Chapter 7 Flashcards
Define pathogenic.
able to cause disease
Define virulence.
relative measure of the harm done to the host
What are some of the factors that interact to determine the outcome of infection?
genetic features of the virus, the route and dose of inoculum, and a variety of host factors
What host factors can determine the outcome of infection?
age, gender, immune status, concomitant infections, nutritional background, and genetic background
When does an acute infection occur?
when a virus firstinfects a susceptible host
What is the duration of an acute infection?
it is generally of short duration
What is the nature of an acute infection?
the infection may be self-limiting, or controlled by innate or adaptive immune responses
What is a persistent infection?
the continuation of infection beyond the time when the immune system might be reasonable expected to clear the infection; the adaptive immune system has not cleared the virus effectively
What happens with virus particles or viral gene products during a persistent infection?
they continue to be produced for long periods of time; either continuous replication or establishment of latency
What happens to the viral genome during latency?
the viral genome is maintained but there is no productive replication
What kind of damage occurs in a persistent infection?
minimal tissue damage either because replication is restricted or replication causes minimal damage to the host cell
What is a persisent latent infection?
A reversible, non-productive infection in which only a few, if any, viral genes are expressed. Periodically, productive replication is reactivated
What is a persistent progressive infection?
Levels of virus replication, and clinical signs increase over time; also called a slow infection
What is a persistent chronic inifection?
Continual virus replication, often at a set level, with or without clinical signs of disease
What are the three main portals of entry for viruses?
mucosal surfaces, skin, and vertical transmission
What mucosal surfaces can viruses enter?
respiratory, digestive, urogenital, and conjunctival mucosal surfaces
What is vertical transmission?
transmission of viruses without passage through the environment
What are the three ways of verticle transmission mentioned in class?
in the germ line of fertilized eggs, across the placenta, and during passage through the birth canal
How do viruses enter skin?
they must enter through abrasions and penetrations
What are some consequences of vertical transmission?
abortion, congenital disease, and persistent infection
What is a localized infection?
Viruses replicate only at or near the body surface through which they entered.
Where does viral entry, replication, lesion formation, and shedding occur in localized infections?
all in the same organ
How does the spread of localized infection in epithelial cells occur?
by sequential infection of neighboring cells
Usually, what is the incubation time of viruses that cause localized infections?
it is usually short
What are some examples of diarrhea causing localized infections?
coronaviruses and rotavirus
What are some localized infections that infect the upper and lower respiratory tract?
influenza and parainfluenza viruses
What are the steps (pathogenesis) of a localized infection with influenza?
Day 0 - initial infection
Day 1- cell-to-cell spread in mucus layer of epithelium (no signs yet)
Day 2- beginning of earliest prodromal signs
Day 3- Time of maximal shedding
Day 4 - Mild feverm dry cough, minimal inflammation
Day 5 - Viral replication begins to wane
Day 6 and 7 - fever and other signs at max, inflammation peaks, productive cough, active immune response, viral replication minimal, beginning of recovery
What is a systemic virus?
Viruses that spread from entry site following lymphatic and hematogenous dissemination to cause generalized infections in specific organs
True or False: In systemic viral infections, site of entry, the target organ, and shedding all occur in the same place.
FALSE
What are some viruses that cause systemic infections?
paramyxoviruses, rotaviruses, papillomaviruses
What happens at 1?
Following entry at one of the body surfaces, the virus is able to penetrate into the underlying tissues.
What happens at 2?
The virus arrives at a regional site and undergoes primary amplification - typically a regional lymph node.
What happens at number 3?
The progeny virus produced at this regional site enters the bloodstream, giving rise to the first viremic period, called the primary viremia. This is transient and subclinical
What happens at number 4?
The primary viermia serves to seed the central organs of replication, which are typically the liver, spleen, bone marrow, and the vascular endothelium
What happens at number 5?
Replication in these central depots liberates large amounts of virus into the blood stream. This is the secondary viremia. About the time the viremia peaks and begins to decline, fever appears.
What happens at number 6?
The secondary viremia seeds the sites of shedding, and the sites in which the primary lesions of the disease are found, which may or may not be the same
What happens at number 7?
Replication in susceptible cells at these sites produces the characteristic signs and lesions of the disease, and results in liberation of the virus into the environment.
What are the four determinants of whether a viral infection remains localized or spreads systemically?
Availability of susceptible cells in deeper tissues, macrophage susceptibility (yes or no), directional release, and temperature range of the virus
What are the four possible outcomes of a virus-macrophage interaction?
- Virus is ingested and degraded by macrophage
- In the presence of virus-specific antibody, the virus is opsonized and degraded
- Macropage is susceptible to virus, and results in lytic or persistent infection
- Macrophage is susceptible to virus, and in the presence of some virus-specific antibodies, replication of macrophages are enhanced
What plays an important role in determining the disease pattern of a virus?
the cell surface
What does virus release form apical surfaces of respiratory or intestinal epithelium result in?
luminal release, back where the infection started
As a general rule, where are enteric viruses that produce localized infections released?
from the apical (luminal) surface, so that their progeny end up in the feces
Generally, where are viruses that cause systemic infections released?
from the basolateral surface of mucosal cells in order to facilitate a more systemic spread and avoid antibody in the mucus layer
Why is depletion of susceptible cell popylations more important in localized infections?
because the population of host cells in these areas is more limited than in systemic infections
What host immunity process seems to be more important in mediating recovery from most active viral infections?
T lymphocyte-mediated cytotoxicity response
In localized infections, what is the primary role of the immune response?
to prevent reinfection
When does peak shedding occur in systemic infections?
during secondary viremia
When should you take diagnostic samples if a viral infection is suspected?
early
In both systemic and localized infections, virus _____ has largely ceased by the time fever, disease signs, and lesions are prominent.
shedding
When should you take ‘post-infection’ samples?
a week or more after clinical signs have declined
What two virus families use restricted gene expression as a mechanism of persistence?
lentiviruses/retroviruses
In high virulence viruses, what are the numbers of LD50 like?
low (5)
In medium virulence viruses, what is the number of LD50 like?
medium (5,000)
In low virulence viruses, what is the number of LD50 like?
very high (1,000,000)
What can replication of latent viruses be induced by?
hormonal changes, pregnancy, stress, steroids, and decline in immune function
What are four ways that persistent virus infections can damage the host?
Cytopathic effects of the virus on the cell
immunopathologic tissue damage
interference with growth or with specialized functions of the cell
oncogenic transformation
What are some survival strategies in which persistent viruses avoid being eliminated by the host?
Immunological tolerance
escape from competent adaptive immune response
infection of tissues with reduced immunosurveillance
infection of the immune system
restricted viral gene expression
