses appear spherical when viewed with the electron microscope. A less common shape in animal viruses is the helical- or rod-shaped structure. Other virions are pleomorphic—they have an irregular shape. The most common type of phage, the complex tailed form, does not occur in animal and plant viruses. ■ complex phages, p. 324

Classification of Animal Viruses

The taxonomy of animal viruses changes as more is learned about their properties. The taxonomy likely will continue to evolve, and so only general principles are considered here. The most widely employed taxonomic criteria for animal viruses are based on a number of characteristics:

1. Genome structure—DNA or RNA, single-stranded or double-stranded, segmented or a single molecule

2. Virus particle structure—isometric (icosahedral), helical (rod-shaped), or pleomorphic (irregular in shape)

3. Presence or absence of a viral envelope

Based on these major criteria, animal viruses are divided into a number of families, whose names end in -viridae. Fourteen families of RNA-containing viruses and seven families of DNA-containing viruses infect vertebrates (tables 14.1 and 14.2, respectively). The members of each family are derived from a common ancestor, as shown by nucleic acid hybridization. Evolutionary relationships between families however, cannot be inferred from this taxonomic scheme. The names of the families come from a variety of sources (see tables 14.1 and 14.2). In some cases, the name comes from the appearance of the virion, for example, Coronaviridae coming from corona, which means crown. In other cases, the name comes from the geographic area the virion was first isolated. Bunyaviridae is derived from Bunyamwera, a locality in Uganda, Africa. Each family contains numerous genera whose names end in -virus, making it a single word, for example, Enterovirus. The species name is the name of the disease the virus causes (for example, polio or poliovirus). The species name is one or two words. In contrast to bacterial nomen-

Figure 14.1 Shapes of Viruses (a) EM of human papillomavirus, an isometric virus whose capsomeres can be clearly seen. (b) EM of rhabdovirus particles, with their characteristic bullet shape. (c) Ebola virus, a filamentous virus that occurs in a number of shapes.

Table 14.1 Classification of RNA Viruses Infecting Vertebrates

14.1 Structure and Classification of Animal Viruses 343


Drawing of Virion

Virion Structure

Genome Representative Pathogenic Members

Structure* and Some Diseases They Cause

Picornaviridae (pico, micro; rna, ribonucleic acid)

Caliciviridae (calix, cup)

Togaviridae (toga, cloak)

Flaviviridae (flavus, yellow)

Coronaviridae (corona, crown)

Rhabdoviridae (rhabdos, rod)

Filoviridae, (filo, threadlike)

Paramyxoviridae, (para, by the side of; myxa, mucus)

Orthomyxoviridae (orthos, straight; myxa, mucus)

Bunyaviridae (Bunyamwera, a locality in Uganda)

Arenaviridae (arena, sand)

Reoviridae (respiratory enteric orphan virus)

Birnaviridae (bi, two)

Retroviridae (retro, backwards)

Naked isometric

Naked isometric

Lipid-containing envelope

Lipid-containing envelope

Lipid-containing envelope


lipid-containing envelope

Long filamentous; sometimes circular; lipid-containing envelope


lipid-containing envelope


lipid-containing envelope

Lipid-containing envelope


lipid-containing envelope


Naked; icosahedral


lipid-containing envelope

1 molecule, ss RNA

1 molecule, ss RNA

1 molecule, ss RNA

1 molecule, ss RNA

1 molecule, ss RNA

1 molecule, ss RNA

1 molecule, ss RNA

1 molecule, ss RNA

7-8 segments of linear ss RNA

3 molecules of ss RNA

2 molecules of ss RNA with hydrogen-bonded ends

Linear ds RNA divided into 10,11, or 12 segments

2 segments of linear ds RNA

2 identical molecules ss RNA

Poliovirus; rhinovirus

Norwalk virus; many members cause gastroenteritis

Many multiply in arthropods and vertebrates; encephalitis in humans

Yellow fever virus; dengue virus Colds and respiratory tract infections Rabies virus

Marburg virus; ebola virus

Mumps virus; parainfluenza virus Influenza virus Hantaan virus Lassa virus

Diarrhea in animals

No human pathogens; diseases in chickens and fish HIV

* ss, single-stranded; ds, double-stranded.

clature in which an organism is referred to by its genus and species name, viruses are commonly referred to only by their species name, the name of the disease they cause.

The classification of the family Picornaviridae, which infects humans, is shown in table 14.3. This family contains three genera. The genus Enterovirus contains four species. Each species contains numerous "types.'' Whether these "types'' should really be different species is in dispute. Rhinovirus contains a single species, rhinovirus, which in turn contains 100 "types.'' ■ nucleic acid hybridization, pp. 224,235

In general, viruses with a similar genome structure replicate in a similar way. For example, animal viruses generally follow the same replication strategies as phages with similar genomes.

344 Chapter 14 Viruses, Prions, and Viroids: Infectious Agents of Animals and Plants

Table 14.2 Classification of DNA Viruses Infecting Vertebrates


Drawing of Virion

Virion Structure

Genome Structure*

Representative Pathogenic Members and Some Diseases They Cause

Hepadnaviridae (hepa, liver; dna, deoxyribonucleic acid)

Parvoviridae (parvus, small)

Lipid-containing envelope


1 molecule, mainly ds DNA but with a single-stranded gap

1 molecule, ss DNA

Hepatitis B virus

Outbreaks of gastroenteritis following eating of shellfish

Papovaviridae (papilloma, polyoma, vacuolating agent

Adenoviridae (adenos, gland)

Herpesviridae (herpes, creeping)

Poxviridae (poc, pustule)

Iridoviridae (irid, rainbow)



Enveloped with surface projections

Enveloped; large brick-shaped


1 molecule, circular ds DNA

1 molecule, ds DNA 1 molecule, ds DNA

ds DNA; covalently closed ends

1 molecule, ds DNA

Human papillomaviruses associated with genital and oral carcinomas

Some cause tumors in animals Herpes simplex virus; cytomegalovirus

Smallpox virus; vaccinia virus

No known human pathogens; only animal pathogens

* ss, single-stranded; ds, double-stranded.

Groupings Based on Routes of Transmission

Viruses that cause disease are often grouped according to their routes of transmission from one individual to another. These are not taxonomic groupings, and members of more than one family may be included in the same group. These groupings, summarized in table 14.4, provide examples of such a scheme.

The enteric viruses are usually ingested on material contaminated by feces, the fecal-oral route. They replicate primarily in the intestinal tract, where they usually remain localized. They often cause gastroenteritis, an inflammation of the stomach and intestine. Some, however, such as the poliovirus, repli-

cate first in the intestines but do not cause gastroenteritis. Rather, they cause a systemic disease.

Respiratory viruses usually enter the body in inhaled droplets and replicate in the respiratory tract. The respiratory viruses include only those viruses that remain localized in the respiratory tract. Viruses that infect via the respiratory tract but then cause systemic diseases are not considered respiratory viruses. These latter include the viruses that cause mumps and measles.

Viral zoonoses, caused by zoonotic viruses, are diseases that are transmitted from an animal to a human or to another animal. Many viruses, such as rabies, are transmitted directly from animals to humans, but humans cannot transmit it to other humans. Others, such as canine distemper, can be transmitted from dogs to African lions. One group of viruses, the arboviruses, are so named because they infect arthropods such as mosquitoes, ticks, and sandflies, where they replicate. Arthropods then bite vertebrates and transmit the virus. Thus, the viruses are arthropod-borne. In many cases, viruses can invade and replicate in widely different species. The same arthropod may bite birds, reptiles, and mammals and transfer viruses among these widely different groups. More than 500 arboviruses are known, and about 80 are known to infect humans. Twenty cause significant diseases such as West Nile fever, yellow fever, Western equine encephalitis, and dengue fever.

Sexually transmitted viruses cause lesions in the genital tract. These include herpesviruses and papillomaviruses. Other viruses that cause systemic infections are often transmitted dur

Table 14.3 Classification of Human Picornaviruses

Family Genus Species

Picornaviridae Enterovirus

polioviruses 1-3

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  • Elisha
    Why plant virus are rarely enveloped?
    4 years ago
  • marco
    What are the basic 3 basic shapes of animal viruses?
    4 years ago
  • Angelika
    How to classify the shape of a protein coat of a virus?
    3 years ago
  • elen
    Why bacterial and plant visuses are rarely enveloped?
    3 years ago
  • Selina
    How to classify viruses?
    3 years ago
  • nino
    How animal viruses can be clasdified according to yhe type of nucleic acid they carry?
    3 years ago
  • Danielle Duncan
    Why are enveloped called pleomorphic?
    3 years ago
  • rina barese
    How to clssify viruses basedon envelope?
    3 years ago
  • annett
    How animal viruse can be classified according to the type of nucleic acid they carry?
    3 years ago
  • Hagos
    How animal viruse can be classifed?
    3 years ago
  • elsie
    What are the criteria for animal virus classification?
    3 years ago
  • edward roddy
    What classify virus as animal?
    3 years ago
  • abdullah
    How are animal viruses classified?
    3 years ago
  • tuula mantila
    How to classify animals?
    2 years ago
  • lidia udinese
    How are animal viruses classifeied?
    2 years ago
  • Madeline
    Why is plant viruses helix naked?
    2 years ago
  • danait
    Who gave the name of animal viruses?
    2 years ago
  • robert stanley
    What are major class of animal viruses?
    2 years ago
  • kerttu
    What are the properties used for the classification of animal viruses?
    2 years ago
  • lucie cameron
    Can animal viruses be referred to as complex viruses?
    2 years ago
  • carl amundson
    What are the 5 major criteria for classifying viruses?
    1 year ago
  • fearne jones
    What do phages plant and animal viruses has in common?
    1 year ago
  • anne
    What are clasification of animal virus?
    1 year ago
  • sharonda
    What structure is found in many animal viruses but isn't found in bacterial viruses?
    1 year ago
  • mathias himmel
    What is the most common structure of animal viruses?
    1 year ago
  • roderick
    What viruses can animals get from icosahedral?
    10 months ago
  • gary
    What are the examples of isometric shaped virus?
    2 months ago

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