Micro. Ch. 13

what is antigenic drift?

- first form of antigenic variation - Involves small changes in virion structure - Results from mutations - Occurs after the infection has begun - Influenza virus changes a little bit every year, increasing the susceptible population every year

what is antigenic shift?

- the second form of antigenic variation - Involves major changes in virion structure - Is due to the acquisition of new genes - This is through co-infection or recombination - Influenza viruses exhibit dramatic changes in H and N; everyone is susceptible

In reference to cytotoxic T lymphocyte (CTL) escape mutants, which of the following statements is correct? A. If immunodominant peptides mutate, the host's cytotoxic T lymphocytes are no longer able to respond B. Escape mutants are associated more commonly with DNA viruses than with RNA viruses C. If escape mutants occur late in an infection, a persistent infection occurs D. Viruses escaping cytotoxic T lymphocyte detection are usually cleared effectively by host antibody responses

A. If immunodominant peptides mutate, the host's cytotoxic T lymphocytes are no longer able to respond

Which of the following explains why some acute viral infections are asymptomatic in individuals exposed to a virus for the first time? A. Innate immune defenses restrict the spread and severity of viral infections B. They have a short incubation period C. The adaptive immune response is deployed quickly D. Acute viruses become avirulent soon after infection

A. Innate immune defenses restrict the spread and severity of viral infections

The duration of a rhinovirus infection and an influenza infection is usually only limited to 4-5 days. What type of viral infection does this represent?

Acute infection Acute infections are typically characterized by rapid production of virions followed by host defenses quickly eliminating the infection, or by host death. Latent and slow infections are both types of persistent infection in which the host defenses are modulated or bypassed so that the virus is never cleared from the infected host.

Explain why an individual who has had a prior infection with a rhinovirus is quite likely to experience another episode of rhinovirus infection at a later date. Why do we not immunize against rhinovirus?

An infection with rhinovirus will develop immunological memory against that virus but this may not be effective against a later infection as rhinoviruses undergo antigentic variation. This is a structural change in the virion, either by antigenic drift or antigenic shift, which alters the specific structures used by the immune system to recognize and destroy virus-infected cells and eliminate free virions. This alteration renders the immunological memory developed during the earlier infection less effective or nonexistent against the new infection, allowing the infection to persist. Vaccines usually require the use of antigens to produce long-lasting immunological memory in the host that can then protect the host from subsequent infection and mimic natural infections in this way. Viruses that undergo frequent antigenic variation, such as rhinovirus, pose difficulties for vaccine development as immunological memory would need to be developed against a large number of antigenic variants, and vaccines would need to be continuously updated to include new variants. Not only does this pose a challenge for vaccine design and production, it is also unlikely to be cost effective for the level of protection it affords and the frequency with which vaccination would be required.

When changes in virion structure occur as a result of major genetic changes, this is called

Antigenic shift

Indicate which of the following statements is false. A. Human immunodeficiency virus (HIV) can be transferred in utero B. Herpes viruses sometimes cause Koplik's spots on mucosal epithelium C. The transmission rate of rubella from pregnant women to the fetus is more than 80% during the first trimester D. Viruses disseminating from the blood to other replication sites are using the hematogenous route of transmission

B. Herpes viruses sometimes cause Koplik's spots on mucosal epithelium

Which of the following is a mismatched pair in reference to viral entry points and dissemination? A. Skin: rabies virus B. Respiratory tract; herpes simplex virus C. Nervous system: poliovirus D. Urogenital system: papillomavirus E. Digestive tract: adenovirus

B. Respiratory tract; herpes simplex virus

Which of the following is not an example of a virus associated with persistent infections of humans? A. Epstein-Barr virus B. Human immunodeficiency virus C. Influenza virus D. Hepatitis C virus E. Herpes simplex virus type 1

C. Influenza virus

Which of the following is not a requirement of an effective vaccine? A. The components of the vaccine should induce protective immune responses B. The host should mount a memory response that has the capacity to respond if the pathogen is encountered at a later time C. The vaccine should induce only mild disease D. The vaccine should be biologically stable E. The cost to produce and administer the vaccine should be minimized to promote effective use in target populations

C. The vaccine should induce only mild disease

Mr. Johnson has an immunodeficiency disease called X-linked agammaglobulinemia owing to a genetic defect in a gene called Bruton's tyrosine kinase (Btk), which is required for B cell development. He lacks B cells and is therefore unable to produce antibodies. He receives regular injections of gammaglobulin to ensure he has protective immunity against microbial diseases. This is an example of

Passive immunity

Viral infections that last the lifetime of the host are referred to as _____ infections.

Persistent

The digestive tract is a common site of entry for many viruses, even though it contains stomach acid, bile, and phagocytic cells, which create a hostile environment. Which of the following viruses require entry through the digestive tract for infection?

Reovirus

Which of the following uses host intestinal proteases to be converted into infectious viral particles? A. Rhinovirus B. Norwalk virus C. Poliovirus D. Reovirus E. Adenovirus

Reovirus

Active viremia describes

Replication of viruses in the blood

What are the cause of over 50% of the common colds we acquire?

Rhinoviruses

Which of the following is a mismatched pair in reference to viral transmission? A. HIV: in utero B. Rabies: zoonotic disease C. Iatrogenic: sexually transmitted D. Rhinovirus: aerosol E. Herpes simplex virus: semen

Iatrogenic: sexually transmitted

Papillomavirus, which causes skin warts, is able to subvert cytotoxic T lymphocytes (CTL) responses by

Infecting tissues that are sequestered from the immune system

Many viruses are able to be reactivated many years after initial entrance into a host. An example of this is the varicella-zoster virus and shingles. This is an example of a(n)

Latent infection A general characteristic of latent infections is the persistence of an intact viral genome so that reactivation can occur at a later time. Individuals who have previously been infected with varicella-zoster virus can later develop shingles when the virus reactivates. Reactivation can occur due to stress, trauma, other illness, or any condition that indicates that the host cell is no longer a suitable place to stay. Slow infections are another type of persistent infection. Acute infections are characterized by rapid production of virions followed by rapid resolution and elimination of the infection by host defenses, or by host death.

An oncogenic virus is one that

Can induce cancer in animals

Rhinovirus and influenza viruses often cause reinfection in people that have already been a host to the disease. How is this possible? A. Antigenic variation B. Antigenic drift C. Antigenic shift D. All of the above

D. All of the above

The varicella-zoster virus establishes latency in the _____, and if reactivated causes the development of _____.

D. Sensory ganglia; shingles

Compare and contrast chicken pox and shingles.

Chicken pox and shingles are caused by the same virus, varicella-zoster (VZV), but in different circumstances and with different effects. After initial infection by VZV, the virus spreads quickly to the lymph nodes where T cells become infected and move into the blood, from where they are disseminated round the body (including the liver, spleen and other organs) and a rash develops all over the skin. This rash is associated with itching. Once the rash disappears, VZV can then move to the peripheral nervous system and establish a latent infection in the sensory ganglia. VZV can become reactivated at any time. When VZV is reactivated, it causes shingles, travelling back down the peripheral nerve and resulting in a localized rash associated with a substantial degree of pain. The pain can persist after the rash is gone, especially in older patients who are also susceptible to post herpetic neuralgia, which can last for years and be very debilitating. The reason for activation is not clearly understood but is thought to be associated with trauma or stress or a condition that indicates to the virus it needs a different host cell. Anyone who has had chickenpox could get shingles. Chicken pox is not a serious illness in children, but it is more debilitating in adults and a vaccine is available. A live virus is used in the vaccination, so anyone vaccinated against chicken pox is also susceptible to shingles but studies suggest that the risk is less than for those who developed chicken pox.

Which of the following is true about persistent viral infections? A. Virions continue to be produced for months B. Such an infection can become chronic C. Such an infection can remain for a lifetime D. A persistent viral infection can occur if the virus mutates to become an escape mutant E. All of the choices are true

E. All of the choices are true

Which of the following is not a characteristic of latent viral infections? A. Survival of intact viral genomes in infected host cells B. Non-productive infection and few virions C. Reactivation can occur years after initial infection D. Absence of or reduction in host immune responses E. Major changes in virion structure due to the acquisition of new viral genes

E. Major changes in virion structure due to the acquisition of new viral genes

All of the following cause skin rashes in humans except A. Rubella virus B. Measles virus C. Parvovirus D. Hepatitis C virus E. Varicella-zoster virus

Hepatitis C virus

An example of a virus that does not use host neurons for the establishment of latency is

Hepatitis C virus

Why is vaccination the most promising approach to the control of viral epidemics? Describe the three classes of viral vaccine administered to humans and provide examples of each.

There are very few drugs available to treat viral infections, none of which provide a definitive cure, and the most effective strategy to control viral epidemics is through prevention by vaccination. Vaccination induces an adaptive immune response in the host producing long-lasting immunological immunity that protects the host in subsequent encounters with the same virus. Vaccination develops immunity without individuals contracting the disease and helps prevent the spread of disease among susceptible hosts by providing herd immunity. In a population with a large proportion of immune individuals, any infection will be quickly resolved and transmission of the infection limited. There are three classes of vaccine: live attenuated vaccines, inactivated vaccines, and subunit vaccines. Live attenuated vaccines are intact viral particles that have been mutated and selected for poor growth in humans, but are still capable of replicating and stimulating active immunity. Examples include vaccines against measles, mumps, rubella, and yellow fever. Inactivated vaccines are composed of virus that is 'dead' or has been rendered non-infectious by chemical or physical treatment. Examples include vaccines for hepatitis A, polio (inactivated), and rabies. Subunit vaccines are comprised of immunogenic parts of the virus, derived through genetic engineering and recombinant DNA techniques, and do not contain any intact virions. Examples include influenza and hepatitis B vaccines.

Virus that use the digestive tract as a site of entry can become systemic by

Using M cells to enter the blood and lymph

What is virulence and what factors have an effect on the virulence of viral infections? Explain why host susceptibility is such an important consideration in viral virulence.

Virulence is the ability of an infectious pathogen to overcome the hosts defenses and establish an infection. Factors that affect virulence include the route of infection, the age, health status, and gender of the host, alterations in the viruses ability to replicate or cause cytotoxicity, and any alterations in a host cell's function required for virion production. Host susceptibility has an important influence on the virulence of viral infections as it affects the extent to which host defenses can control the viral infection. An immune host cannot be infected and will also not transmit the virus, but a susceptible host is one that can be infected and can also transmit the disease. The extent to which the disease can spread depends on how efficient the viral infection is and the proportion of susceptible hosts present in the population. If a population has a large proportion of immune hosts, the chance of encountering and transmitting the disease to a susceptible host is low, and the population exhibits herd immunity. The health status of susceptible hosts is important in determining the extent of disease and the period of transmissibility. In a population with healthy immune response capabilities, an infection can be resolved quickly with limited transmission, while in an immunocompromised population transmission of the infection can be rapid. Factors such as malnutrition, age, gender, and pregnancy can all influence a hosts susceptibility and the virulence of viral infection.