Lecture 34 - Viral Infections and Disease

what do most viral infections result in?

Most infections are inapparent (unknown to exist) or asymptomatic (no detectable symptoms)

what is horizontal transmission?

between membranes of the same species; this includes direct contact, vehicle-borne (air, water), or vector-borne

what is vertical transmission?

transfer of infection between mother and child

what is iatrogenic transmission?

transmission caused by a medical procedure or treatment

what is noscomial transmission?

when an individual is infected while in a hospital or health care facility

what is germ-line transmission?

agent is transmitted as part of the genome (e.g. proviral DNA)

what is the difference between signs and symptoms?

• Signs: evidence of disease thart can be measured or recorded by others

• Symptoms: impacts that are apparent to only the patient

what is the incubation period?

Initial period between infection and when symptoms become apparent

what happens during the incubation period?

• During this period, signs are present and can be detected

• Signs include virus replication, innate immune signaing, and other host responses

• For some virus infections, an individual may be contagious during the incubation period

what is a short incubation period?

on the order of hours to days; typically associated with replication at the primary site of infection which produces symptoms

what is a long incubation period?

Long incubation periods: on the order of weeks to months; symptoms develop beyond the primary site of infection (virus needs to spread to cause apparent symptoms)

what must viruses do in order to be successful?

• Produce enough viral progeny

• Be able to access susceptible and permissive cell populations to sustain cell growth

• Overcome or suppress the local antiviral hosts defenses

what does virus-induced disease look like?

• Effects of viral replication on the host → as the virus steeals nutrients, destroys cells, and disrupts homeostasis in cells and tissues, these impacts are directly felt in the host

• Effects on host response on the virus and host → upon recognition of the pathogen, the host response (immunopathology) is often just as destructive as the pathogen itself due to dysregulation andn overactive cell signaling

what does viral damage to cells look like?

• Nutrient and resource deprivation: cells lacking sufficient anabolic resources are more susceptible to damage and lack of repair

• Cytolytic viruses: cytopathic effects (apoptosis, necrosis)

• Viroporins: viral proteins or molecules that generate pores in host membranes

• Viral inhibition of host protein and RNA synthesis: leads to loss of membrane integrity, leakage of enzymes from lysosomes. Cytoplasmic degradation

• Syncytium formation by enveloped viruses (parainfluenza, HIV)

what are cytokine storms?

extensive signaling by the host that leads to significant disease

what is an acute infection?

• Associated with rapsid onset of viral replication with large production of virus 

• These infections are initiated and primarily remain at the site of infection 

• Generally short but can result in possibly severe course of disease 

• Can cause serious public health problems

what is a latent infection?

• Occur when viruses enter a non-productive phase of the replication cycle and go dormant

• During these times of dormancy, viral gene products that promote productive replication are not made or are found in low concentrations

• Cells harboring latent viral genomes are poorly recognized by the immune system allowing the pathogen to persist

what is a persistent infection?

• Occur when the primary infection is not cleared by the immune response

• During persistent infections, virus particles, proteins, and genomes continue to be produced and may last beyond virus production

• When cytopathic effects are absent and host defenses are reduced, persistent infections are more likely to occur

what is virulence?

Virulence is defined as the extent of pathogenciity (or disease potenital) attributed to a pathogen

how is virulence quanified?

• Virus titer

• Mean time to death

• Mean time to appearance of signs

• Measurement of fever, weight loss

• Measurement of pathological lesions

• Changes in immune factors or cells

what is incidence?

number of people infected/number in population/time

what is the morbidity rate?

number of people ill/number in population

what is the mortality rate?

number of deaths/number in population

what is the case fatality rate?

number of dearths/number of confirmed infected

what is the infection fatality rate?

number of deaths/number of actual infections

what is R0?

• Number of secondary infections that can arise in population of susceptible hosts from a single infected individual

• Influenced by the time of contact between individuals, length of infectious period, and can be affected significantly by interventions like social distancing, mask wearing, and vaccinations

what is virulence influenced by?

• Influenced by dose, route of infection, species, age, sex, and susceptibility of host

• Not correct to compare virulence of different virus

what are virulence genes?

usually identified by mutation: a virus that causes reduced or no disease in a specificed system