Other Infectious Agents

with the virus from infected plants or by people who smoke. The most important plant virus vectors are probably insects; thus, insect control is a potent tool for controlling the spread of plant viruses.

Insect Transmission of Plant Viruses

Plant viruses can be transmitted by insects in several ways. First, in external or temporary transmission, a virus is associated with the external mouthparts of the vector. In this case, the ability to transmit the virus lasts only a few days. Second, in circulative transmission, the virus circulates but does not multiply in the body of the insect; the virus may be transmitted during the lifetime of the insect. Third, the transmission may involve actual multiplication of the virus within the insect. In this case, the virus is infectious for both the insect and a plant cell.

In many instances of insect infection with plant viruses, the viruses are passed from generation to generation of the insect and may be transmitted to plants at any time. The existence of insect-transmitted plant viruses raises several interesting

Although viruses are composed of only one type of nucleic acid surrounded by a protective protein coat, other agents that cause serious diseases are even simpler in structure. These are the prions and the viroids.


In addition to viruses, another group of agents that cause slow diseases are prions, proteinaceous infectious agents that apparently contain only protein and no nucleic acid (see figure 1.14). These agents have been linked to a number of fatal human diseases as well as to diseases of animals. In all of these afflictions, brain function degenerates as neurons die, and brain tissue develops spongelike holes. Thus, the general term transmissible spongiform encephalopathies has been given to all of these diseases. The time after infection before symptoms appear is many years. Some of the slow but always fatal infections that have been attributed to prions are listed in table 14.12. ■ prions, p. 11

Prions have many properties of viruses, but their evolutionary relationship to viruses is unclear. Both viruses and prions

Table 14.12 Slow Infections Caused by Prions



Site of Infection


Scrapie agent


Central nervous system

Scrapie spongiform encephalopathy

Kuru agent


Central nervous system

Kuru spongiform encephalopathy

Creutzfeldt-Jakob agent


Central nervous system

Creutzfeldt-Jakob spongiform encephalopathy

Mad cow agent

Cows and humans

Central nervous system

Mad cow spongiform encephalopathy

Chronic wasting disease agent

Deer and Elk

Central nervous system

Chronic wasting disease

366 Chapter 14 Viruses, Prions, and Viroids: Infectious Agents of Animals and Plants are obligate intracellular parasites, but prions are smaller than the smallest viruses and, unlike viruses or any other replicating agent, contain no nucleic acid. As a result, prions are not inactivated by UV light or nucleases, but are inactivated by chemicals that denature proteins, as well as by heat.

One of the most intriguing questions regarding prions is how can they replicate if they do not contain any nucleic acid. Recall that only nucleic acids can replicate and code for proteins. An answer to this question is now emerging. It is clear that a protein similar in its amino acid composition to the prion protein is synthesized in normal, uninfected cells of the body. Its function is unknown. However, the tertiary structure of the prion protein differs from the normal protein, which results in their having different properties. The prion protein replicates by converting the normal host protein into prion protein, thereby creating more prion protein molecules (figure 14.22). Thus, the prion protein is "infectious" because it catalyzes the conversion of normal protein into prion protein by changing the folding properties of the protein. Therefore, only if the cell is synthesizing normal protein is the prion protein able to replicate. ■ tertiary structure, p. 28

In most cases, the prion disease is only transmitted to members of the same species, because the amino acid sequence of different prion proteins in different species differs from one another. However, the barrier to prion transmission between species also depends on the strain of prion. It is now clear that the prion that caused mad cow disease in England has killed more than 100 people by causing a disease very similar to Creutzfeldt-Jakob disease. Presumably these people ate beef of infected animals. Thus far, no human deaths have been attributed to eating sheep infected with the scrapie agent or deer and elk infected with the prion causing chronic wasting disease. However, because the incubation period extends over many years, the possibility that cross infection can occur in these situations also has not been ruled out entirely. Additional information about the role of prions in human disease is covered in chapter 26.


The term viroid defines a group of pathogens that are also much smaller and distinctly different from viruses (see figure 1.13). Viroids that have been characterized consist solely of a small, single-stranded RNA molecule that varies in size from 246 to 375 nucleotides. This is about one-tenth the size of the smallest infectious viral RNA known. They have no protein coat and therefore are resistant to proteases. Their other properties include:

■ Viroids replicate autonomously within susceptible cells. No other virions or viroids are required for their replication.

■ A single viroid RNA molecule is capable of infecting a cell.

■ The viroid RNA is circular and is resistant to digestion by nucleases.

Both normal protein (NP) and abnormal prion protein (PP) are present.

Both normal protein (NP) and abnormal prion protein (PP) are present.

Step 1 Abnormal prion protein interacts with the normal protein


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