General Virology 3- Epidemiology of Viral Infections Flashcards
Definition of viral transmission
Passing of an infectious virus from an infected host to a susceptible host, regardless of whether the other individual was previously infected
Define horizontal transmission
Direct or indirect contact between animals within the population at risk
Define vertical transmission
Transmission in utero, during birth, colostrum or milk (during the first few weeks)
I.e. In embryo, fetus or newborn
Diseases that transmit vertically before birth
BVDV
blue tongue
feline parvo
Disease that transmit vertically during birth
canine herpes virus
Diseases that transmit vertically shortly after birth
Caprine arthritis encephalitis (CAE), and Visna-maedi virus
What are the consequences of vertical transmission?
Embryonic death, abortions (PRRS), congenital diseases (BVDV, border disease), and congenital defects (blue tongue and feline Parvo)
What congenital defect does feline Parvo cause?
cerebella hypoplasia
Give examples of horizontal transmission
-Direct contact
-Indirect contact
-Airborne/Aerosols
-Vector-borne
-Iatrogenic
-Fomites
What is a fomite?
non-living objects or materials which are able to carry carry viruses
Name some common vehicles for viruses
-Colostrum and milk
-virus contaminated meat
-virus contaminated bone products
What are NoSocomial infections?
Animal acquires infection while in clinic/hospital
Physical ______ of the virus affects survival of the virus
Physical stability
Virus transmitted by respiratory route have what stability?
low
Virus transmitted by fecal oral route have what stability?
higher
Enveloped viruses are labile, with the exception of? And why?
Orf (pox) virus and Marek’s disease
Thought to be protected by the karatin
Pox virus has what distinct histological feature ?
Viral inclusion bodies in the cytoplasm
To survive in nature viruses what must be must maintained by ? And how is this done?
Serial infections via Chain of transmission
Serial interval is the time between illness onset in the first case till illness onset in the second
What is an example of biological gradient?
Due to close proximity with other species, viruses can mutate to make a variant that infects a new species.
Example: Equine H3N8 that causes equine influenza, the virus mutated and produced canine influenza at the race track both species were at
chain of transmission/serial infections depend on what?
clinical and subclinical infections
The more productive source of the virus is clinical or subclinical infection
Clinical infection
(is very productive but will also be identified and treated)
What is going on during subclinical infection?
more numerous and better opportunity for virus to disseminate
more important
What are the three major patterns that allow for serial transmission?
1) Acute self-limiting infection/ hit and run strategy
2) persistant infection
3) vector maintenance
Describe acute self-limiting infection
Called hit and run because causes disease then virus is gone (infectious virus disappears w clinical recovery)
- high viral load during disease
- a continuous supply of susceptible hosts are required
- after infection there is a strong antibody response
Give 3 example of viruses that cause acute self limiting infection
-Infliuenza
-Rota viral diarrhea
-infectious bursal disease
There are 3 persistent infection patterns. What are they?
-Persistent infection
-Chronic infection
-Latent infection
Describe persistent infections
Animal will have years of shedding the virus with subclinical infection, before clinical disease or death happens. The animal cannot clear the infection
What is an example of a persistent infection?
Bovine viral diarrhea virus (BVDV)
Describe chronic infections
Initially have clinical disease and high viral load (resembling acute infection) but after clinical recovery there are years of shedding the virus at low levels
Give two examples of viruses that are chronic infection patterns
Foot and mouth disease
Feline calici virus
Describe the latent infection pattern
-cycles of reactivation and latency
-will have intermittent shedding with or without clinical signs
-reactivation periods happen due to immune response
-there is NO shedding (productive infection) during the latency periods
-infection is life long
Describe how infection and reactivation period in herpes virus can be stimulated
Herpes virus will establish infection in the primary site (ex BHV1 in nasal mucosa), the virus will then establish life long latency trigeminal ganglia (neurons) and lymphoid follicles (t-cells)
An immune suppersive event (ex stress) will reactivate the virus and establish a productive period in the primary replication site (ex. nasal mucosa)
What is the ONLY virus that has a latent infection pattern
Herpes viruses
What are emerging viral diseases?
Diseases that are newly recognized, newly evolved or occurred previously but is showing an increase in incidence
What are the three factors that affect emerging viral diseases?
1) host determinants
2) constant changes in the environment
3) viral determinants
What are host factors that affect emerging viruses?
The virus must successfully…
-invade the host
-evade the immune system
-enter, replicate and exit the cell
-find another cell
-find another host
What are environmental factors that affect emerging viruses?
-ecological factors- can lead to arthropod borne infections
-Human activities- lead to translocations of viruses and vectors
What are viral determinants that cause new emerging diseases?
short generation times and higher mutations rates (especially in enzootic variants)
What causes minor genetic changes
antigenic drift
What is antigenic drift?
the accumulation of a series of minor genetic mutations due to error in copying the viral nucleic acid
can be lethal or non lethal to virus
What viruses (RNA or DNA) have more mutations? and Why?
RNA viruses. B/c DNA viruses use the cellular machinery in the nucleus to replicate and this has proof reading ability
RNA viruses typically have their own transcription enzymes and replicate in the cytoplasm. So no proof reading =every progeny will be different from parent viruses
Of all viruses which have the greatest mutation rate?
Retroviruses
What is the relationship between genome size and mutation rate
Smaller the genome the greater the mutation rates
How can viral variants change in biological transmission and species tropism?
1) There is longterm host and virus co-evelution (ex Virus in bats)
2) because of this co-evolution the virus won’t be cleared and prolonged so gives it lots of time to evolve (antigenic drift)
3) This evolution creates a novel pathogen with higher spillover risk and transmission potential
4) Infection in novel host- causing an overreactive immune response with high cytotoxic effects (cytokine storm)
What are two things that can cause drastic genetic changes?
1) template switching between closely related viruses
2) Reassortment (exchange of entire gene segments)
How does template switching occur?
Co-infection of a cell by two different viral strains can lead to the generation of recombinant viruses due to templates switching
How does reassortment occur?
Co-infection of a cell by genetically distinct strains of a SEGMENTED virus can generate different combinations of reasserted progenies
