General virology 3 - Aimee Flashcards
What are the different types of transmission?
- Horizontal (direct or indirect)
- Vertical (in utero, during birth, colostrum, milk)
Define virus transmission.
The passing of an infectious virus from an infected host to a susceptible host, regardless of whether the other individual was previously infected or naive.
What is vertical transmission?
Virus transmission from a parent to offspring (embryo, fetus, or newborn).
When does vertical transmission occur?
Before, during, or shortly after birth.
What are the consequences of vertical transmission?
- Embryonic death / abortion
- Congenital disease
- Congenital defects
What is horizontal transmission?
Virus transmission between animals within the population at risk.
What are the modes of horizontal transmission?
- Direct contact
- Indirect contact (fomites)
- Common vehicles
- Airborne
- Vector-borne
- Iatrogenic
What are examples of direct contact virus transmission?
- Licking
- Rubbing
- Biting
- Sexual contact
- Skin abrasions
What are examples of indirect contact virus transmission?
Fomites:
- Shared eating containers
- Bedding
- Restraint devices
- Vehicles
- Clothing
- Improperly sterilized surgical equipment (iatrogenic)
What are common vehicles of transmission?
- Colostrum and milk
- Virus contaminated meat
- Virus contaminated bone products
How does airborne transmission of large droplets occur?
Large droplets settle down.
How does airborne transmission of micro droplets occur?
Micro droplets evaporate and form a droplet nucleus, becomes an aerosol and remain suspended in air for long periods. They can also travel long distances with certain wind and weather conditions.
What is an example of an arthropod-borne virus?
Mosquito - Equine encephalitis virus.
Soft ticks - African swine fever virus.
Culicoides - Blue tongue virus.
What does iatrogenic transmission mean?
Caused by the doctor or animal handler.
What are examples of iatrogenic transmission?
- Non-sterile equipment
- Multi-use syringe
- Inadequate hand washing
(BLV, equine infectious anemia)
What are nosocomial infections?
Infection acquired while in clinics or hospitals.
Give an example for a virus that undergoes vertical transmission (one for each: before, during, and after birth).
Before - Feline parvovirus, blue tongue, BVDV.
During - Canine herpesvirus.
After - Maedi-Visna, CAE.
How stable are viruses transmitted by the respiratory route in the environment?
Low stability
How stable are viruses transmitted by the fecal-oral route in the environment?
Higher stability
Are enveloped viruses environmentally stable?
No, labile in environment.
Which enveloped virus is stable in the environment?
Orf virus and Marek’s disease virus.
Maybe because it is protected by skin keratin???
Are non-enveloped viruses environmentally stable?
Yes.
Survival of a virus in nature depends on…
maintenance of serial infections (chain of transmission)
What is the difference between clinical and subclinical infections in terms of transmission?
Clinical infection - Virus is more productive.
Subclinical infection - More numerous and more important for transmission. Gives a better opportunity for virus to be transmitted.
What are the major patterns that viruses use to maintain serial transmission?
- Acute self limiting
- Persistent infection
- Vector maintenance
What is acute self limiting transmission limited by?
Population size
What viruses are an example of a virus that is an acute self-limited infection?
- Influenza
- Rota viral diarrhea
- Infectious bursal disease
What are the features of an acute self-limiting infection?
- Requires a continuous supply of susceptible hosts
- High yield of virus during disease
- The infectious virus disappears with clinical recovery
What is the hit and run strategy referring to?
Acute self-limiting infection.
What are the three types of persistent infection patterns?
- Persistent
- Chronic
- Latent
What is the hit and stay strategy referring to?
Persistent infections.
What are examples of viruses that cause persistent infection?
- Bovine viral diarrhea virus
What are examples of viruses that cause chronic infection?
- Foot and mouth diseases
- Feline calici virus
What are the features of a persistent infection?
The virus is shed for years with a subclinical infection. Clinical disease and death occur after years of subclinical infection and shedding.
What are the features of a chronic infection?
- Initially, there is clinical disease and high yield of infectious virus. This resembles acute infection.
- Recovery from clinical disease is followed by years of low-level virus shedding.
What is an example of a latent infection?
Herpesvirus is the ONLY example.
What are the features of a latent infection?
- The virus is shed intermittently either with or without clinical disease.
- The periods without disease shed are called latency periods.
- Weakened immune systems can reactivate the virus from latency.
- The initial infection mimics a typical acute infection.
What is the site of productive infection of BHV-1?
Respiratory epithelium (site of replication)
Where does BHV-1 establish latency?
- Trigeminal ganglia neurons
- Tonsillar lymphoid follicles (T cells)
Describe what happens in cases of stress or decreased immunity in animals infected with BHV-1.
The latent virus becomes reactivated and reestablishes productive infection in primary replication sites.
What are emerging viral diseases?
Diseases that are newly recognized, newly evolved, or occurred previously but now has increased incidence (geographical/host/vector range).
What are the factors contributing to emerging viral diseases?
- Host factors
- Environmental changes
- Viral factors
What are the host factors of emerging viral diseases?
The virus must infect a host and successfully invade the host, bypassing its antiviral defenses.
Host must have cell machinery and receptors to support viral replication.
How do environmental changes contribute to emerging viral diseases?
- Ecological changes (ex. temperature changes can increase the range of arthropods in vector borne infections).
- Human activities (viruses and their vectors can be translocated as a result of human travel).
How do viral determinants contribute to emerging viral diseases?
Novel viruses may arise from enzootic viruses due to:
- Short generation time
- Higher mutation rates
How are viral genetic/biological variants different?
Differences in nucleotide sequence lead to differences in virulence (severity of disease) and tropism (location/host)
Will all progeny of viral genomes with new mutations survive?
No, some will contain lethal mutations.
Do RNA or DNA viruses mutate faster?
RNA viruses as they do not have proofreading ability whereas DNA viruses typically use host machinery in the nucleus
What is the implication of viral diversity?
Virus becomes difficult to control due to the presence of many variants (vaccine failure) and virus can become more virulent or can infect different species.
Which viruses have the fastest mutation rate?
Reverse transcription viruses
How is viral diversity generated?
- One mutation is added to each new genome with each round of replication.
- Since viral replication is fast, there are many new quasi species produced quickly.
What is an example of a virus changing tropism?
Equine influenza H3N8 mutated to produce the canine influenza virus.
How can viruses evolve via drastic genetic changes?
- Recombination - template switching between closely related viruses
- Reassortment - Exchange of entire gene segments
Describe viral reassortment.
- Reassortment happens in viruses with segmented genomes.
- When two different segmented viruses infect the same cell, the segments can mix and match during the assembly of new virions.
- Whole gene segments are swapped between the viruses.
This can lead to rapid changes in the virus’s characteristics, such as its ability to infect hosts or its resistance to vaccines.
What are the differences between viral recombination and viral reassortment?
- Both involve co-infection of a cell.
- Recombination involves an exchange of genes.
- Reassortment requires a virus with a segmented genome.
- Entire gene segments are exchanged with reassortment.
Describe viral recombination.
- When two similar (closely related) viruses infect the same cell, they can undergo a process during replication where their genetic material mixes. Which usually involves the breaking and rejoining of their DNA or RNA strands.
As a result, a new viral genome is formed that has segments from both parent viruses. (a recombinant virus is formed.
Co-infection can occur with…
- Two wildtype viral strains
- A live attenuated vaccine strain and a wildtype virus
- Two attenuated live vaccine strains
What receptors are involved in influenza A viruses and what is the ligand?
Ligand = HA
Receptor = Sialic acid on host cells
Avian influenza strains have the highest affinities for which receptor?
Alpha 2-3 linked sialic acid receptor (gut and lung)
Human influenza strains have the highest affinities for which receptor?
Alpha 2-6 linked sialic acid receptor (non-ciliated respiratory epithelium)
Which species have both the alpha 2-3 and alpha 2-6 sialic acid receptors?
Pigs and quails
What is the significance of pigs and quails having receptors for both avian and human influenza?
Allow co-infections which can lead to reassortment or antigenic shift
Do dogs have more avian or human influenza receptors?
Only have avian.
What type of reassortment occurred with the 2009 H1N1 virus?
A triple assortment with swine flu virus, human flu virus, and avian flu virus.
