21: Viruses Flashcards
Viral Evolution, Morphology, and Classification, Virus Infections and Hosts, Prevention and Treatment of Viral Infections, Other Acellular Entities - Prions and Viroids (151 cards)
1
Q
What does it mean to be acellular?
A
Lacking cells.
2
Q
What is a capsid?
A
Protein coating of the viral core. Some viral capsids are simple polyhedral “spheres”, whereas others are quite complex in structure.
3
Q
What is a capsomere?
A
Protein subunit that makes up the capsid.
4
Q
What is an envelope?
A
Lipid bilayer that envelopes some viruses.
5
Q
What is a group I virus?
A
Virus with a dsDNA genome. Their mRNA is produced by transcription directly from the DNA template in much the same way as with cellular DNA.
6
Q
What is a group II virus?
A
Virus with a ssDNA genome. They convert their single-stranded genomes into a dsDNA intermediate before transcription to mRNA can occur.
7
Q
What is a group III virus?
A
Virus with a dsRNA genome. The strands separate, and one of them is used as a template for the generation of mRNA using the RNA-dependent RNA polymerase encoded by the virus.
8
Q
What is a group IV virus?
A
Virus with a ssRNA genome with positive polarity. Intermediates of dsRNA, called replicative intermediates, are made in the process of copying the genomic RNA. Multiple, full-length RNA strands of negative polarity are formed from these intermediates, which may then serve as templates for the production of RNA with positive polarity, including both full-length genomic RNA and shorter viral mRNAs.
9
Q
What is a group V virus?
A
Virus with a ssRNA genome with negative polarity. dsRNA intermediates are used to make copies of the genome and produce mRNA. In this case, the negative-stranded genome can be converted directly to mRNA. Additionally, full-length positive RNA strands are made to serve as templates for the production of the negative-stranded genome.
10
Q
What is a group VI virus?
A
Virus with a ssRNA genome converted into dsDNA by reverse transcriptase. Group VI viruses have diploid (two copies) ssRNA genomes that must be converted to dsDNA, which is then transported to the nucleus of the host cell and inserted into the host genome. Then, mRNA can be produced by transcription of the viral DNA that was integrated into the host genome.
11
Q
What is a group VII virus?
A
Viruses with partial dsDNA genomes that make ssRNA intermediates that act as mRNA, but are also converted back into dsDNA genomes by reverse transcriptase, necessary for genome replication.
12
Q
What is a matrix protein?
A
Envelope protein that stabilizes the envelope and often plays a role in the assembly of progeny virions.
13
Q
What is negative polarity?
A
ssRNA viruses with genomes complimentary to their mRNA.
14
Q
What is positive polarity?
A
ssRNA viruses with genomes that contain the same base sequences and codons found in their mRNA. It means that the genomic RNA can serve directly as mRNA.
15
Q
What is a replicative intermediate?
A
dsRNA intermediate made in the process of copying genomic RNA.
16
Q
What is reverse transcriptase?
A
Enzyme found in Baltimore groups VI and VII that converts single-stranded RNA into double-stranded DNA.
17
Q
What is a viral receptor?
A
Glycoprotein used to attach a virus to host cells via molecules on the cell.
18
Q
What is a virion?
A
Individual virus particle outside a host cell.
19
Q
What is the virus core?
A
Contains the virus genome. It may be either DNA or RNA. Viral genomes tend to be small, containing only those genes that encode proteins that the virus cannot get from the host cell. The genetic material may be single- or double-stranded. It may also be linear or circular. While most viruses contain a single nucleic acid, others have genomes that have several, which are called segments.
20
Q
How were viruses first discovered?
A
Viruses were first discovered after the development of a porcelain filter, called the Chamberland-Pasteur filter, which could remove all bacteria visible in the microscope from any liquid sample. In 1886, Adolph Meyer demonstrated that a disease of tobacco plants, tobacco mosaic disease, could be transferred from a diseased plant to a healthy one via liquid plant extracts. In 1892, Dmitri Ivanowski showed that this disease could be transmitted in this way even after the Chamberland-Pasteur filter had removed all viable bacteria from the extract. Still, it was many years before it was proven that these “filterable” infectious agents were not simply very small bacteria but were a new type of very small, disease-causing particle.
21
Q
How large are viruses?
A
Virions are very small, about 20-250 nanometers in diameter, and are thus, unlike bacteria (which are about 100 times larger), not visible with a light microscope, with the exception of some large virions of the poxvirus family. It was not until the development of the electron microscope in the late 1930s that scientists got their first good view of the structure of the tobacco mosaic virus (TMV) and other viruses. The surface structure of virions can be observed by both scanning and transmission electron microscopy, whereas the internal structures of the virus can only be observed in images from a transmission electron microscope.
22
Q
How might viruses have evolved?
A
Little is known about how viruses originated, because viruses do not fossilize. Researchers must conjecture by investigating how today’s viruses evolve and by using biochemical and genetic information to create speculative virus histories. While most findings agree that viruses don’t have a single common ancestor, scholars have yet to find a single hypothesis about virus origins that is fully accepted in the field. One such hypothesis, called devolution or the regressive hypothesis, proposes to explain the origin of viruses by suggesting that viruses evolved from free-living cells. However, many components of how this process might have occurred are a mystery. A second hypothesis (called escapist or the progressive hypothesis) accounts for viruses having either an RNA or a DNA genome and suggests that viruses originated from RNA and DNA molecules that escaped from a host cell. A third hypothesis posits a system of self-replication similar to that of other self-replicating molecules, likely evolving alongside the cells they rely on as hosts; studies of some plant pathogens support this hypothesis. The emerging field called virus molecular systematics attempts to explain the origin of viruses through comparisons of sequenced genetic material.
23
Q
What are some characteristics of viral morphology?
A
They are acellular. A virion consists of a nucleic acid core, an outer protein coating or capsid, and sometimes an outer envelope made of protein and phospholipid membranes derived from the host cell. Viruses may also contain additional proteins, such as enzymes. Viral morphology is quite diverse. Virion complexity does not correlate with complexity of the host, with some of the most complex virion structures observed in bacteriophages.
24
Q
What are filamentous viruses?
A
Viruses that are long and cylindrical.
25
What are isometric or icosahedral viruses?
Viruses that have shapes that are roughly spherical.
26
What are enveloped viruses?
Viruses that have membranes surrounding capsids. Animal viruses, such as HIV, are frequently enveloped. Glycoproteins embedded in the viral envelope are used to attach to host cells. Other envelope proteins are the matrix proteins that stabilize the envelope and often play a role in the assembly of progeny virions. Because of the fragility of the envelope, non-enveloped viruses are more resistant to changes in temperature, pH, and some disinfectants than enveloped viruses.
27
What are head and tail viruses?
Viruses that infect bacteria and have a head that is similar to icosahedral viruses and a tail shape like filamentous viruses.
28
What is viral reception?
Many viruses use some sort of glycoprotein to attach to their host cells via molecules on the cell called viral receptors. For these viruses, attachment is a requirement for later penetration of the cell membrane, so they can complete their replication inside the cell. The receptors that viruses use are molecules that are normally found on cell surfaces and have their own physiological functions. Viruses have simply evolved to make use of these molecules for their own replication.
29
How does HIV infect a cell?
HIV uses the CD4 molecule on T lymphocytes as one of its receptors. CD4 is a cell adhesion molecule which functions to keep different types of immune cells in close proximity to each other during the generation of a T lymphocyte immune response. After attaching to the CD4 receptor, HIV fuses with the cell membrane, releasing viral contents into the cell, such as HIV RNA, reverse transcriptase, integrase, and other viral proteins. A preinitiation complex is formed between viral ssRNA and reverse transcriptase, and produces viral DNA. The viral DNA is transported across the nucleus and integrated into the host DNA by integrase. New viral RNA produced by transcription of viral DNA is used as genomic RNA and to make viral proteins. New viral RNA and proteins move to the cell surface and a new, immature HIV forms. Finally, the virus matures by protease, releasing individual HIV proteins.
30
How does KSHV (Kaposi's sarcoma-associated herpesvirus) infect a cell?
The KSHV virus bind the xCT receptor on the surface of human cells. xCT receptors protect cells against stress. Stressed cells express more xCT receptors than non-stressed cells. The KSHV virion causes cells to become stressed, thereby increasing expression of the receptor to which it binds.
31
What is the T4 bacteriophage's morphology?
The T4 bacteriophage is among the most complex virions known, which infects *E. coli*, has a tail structure that the viruses uses to attach to host cells and a head structure that houses its DNA.
32
What is an adenovirus?
A non-enveloped animal virus that causes respiratory illnesses in humans. It uses glycoprotein spikes protruding from its capsomeres to attach to host cells. It has a double-stranded DNA genome enclosed in an icosahedral capsid that is 90-100 nm across. It is transmitted orally and causes a variety of illnesses in vertebrates, including human eye and respiratory infections.
33
What is HIV?
Human immunodeficiency viruses. They are enveloped, icosahedral retroviruses that infect humans, and are the causative agent in AIDS. They have ssRNA genomes, and are in group VI of the Baltimore classification scheme.
34
In general, how do DNA viruses replicate?
In DNA viruses, the viral DNA directs the host cell's replication proteins to synthesize new copies of the viral genome and to transcribe and translate that genome into viral proteins.
35
How do RNA viruses replicate and why are mutations more common in RNA viruses than in DNA viruses?
To replicate their genomes in the host cell, RNA viruses encode enzymes that can replicate RNA from the RNA genome (RNA-dependent RNA polymerase, RdRp), which cannot be done by the host cell. These RNA polymerase enzymes are more likely to make copying errors than DNA polymerases, and therefore often make mistakes during transcription. For this reason, mutations in RNA viruses occur more frequently than in DNA viruses. This causes them to change and adapt more rapidly to their host.
36
What is the Baltimore classification scheme?
The most commonly used classification method today. It groups viruses according to how the mRNA is produced during the replicative cycle of the virus.
37
What is a non-segmented genome?
Viral genome that consists of a single segment of genetic material.
38
What is a segmented genome?
Viral genome that is divided into multiple segments.
39
What is the rabies virus?
A virus with a single-stranded RNA (ssRNA) core and an enveloped helical capsid. Rabies transmission occurs when saliva from an infected mammal enters a wound. The virus travels through neurons in the peripheral nervous system to the central nervous system where it impairs brain function, and then travels to other tissues. The virus can infect any mammal, and most die within weeks of infection.
40
What is the variola virus?
A virus with a double-stranded DNA (dsDNA) core and a complex capsid. It is the causative agent of smallpox.
41
What is smallpox?
A human virus transmitted by inhalation of the variola virus, localized in the skin, mouth, and throat, which causes a characteristic rash. Before its eradication in 1979, infection resulted in a 30-35% mortality rate.
42
What are the classifications of capsids?
Naked icosahedral, enveloped icosahedral, enveloped helical, naked helical, and complex.
43
What is a complex capsid?
Complex with many proteins; some have combinations of icosahedral and helical capsid structures.
44
Who developed the Baltimore classification scheme?
Nobel Prize-winning biologist David Baltimore in the early 1970s.
45
What is an acute disease?
Disease where the symptoms rise and fall within a short period of time.
46
What is an asymptomatic disease?
Disease where there are no symptoms and the individual is unaware of being infected unless lab tests are performed.
47
What is AZT?
Anti-HIV drug that inhibits the viral enzyme reverse transcriptase.
48
What is a bacteriophage?
Virus that infects bacteria.
49
What is budding?
Method of exit from the cell used in certain animal viruses, where virions leave the cell individually by capturing a piece of the host plasma membrane. During the budding process, the cell does not undergo lysis and is not immediately killed, however, the damage to the cells that the virus infects may make it impossible for the cells to function normally, even though they remain alive for a period of time. Budding is the process whereby many enveloped viruses acquire their lipid bilayer.
50
What is cell necrosis?
Cell death.
51
What is a chronic infection?
Describes when the virus persists in the body for a long period of time.
52
What does it mean to be cytopathic?
Causing cell damage.
53
What is fusion?
Method of entry by some enveloped viruses, where the viral envelope fuses with the plasma membrane of the host cell.
54
What is a gall?
Appearance of a plant tumor.
55
What is horizontal transmission?
Transmission of disease between unrelated individuals.
56
What is hyperplasia?
Abnormally high cell growth and division.
57
What is hypoplasia?
Abnormally low cell growth and division.
58
What is an intermittent symptom?
Symptom that occurs periodically.
59
What is latency?
Virus that remains in the body for a long period of time but only causes intermittent symptoms.
60
What is lysis?
Bursting of a cell.
61
What is the lytic cycle?
Type of virus replication in which virions are released through lysis, or bursting, of the cell. In the lytic cycle, the phage first infects a cell. The phage DNA then circularizes, remaining separate from the host DNA. The phage DNA replicates and phage proteins are made. New phage particles are assembled. The cell eventually lyses, releasing the phage.
62
What is the lysogenic cycle?
Type of virus replication in which the viral genome is incorporated into the genome of the host cell. The term lysogenic cycle is usually reserved to describe bacteriophages. In the lysogenic cycle, the phage first infects a cell. The phage DNA becomes incorporated into the host genome. The cell divides, and prophage DNA is passed on to daughter cells. Under stressful conditions (such as starvation or exposure to toxic chemicals), the phage DNA is excised from the bacterial chromosome and enters the lytic cycle.
63
What is an oncogenic virus?
Virus that has the ability to cause cancer. These viruses interfere with the normal regulation of the host cell cycle either by introducing genes that stimulate unregulated cell growth (oncogenes) or by interfering with the expression of genes that inhibit cell growth. Oncogenic viruses can be either RNA or DNA viruses.
64
What does it mean to be permissive?
Cell type that is able to support productive replication of a virus.
65
What does it mean to be productive?
Viral infection that leads to the production of new virions.
66
What is a prophage?
Phage DNA that is incorporated into the host cell genome.
67
What is vertical transmission?
Transmission of a disease from parent to offspring.
68
What are the steps in the typical viral replication cycle?
Attachment, penetration, uncoating, replication, assembly, and release.
69
What is viral attachment?
A virus attaches to a specific receptor site on the host cell membrane through attachment proteins in the capsid or via glycoproteins embedded in the viral envelope. The specificity of this interaction determines the host - and the cells within the host - that can be infected by a particular virus.
70
What is viral entry?
The nucleic acid of bacteriophages enters the host cell naked, leaving the capsid outside the cell. Plant and animal viruses can enter through endocytosis, in which the cell membrane surrounds and engulfs the entire virus. Some enveloped viruses enter the cell when the viral envelope fuses directly with the cell membrane. Once inside the cell, the viral capsid is degraded, and the viral nucleic acid is released, which then becomes available for replication and transcription.
71
What is a retrovirus?
A type of virus that inserts a copy of its RNA genome into the DNA of a host cell that it invades, thus changing the genome of that cell. Once inside the host cell's cytoplasm, the virus uses its own reverse transcriptase enzyme to produce DNA from its RNA genome, the reverse of the usual pattern, thus retro (backwards).
72
What is reverse transcription?
The process of creating double-stranded DNA from an RNA template. Reverse transcription never occurs in uninfected host cells - reverse transcriptase is only derived from the expression of viral genes within the infected host cells.
73
What are some ways that HIV is treated?
HIV produces some of its own enzymes not found in the host cells, such as reverse transcriptase, which allows researchers to develop drugs that inhibit these enzymes. These drugs, including the reverse transcriptase inhibitor AZT, inhibit HIV replication by reducing the activity of the enzyme without affecting the host's metabolism. This approach has led to the development of a variety of drugs used to treat HIV and has been effective at reducing the number of infectious virions (copies of viral RNA) in the blood to non-detectable levels in many HIV-infected individuals.
74
What is viral replication and assembly?
The replication mechanism depends on the viral genome. DNA viruses usually use host cell proteins and enzymes to make additional DNA that is transcribed to mRNA, which is then used to direct protein synthesis. RNA viruses usually use the RNA core as a template for synthesis of viral genomic RNA and mRNA. The viral mRNA directs the host cell to synthesize viral enzymes and capsid proteins, and assemble new virions. Of course, there are exceptions to this pattern. If a host cell does not provide the enzymes necessary for viral replication, viral genes supply the information to direct synthesis of the missing proteins.
75
What is viral egress?
The last stage of viral replication is the release of the new virions produced in the host organism, where they are able to infect adjacent cells and repeat the replication cycle. Some viruses are released when the host cell dies, and other viruses can leave the infected cells by budding through the membrane without directly killing the cell.
76
What is a lambda virus?
A lysogenic bacteriophage that infects the E. coli bacterium.
77
What are herpesviruses?
A large family of DNA viruses that cause infections and certain diseases in animals, including humans. They are isometric/icosahedral, double-stranded, with a linear genome, a complex capsid, and a latent replication cycle.
78
What is sense?
The sense of a nucleic acid molecule, particularly of a strand of DNA or RNA, refers to the nature of the roles of the strand and its complement in specifying a sequence of amino acids. DNA is positive-sense if an RNA version of the same sequence is translated or translatable into protein, negative-sense if not.
79
What are herpes simplex viruses?
Herpesviruses that cause oral and genital herpes in humans. They are enveloped icosahedral, group I viruses. They can exist in the nervous tissue for long periods of time without producing new virions, only to leave latency periodically and cause lesions in the skin where the virus replicates.
80
What is non-enveloped viral entry?
Non-enveloped or "naked" viruses may enter the cell in two different ways. As a protein in the viral capsid binds to its receptor on the host cell, the virus may be taken inside the cell via a vesicle during the normal cell process of receptor-mediated endocytosis. An alternative method of cell penetration used by non-enveloped viruses is for capsid proteins to undergo shape changes after binding to the receptor, creating channels in the host cell membrane. The viral genome is then "injected" into the host cell through these channels in a manner analogous to that used by many bacteriophages.
81
What is enveloped viral entry?
Enveloped viruses have two ways of entering cells after binding to their receptors: receptor-mediated endocytosis, or fusion. Many enveloped viruses enter the cell by receptor-mediated endocytosis in a fashion similar to some non-enveloped viruses. On the other hand, fusion only occurs with enveloped virions. These viruses use special fusion proteins in their envelopes to cause the envelope to fuse with the plasma membrane of the cell, thus releasing the genome and capsid of the virus into the cell cytoplasm.
82
What is non-enveloped viral release?
Non-enveloped viral progeny accumulate in infected cells until there is a signal for lysis or apoptosis, and all virions are released together.
83
What is a rhinovirus?
The most common viral infectious agent in humans and the predominant cause of the common cold. They are non-enveloped.
84
What are some examples of acute viral diseases?
The common cold and influenza.
85
What is an example of a long-term infection caused by a virus?
Hepatitis C.
86
What are human herpesviruses 6 and 7?
Human herpesviruses that in some cases can cause the minor childhood disease roseola, often successfully causes productive infections without causing any symptoms at all in the host (asymptomatic infections).
87
What happens during a hepatitis C infection?
In hepatitis C infections, the virus grows and reproduces in liver cells, causing low levels of liver damage. The damage is so low that infected individuals are often unaware that they are infected, and many infections are detected only by routine blood work on patients with risk factors such as intravenous drug use. However, a lack of symptoms is an indication of a weak immune response to the virus. This allows the virus to escape elimination by the immune system and persist in individuals for years, all the while producing low levels of progeny virions in what is known as a chronic viral disease. Chronic infections of the liver by this virus leads to a much greater chance of developing liver cancer, sometimes as much as 30 years after the initial infection.
88
What happens during a herpes simplex infection?
The herpes simplex virus can remain in a state of latency in nervous tissue for months, even years. As the virus "hides" in the tissue and makes few if any viral proteins, there is nothing for the immune response to act against, and immunity to the virus slowly declines. Under certain conditions, including various types of physical and psychological stress, the latent herpes simplex virus may be reactivated and undergo a lytic replication cycle in the skin, causing the lesions associated with the disease. Once virions are produced in the skin and viral proteins are synthesized, the immune response is again stimulated and resolves the skin lesions in a few days by destroying viruses in the skin. As a result of this type of replicative cycle, appearances of cold sores and genital herpes outbreaks only occur intermittently, even though the viruses remain in the nervous tissue for life.
89
What is the varicella-zoster virus?
Herpesvirus that causes chickenpox. After having a chickenpox infection in childhood, the varicella-zoster virus can remain latent for many years and reactivate in adults to cause the painful condition known as "shingles". It has an enveloped icosahedral capsid with a dsDNA genome that becomes incorporated in the host DNA and can reactivate after latency in the form of shingles, often exhibiting a rash.
90
What are some cancers that are associated with viral infections?
Cervical cancer caused by human papillomavirus (HPV), liver cancer caused by hepatitis B virus, T-cell leukemia, and several types of lymphoma.
91
What is human papillomavirus (HPV)?
A papillomavirus with a naked icosahedral capsid and a dsDNA genome that is incorporated into the host DNA. The virus, which is sexually transmitted, is oncogenic and can lead to cervical cancer.
92
What are plant viruses?
Viruses that infect plants, which contains a core of either DNA or RNA. As plant viruses have a cell wall to protect their cells, these viruses do not use receptor-mediated endocytosis to enter host cells like with animal viruses. Plant offspring may inherit viral diseases from parent plants.
93
What are some ways that plant viruses are transmitted?
Plant viruses can be transmitted by a variety of vectors, through contact with an infected plant's sap, by living organisms such as insects and nematodes, and through pollen transmission.
94
What increases the susceptibility of a plant to a plant virus?
For many plant viruses to be transferred from plant to plant, damage to some of the plants' cells must occur to allow the virus to enter a new host. This damage is often caused by weather, insects, animals, fire, or human activities like farming or landscaping.
95
How does hyperplasia appear as?
Galls (tumors).
96
How does hypoplasia appear as?
Thinned, yellow splotches on leaves.
97
How does cell necrosis appear as?
Dead, blackened stems, leaves, or fruit
98
How do abnormal growth patterns appear as?
Malformed stems, leaves, or fruit.
99
How does discoloration appear as?
Yellow, red, or black lines, or rings in stems, leaves, or fruit.
100
How do plant viruses cause economic damage?
Plant viruses can seriously disrupt crop growth and development, significantly affecting food supply. They are responsible for poor crop quality and quantity globally, and can bring about huge economic losses annually. Other viruses may damage plants used in landscaping.
101
What are some examples of viruses that infect agricultural food plants?
Tomato spotted wilt virus, bean common mosaic virus, and cucumber mosaic virus.
102
What are two of the most common viruses found in plants used for landscaping?
Peony ring spot, rose mosaic virus.
103
What are some of the symptoms of a bean common mosaic virus infection?
Lowered bean production and stunted, unproductive plants.
104
What are some of the symptoms of rose mosaic disease?
Wavy yellow lines and colored splotches on the leaves of the plant.
105
What is a mosaic virus?
Any virus that causes infected plant foliage to have a mottled appearance.
106
What is attenuation?
Weakening of a virus during vaccine development.
107
What is a back mutation?
When a live virus vaccine reverts back to its disease-causing phenotype, which can then be spread to other humans in an epidemic.
108
What is gene therapy?
Treatment of genetic disease by adding genes, using viruses to carry the new genes inside the cell.
109
What is an oncolytic virus?
Virus engineered to specifically infect and kill cancer cells.
110
What is phage therapy?
Treatment of bacterial diseases using bacteriophages specific to a particular bacterium.
111
What is a vaccine?
Weakened solution of virus components, viruses, or other agents that produce an immune response.
112
What are some ways to prepare vaccines?
Live viruses, killed viruses, or molecular subunits of the virus. The killed viral vaccines and subunit viruses are both incapable of causing disease.
113
What are live viral vaccines?
Vaccines designed in the laboratory to cause few symptoms in recipients while giving them protective immunity against future infections.
114
What was the impact of Polio vaccination?
Polio was one disease that represented a milestone in the use of vaccines. Mass immunization campaigns in the 1950s (killed vaccine) and 1960s (live vaccine) significantly reduced the incidence of the disease, which caused muscle paralysis in children and generated a great amount of fear in the general population when regional epidemics occurred.
115
What are some childhood vaccines?
The success of the polio vaccine paved the way for the routine dispensation of childhood vaccines against measles, mumps, rubella, chickenpox, and other diseases.
116
What is the danger of using live vaccines?
Though live vaccines are usually more effective than killed vaccines, they carry a low but significant danger that the viruses will revert to their disease-causing form by back mutations.
117
How are live vaccines made?
Live vaccines are usually made by attenuating (weakening) the "wild-type" (disease-causing) virus by growing it in the laboratory in tissues or at temperatures different from what the virus is accustomed to in the host. Adaptations to these new cells or temperatures induce mutations in the genomes of the virus, allowing it to grow better in the laboratory while inhibiting its ability to cause disease when reintroduced into conditions found in the host. These attenuated viruses thus still cause infection, but they do not grow very well, allowing the immune response to develop in time to prevent major disease.
118
What is an example of back mutation causing an epidemic?
In 2007 in Nigeria, mutations in a polio vaccine led to an epidemic of polio in the country.
119
Why are some vaccines under continuous development?
Certain viruses, such as influenza and HIV, have a high mutation rate compared to other viruses and normal host cells.
120
Why is a new influenza vaccine produced each season?
With influenza, mutations in the surface molecules of the virus help the organism evade the protective immunity that may have been obtained in a previous influenza season, making it necessary for individuals to get vaccinated every year.
121
What are some viruses for which the same vaccine is used each year?
Viruses that cause the childhood diseases measles, mumps, and rubella.
122
How are vaccines used to treat rabies infections?
In rabies, a fatal neurological disease transmitted via the saliva of rabies virus-infected animals, the progression of the disease from the time of the animal bite to the time it enters the central nervous system may be 2 weeks or longer. This is enough time to vaccinate an individual who suspects that they have been bitten by a rabid animal, and their boosted immune response is sufficient to prevent the virus from entering nervous tissue. Thus, the potentially fatal neurological consequences of the disease are averted, and the individual only has to recover from the infected bite.
123
How are vaccines used to treat ebola?
In the same way as the rabies vaccine. Ebola is one of the fastest and most deadly viruses on earth. Transmitted by bats and great apes, this disease can cause death in 70-90% of infected humans within 2 weeks. Using newly developed vaccines that boost the immune response in this way, there is hope that affected individuals will be better able to control the virus, potentially saving a greater percentage of infected persons from a rapid and very painful death.
124
How are antiviral drugs used to treat viral infections?
Though these drugs often have limited success in curing viral disease, in many cases, they have been used to control and reduce symptoms for a wide variety of viral diseases. For most viruses, these drugs can inhibit the virus by blocking the actions of one or more of its proteins. It is important that the targeted proteins be encoded by viral genes and that these molecules are not present in a healthy host cell. In this way, viral growth is inhibited without damaging the host. There are large numbers of antiviral drugs available to treat infections, some specific for a particular virus and others that can affect multiples viruses.
125
How are antiviral drugs used to treat genital herpes?
For genital herpes, drugs such as acyclovir can reduce the number and duration of episodes of active viral disease, during which patients developed viral lesions in their skin cells. As the virus remains latent in nervous tissue of the body for life, this drug is not curative but can make the symptoms of the disease more manageable.
126
How are antiviral drugs used to treat influenza?
For influenza, drugs like Tamiflu (oseltamivir) can reduce the duration of "flu" symptoms by 1 or 2 days, but the drug does not prevent symptoms entirely. Tamiflu inhibits a viral enzyme called neuraminidase found in the influenza viral envelope. Neuraminidase cleaves the connection between viral hemagglutinin, also found in the viral envelope, and glycoproteins on the host cell surface. Inhibition of neuraminidase prevents the virus from detaching from the host cell, thereby blocking further infection.
127
What is the most successful use of antivirals?
By far, the most successful use of antivirals has been in the treatment of the retrovirus HIV, which causes a disease that, if untreated, is usually fatal within 10-12 years after infection. Anti-HIV drugs have been able to control viral replication to the point that individuals receiving these drugs survive for a significantly longer time than the untreated.
128
How are antiviral drugs used to treat HIV?
Anti-HIV drugs inhibit viral replication at many different phases of the HIV replicative cycle. Drugs have been developed that inhibit the fusion of the HIV viral envelope with the plasma membrane of the host cell (fusion inhibitors), the conversion of its RNA genome into dsDNA (reverse transcriptase inhibitors), the integration of the viral DNA into the host genome (integrase inhibitors), and the processing of viral proteins (protease inhibitors).
129
Why are multiple drugs used to treat HIV?
When any individual drug is used, the high mutation rate of HIV allows it to easily and rapidly develop resistance to the drug, limiting the drug's effectiveness. The breakthrough in the treatment of HIV was the development of HAART, highly active anti-retroviral therapy, which involves a mixture of different drugs, sometimes called a drug "cocktail". By attacking the virus at different stages of its replicative cycle, it is much more difficult for the virus to develop resistance to multiple drugs at the same time. Still, even with the use of combination HAART therapy, there is concern that, over time, the virus will develop resistance to this therapy. Thus, new anti-HIV drugs are constantly being developed with the hope of continuing the battle against this highly fatal virus.
130
How are viruses used in gene therapy to treat severe combined immunodeficiency (SCID)?
SCID is a heritable, recessive disease in which children are born with severely compromised immune systems. One common type of SCID is due to the lack of an enzyme, adenosine deaminase (ADA), which breaks down purine bases. To treat this disease by gene therapy, bone marrow cells are taken from a SCID patient and the ADA gene is inserted. Viruses such as adenovirus are modified by the addition of the ADA gene, and the virus transports this gene into the cell. The modified cells, now capable of making ADA, are then inserted back into the patients in the hope of curing them.
131
What is the current state of gene therapy using viral vectors?
Gene therapy using viral vectors is still experimental, but holds promise for the treatment of many genetic diseases. Many technological problems still need to be solved for this approach to be a viable method for treating genetic disease.
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What is H101?
A genetically modified adenovirus which has been used since 2005 in clinical trials in China to treat head and neck cancers. The results have been promising, with a greater short-term response rate to the combination of chemotherapy and viral therapy than to chemotherapy treatment alone.
133
Since when have bacterial diseases been treated with antibiotics?
Since the 1940s.
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What is staphylococcus aureus (MRSA)?
A virus that causes an infection commonly acquired in hospitals. It has developed a resistance to the methicillin, as well as a variety of other antibiotics, which makes it difficult to treat.
135
How are bacteriophages used to treat bacterial infections?
Bacteriophages specific to each bacteria bypass resistance to antibiotics and specifically kill them. This is called phage therapy.
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What is the regulatory status of phage therapy?
Although phage therapy is in use in the Republic of Georgia to treat antibiotic-resistant bacteria, its use to treat human diseases has not been approved in most countries. However, the safety of the treatment was confirmed in the United States when the U.S. Food and Drug Administration approved spraying meats with bacteriophages to destroy the food pathogen Listeria.
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What is a pathogen?
Agent with the ability to cause disease.
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What is a prion?
Infectious particle that consists of proteins that replicate without DNA or RNA.
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What is PrPC?
Normal prion protein.
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What is PrPSC?
Infectious form of a prion protein.
141
What is a viroid?
Plant pathogen with a single-stranded, circular RNA that is much simpler than a virus. They do not have a capsid or outer envelope, but like viruses can reproduce only within a host cell. Viroids do not manufacture any proteins, and only produce a single, specific RNA molecule. Human diseases caused by viroids have yet to be identified.
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Who discovered prions?
Historically, the idea of an infectious agent that did not use nucleic acids was considered impossible, but pioneering work by Nobel Prize-winning biologist Stanley Prusiner has convinced the majority of biologists that such agents do exist.
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Which diseases are known to be transmitted by prions?
Fatal neurodegenerative diseases, such as kuru in humans and bovine spongiform encephalopathy (BSE) in cattle (commonly known as "mad cow disease"). The diseases are spread by the consumption of meat, nervous tissue, or internal organs between members of the same species.
144
What is kuru?
A fatal neurodegenerative disease native to humans that is spread by consuming human meat, nervous tissue, or internal organs. Kuru is native to humans in Papua New Guinea and spread from human to human via ritualistic cannibalism. Kuru was controlled by inducing the population to abandon its ritualistic cannibalism.
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What is bovine spongiform encephalopathy (BSE) ("mad cow disease")?
A fatal neurodegenerative disease that affects cattle. It was originally detected in the United Kingdom, and was spread between cattle by the practice of including cattle nervous tissue in feed for other cattle. BSE was initially thought to only affect cattle. Cattle dying of the disease were shown to have developed lesions or "holes" in the brain, causing the brain tissue to resemble a sponge. Later on in the outbreak, however, it was shown that a similar encephalopathy in humans known as variant Creutzfeldt-Jakob disease (CJD) could be acquired from eating beef from animals with BSE, sparking bans by various countries on the importation of British beef and causing considerable economic damage to the British beef industry. BSE still exists in various areas, and although a rare disease, individuals that acquire CJD are difficult to treat. The disease can be spread from human to human by blood, so many countries have banned blood donation from regions associated with BSE.
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What are some symptoms of kuru and BSE?
Individuals with kuru and BSE show symptoms of loss of motor control and unusual behaviors, such as uncontrolled bursts of laughter with kuru, followed by death.
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What is the cause of spongiform encephalopathies, such as kuru and BSE?
An infectious structural variant of a normal cellular protein called PrP (prion protein), PrPSC. Once introduced into the body, the PrPSC contained within the prion binds to PrPC and converts it to PrPSC. This leads to an exponential increase of the PrPSC protein, which aggregates. PrPSC is folded abnormally, and the resulting conformation is directly responsible for the lesions seen in the brains of infected cattle.
148
What is the impact of viroids on crops and which crop plants are known to be affected by viroids?
Viroids are responsible for crop failures and the loss of millions of dollars in agricultural revenue each year. Some of the plants they infect include potatoes, cucumbers, tomatoes, chrysanthemums, avocados, and coconut palms.
149
How is the potato spindle tuber viroid (PSTV) spread?
PSTV is typically spread when infected knives are used to cut healthy potatoes, which are then planted.
150
Why aren't people vaccinated for rabies like dogs and cats are?
Adults are not routinely vaccinated for rabies for two reasons: first, because the routine vaccination of domestic animals makes it unlikely that humans will contract rabies from an animal bite; second, if one is bitten by a wild animal or a domestic animal that one cannot confirm has been immunized, there is still time to get the vaccine and avoid the often fatal consequences of the disease.
151
What is an example of a filamentous virus?
Many plant viruses are filamentous, including TMV (tobacco mosaic virus).
