Lecture 8: Virulence of Viruses Flashcards
Pathogenicity
the ability of a virus to cause disease in the host
Pathogen
The virus which causes disease
Pathogenesis
the manner/mechanism of development of a disease
Virulence
quantitative or relative measure of the degree of pathogenicity of the infecting virus
Avirulent
not harmful to the host
Virulence factors related to the virus
- Genetic variation of virus
- Route of entry of virus in host organs
- Affinity of virus to host organs
- Dose of infection
- Immuno evasion
Virulence factors related to the host
- Host species
- Host immunity
- Host physiological factors (nutrition, age, hormones, stages of cell differentiation)
- Fever
Lethal dose 50 (LD50)
The dose of the virus required to cause death in 50% of animals
Three routes of entry of viruses into host-skin
- Bite of arthropods
- Bite of infected animal
- Contaminated objects
Three routes of entry of viruses into host-mucous membrane
- Conjunctiva
- Oropharynx
- Genitourinary tract/rectum
Two defenses of mucous membranes
IgA (antibodies)
Virucidal proteins
Local spread of viruses on epithelial surfaces
- Causes localized infection
- May or may not proceed to subepithelial/underlying tissues
Subepithelial invasion
- Viruses get access to lymphatics, phagocytic cells, and tissue fluids
- Help carry virus to blood stream
Viremia
Presence of virus in blood stream
Primary viremia
Initial entry of virus into the blood
Two ways primary viremia may occur
- Spread of virus to bloof from subepithelial tissue/lymphatics
- Direct injection into blood stream
Secondary viremia
Virus has replicated in major organs and has once more entered circulation
Disseminated infection
Spreads beyond primary site of infection
Systemic infection
A number of organs or tissues are infected
Neurotropic virus
Viruses that can infect neural cells. Infection may occur by neural or hematogenous spread
Neuroinvasive virus
Viruses enter the CNS after injection at a peripheral site
Neurovirulent virus
Cause disease of nervous tissue, manifested by neurological symptoms and often death
How do viruses cross the blood-brain barrier
they hide in monocytes
Tropism
The specificity/affinity of a virus for a particular host tissue
Pantropic
can replicate in more than one host tissue/organ
Cell lysis allows
release of new viruses
occurs after viral replication
Apoptosis
Programmed cell death
Host will activate as last resort to eliminate viral factories before new virus production is complete
Oncoviruses
Cause cancer
Persistant infection
Remain dormant
Can cause chronic stimulation of host immune system, resulting in immunopathology
Infectious bursal disease
Viral replication causes atrophy of bursa and severe deficiency of B lymphocytes, resulting in immunosuppression, causing host to become susceptible to other pathogens
Acute infection
intensive shedding over a short period of time
Persistent infection
Shed at lower titers for months-years
Skin infection
Localized (papilloma) or disseminated (lumpy skin disease)
Warts
Benign skin growths that appear when a virus infects the top layer of the skin
Erythema
Reddening of ski
Neuronophagia
killing of neuronal cells by phagocytic cells
Perivascular cuffing
inflammatory cells around blood vessels in CNS
Disseminated intravascular coagulation
- Widespread activation of clotting mechanism that results in the formation of blood clots in small blood vessels throughout the body
- Triggered by viral infection
- Over time, clotting proteins in blood are used up and severe bleeding can occur
Vasculitis
inflammation and damage to blood vessels
Vasculitis can be regulated by
immune-complex formation
Teratogenic viruses
cause developmental defects o embryo or fetus after in-utero infection
7 viral immune-evasion strategies
- Negative cytokine regulation
- Alterations in antigen processing pathways
- Evasion of natural killer cells
- Alterations in the B cell system
- Viral evasion through latency
- Inhibition of apoptosis
