18.2 Viruses, viroids, and prions are formidable pathogens in ...

18.2 Viruses, viroids, and prions are formidable pathogens in animals and plants
Section Vocabulary Vaccine - A harmless variant or derivative of a pathogen that stimulates a host’s immune system
to mount defenses against the pathogen.
Viroid - A plant pathogen composed of molecules of naked circular RNA only several hundred
nucleotides long.
Prion - An infectious form of protein that may increase in number by converting related proteins
to more prions.
Viral Diseases in Animals The link between a viral infection and the symptoms it produces is often obscure. Viruses may
damage or kill cells by causing the release of hydrolytic enzymes from lysosomes. Some viruses
cause infected cells to produce toxins that lead to disease symptoms, and some have molecular
components that are toxic, such as envelope proteins. How much damage a virus causes depends
partly on the ability of the infected tissue to regenerate by cell division. People usually recover
completely from colds because the epithelium of the respiratory tract, which the viruses infect,
can efficiently repair itself. In contrast, damage inflicted by poliovirus to mature nerve cells is
permanent, because these cells do not divide and usually cannot be replaced. Many of the
temporary symptoms associated with viral infections, such as fever and aches, actually result
from the body’s own efforts at defending itself against infection.
The immune system is a complex and critical part of the body’s natural defenses. The immune
system is also the basis for the major medical tool for preventing viral infections—vaccines.
Vaccines are harmless variants or derivatives of pathogenic microbes that stimulate the immune
system to mount defenses against the actual pathogen. Vaccination has eradicated smallpox, at
one time a devastating scourge in many parts of the world. The viruses that cause smallpox,
polio, and measles infect only humans. This very narrow host range was critical to the successful
effort of the World Health Organization to eradicate smallpox; similar worldwide vaccination
campaigns currently are under way to eradicate the other two viruses as well. Effective vaccines
are also available against rubella, mumps, hepatitis B, and a number of other viral diseases.
Although vaccines can prevent certain viral illnesses, medical technology can do little, at present,
to cure most viral infections once they occur. The antibiotics that help us recover from bacterial
infections are powerless against viruses. Antibiotics kill bacteria by inhibiting enzyme–catalyzed
processes specific to the pathogens, but viruses have few or no enzymes of their own. However,
a few drugs effectively combat certain viruses. Most antiviral drugs resemble nucleosides and as
a result interfere with viral nucleic acid synthesis. One such drug is acyclovir, which impedes
herpesvirus reproduction by inhibiting the viral polymerase that synthesizes viral DNA. Similarly,
azidothymidine (AZT) curbs HIV reproduction by interfering with the synthesis of DNA by reverse
transcriptase. In the past ten years, much effort has gone into developing drugs against HIV.
Currently, multidrug treatments, sometimes called “cocktails,” have been found to be most
effective. Such a regimen commonly includes a combination of two nucleoside mimics and a
protease inhibitor, which interferes with an enzyme required for assembly of virus particles.
Emerging Viruses Viruses that appear suddenly or that suddenly come to the attention of medical scientists are
often referred to as emerging viruses. HIV, the AIDS virus, is a classic example: This virus
appeared in San Francisco in the early 1980s, seemingly out of nowhere. The deadly Ebola virus,
recognized initially in 1976 in central Africa, is one of several emerging viruses that cause
hemorrhagic fever, an often fatal syndrome characterized by fever, vomiting, massive bleeding,
and circulatory system collapse. A number of other dangerous new viruses cause encephalitis,
inflammation of the brain. One example is the West Nile virus, which appeared for the first time
in North America in 1999 and has spread to all 48 contiguous states in the U.S.
An even more recent viral disease to emerge is severe acute respiratory syndrome (SARS), which
first appeared in southern China in November 2002 (Figure 18.11a). During a global outbreak
from November 2002 to July 2003, about 8,000 people were known to be infected, of whom
more than 700 subsequently died. Researchers quickly identified the agent causing SARS as a
coronavirus, a virus with a single–stranded RNA genome (class IV) that was not previously known
to cause disease in humans (Figure 18.11b).
From where and how do such viruses burst on the human scene, giving rise to previously rare or
unknown diseases? Three processes contribute to the emergence of viral diseases. First, the
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18.2 Viruses, viroids, and prions are formidable pathogens in animals and plants
mutation of existing viruses is a major source of these new diseases. RNA viruses tend to have an
unusually high rate of mutation because errors in replicating their RNA genomes are not
corrected by proofreading. Some mutations enable existing viruses to evolve into new genetic
varieties (strains) that can cause disease in individuals who had developed immunity to the
ancestral virus. Flu epidemics, for instance, are caused by new strains of influenza virus
genetically different enough from earlier strains that people have little immunity to them.
Another source of new viral diseases is the spread of existing viruses from one host species to
another. Scientists estimate that about three–quarters of new human diseases originate in other
animals. For example, hantavirus is common in rodents, especially deer mice. The population of
deer mice in the southwestern United States exploded in 1993 after unusually wet weather
increased the rodents′ food supply. Many people who inhaled dust containing traces of urine and
feces from infected mice became infected with hantavirus, and dozens died. The source of the
SARS–causing virus was still undetermined as of spring 2004, although candidates include the
exotic animals found in food markets in China. And early 2004 brought reports of the first cases
of people in southeast Asia infected with a flu virus previously seen only in birds. If this virus
evolves so that it can spread easily from person to person, the potential for a major human
outbreak is significant. Indeed, evidence is strong that the flu pandemic of 1918–1919, which
killed about 40 million people, originated in birds.
Finally, the dissemination of a viral disease from a small, isolated population can lead to
widespread epidemics. For instance, AIDS went unnamed and virtually unnoticed for decades
before it began to spread around the world. In this case, technological and social factors,
including affordable international travel, blood transfusions, sexual promiscuity, and the abuse of
intravenous drugs, allowed a previously rare human disease to become a global scourge.
Thus, emerging viruses are generally not new; rather, they are existing viruses that mutate,
spread to new host species, or disseminate more widely in the current host species. Changes in
host behavior or environmental changes can increase the viral traffic responsible for emerging
diseases. For example, new roads through remote areas can allow viruses to spread between
previously isolated human populations. Another problem is the destruction of forests to expand
cropland, an environmental disturbance that brings humans into contact with other animals that
may host viruses capable of infecting humans.
Viral Diseases in Plants More than 2,000 types of viral diseases of plants are known, and together they account for an
estimated loss of $15 billion annually worldwide due to agricultural and horticultural crop
destruction. Common symptoms of viral infection include bleached or brown spots on leaves and
fruits, stunted growth, and damaged flowers or roots, all tending to diminish the yield and
quality of crops (Figure 18.12).
Plant viruses have the same basic structure and mode of replication as animal viruses. Most plant
viruses discovered thus far, including tobacco mosaic virus (TMV), have an RNA genome. Many
have a rod–shaped capsid, like TMV (see Figure 18.4a); others have a polyhedral capsid.
Plant viral diseases spread by two major routes. In the first route, called horizontal transmission,
a plant is infected from an external source of the virus. Because the invading virus must get past
the plant’s outer protective layer of cells (the epidermis), the plant becomes more susceptible to
viral infections if it has been damaged by wind, injury, or insects. Insects pose a double threat,
because they can also act as carriers of viruses, transmitting disease from plant to plant. Farmers
and gardeners may transmit plant viruses inadvertently on pruning shears and other tools. The
other route of viral infection is vertical transmission, in which a plant inherits a viral infection
from a parent. Vertical transmission can occur in asexual propagation (for example, by taking
cuttings) or in sexual reproduction via infected seeds.
Once a virus enters a plant cell and begins reproducing, viral components can spread throughout
the plant by passing through plasmodesmata, the cytoplasmic connections that penetrate the
walls between adjacent plant cells (see Figure 6.28). Proteins encoded by viral genes are capable
of altering the diameter of plasmodesmata to allow passage of viral proteins or genomes.
Scientists have not yet devised cures for most viral plant diseases. So their efforts are focused
largely on reducing the incidence and transmission of such diseases and on breeding varieties of
crop plants that are relatively resistant to certain viruses.
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18.2 Viruses, viroids, and prions are formidable pathogens in animals and plants
Viroids and Prions: The
Simplest Infectious Agents As small and simple as viruses are, they dwarf another class of pathogens: viroids. These are
circular RNA molecules, only several hundred nucleotides long, that infect plants. One viroid
disease has killed over 10 million coconut palms in the Philippines. Viroids do not encode
proteins but can replicate in host plant cells, apparently using cellular enzymes. These small RNA
molecules seem to cause errors in the regulatory systems that control plant growth, and the
symptoms typically associated with viroid diseases are abnormal development and stunted
growth.
An important lesson from viroids is that a single molecule can be an infectious agent that spreads
a disease. But viroids are nucleic acid, whose ability to be replicated is well known. Even more
surprising is the evidence for infectious proteins, called prions, which appear to cause a number
of degenerative brain diseases in various animal species. These diseases include scrapie in sheep;
mad cow disease, which has plagued the European beef industry in recent years; and
Creutzfeldt–Jakob disease in humans, which has caused the death of some 125 British people in
the past decade. Prions are most likely transmitted in food, as in the consumption by people of
prion–laden beef from cattle with mad cow disease. Two characteristics of prions are especially
alarming. First, prions are very slow–acting agents; the incubation period until symptoms appear
is around ten years. Second, prions are virtually indestructible; they are not destroyed or
deactivated by heating to normal cooking temperatures. To date, there is no known cure for
prion diseases, and the only hope for developing effective treatments lies in understanding the
mechanism of infection.
How can a protein, which cannot replicate itself, be a transmissible pathogen? According to the
leading hypothesis, a prion is a misfolded form of a protein normally present in brain cells. When
the prion gets into a cell containing the normal form of the protein, the prion converts the
normal protein to the prion version (Figure 18.13). In this way, prions may repeatedly trigger
chain reactions that increase their numbers. This model, first proposed in the early 1980s, is now
widely accepted.
1. Describe two ways a preexisting virus can become an emerging virus.
a. Mutations can lead to a new strain of a virus that can no longer be recognized by the immune
system, even if an animal has been exposed to the original strain; a virus can jump from one
species to a new host; and a rare virus can spread if a population becomes less isolated.
2. Compare horizontal versus vertical transmission of viruses in plants.
a. In horizontal transmission, a plant is infected from an external source of virus, which could enter
through a break in the plant’s epidermis due to damage by insects or other animals. In vertical
transmission, a plant inherits viruses from its parent either via infected seeds (sexual
reproduction) or via an infected cutting (asexual reproduction).
3. Why does the long incubation period of prions increase their danger as a cause of human disease?
a. A source of infection, such as prion–infected cattle, may show no symptoms for many years. Beef
prepared from such animals before symptoms appear would not be recognized as hazardous and
could transmit infection to people who eat the meat.
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