36. Emerging Viruses Flashcards
what is an emerging virus?
causes new or previously unrecognized infection
5 examples of emerging virus
- 1918 influenza
- 2002 SARS
- 2009 swine flu (H1N1)
- 2014 chikungunya
- 2015 zika
how do emerging viruses expand their host range?
- secondary hosts or vector
- spillover
what is zoonosis?
transmission of virus from wild or domesticated animal to humans
what % of human emerging infectious viruses are zoonotic?
60-80%
example of virus that expanded host range by secondary host
rhabdovirus
example of virus that expanded host range by spillover
explain
Nipah virus –> natural host is bats but switch to other animals then humans
2 roles of evolution for emerging viruses
- mutations allow for biodiversity of pathogens in nature
- selection allows for pathogens to adapt to new hosts and environments
4 types of host-virus interactions
- stable
- evolving
- dead-end
- resistant
what is a STABLE host-virus interaction?
virus is maintained in the ecosystem –> both the host and virus populations survive and multiply
describe host and virus populations vs individual host and virus in STABLE host-virus interactions
does the population or individual matter in STABLE host-virus interaction?
virus population: more or less virulent
host population: develop mechanism to attenuate viral effects (more tolerant to infection, dampen immune response)
individual virus: can infect but may be cleared or establish infection
individual host: get infection and become sick
host and virus POPULATION that survives and multiplies is most importrant
when can stable host-virus interactions be permanent?
if there is only 1 host
4 examples where humans are the only host
- measles
- HSV
- HCMV
- smallpox
3 examples of viruses where stable host-virus interactions include infection of >1 species
- influenza A
- flavivirus
- togavirus
why are there are rarely influenza pandemics?
influenza maintains stable infections in their hosts so only get outbreaks rather than pandemics
what are EVOLVING host-virus infections?
virus transmits to naive populations
2 hallmarks of EVOLVING host-virus interactions?
- instability
- unpredictability
characteristic of pathogen in EVOLVING host-virus interaction
mutations select for increased spread or virulence in best environment
characteristic of host in EVOLVING host-virus interaction
reduced resistance to virus
outcome of EVOLVING host-virus interaction
range from benign to death
3 examples of EVOLVING host-virus interaction
- introduction of smallpox and measles to Indigenous peoples by colonizers
- introduction of west nile virus to western hemisphere
- introduction of pox virus to rid Australia of rabbits
what is a DEAD-END host-virus interaction?
transmission is 1-way to 1 individual of a species, does not spread within species
when do DEAD-END host-virus interactions occur?
frequent outcome of cross-species infection
how do DEAD-END host-virus interactions contribute to the spread of natural infection?
very little contribution to the spread of natural infection
what is a RESISTANT host-virus infection?
host blocks infection completely –> host cells are not susceptible and body’s defenses are strong
how common are DEAD-END host-virus interactions?
most human encounters with viruses are dead-end
what is the relationship between the different host-virus interactions?
very dynamic –> can switch from one type of interaction to other
based on host-virus interactions which viruses have the best chance at being emerging?
stable –> evolving
stable –> dead-end
3 things that affect emerging infections
- introduction of virus into cell (role of the virus)
- establishment and dissemination (role of the host)
- cross-species transmission and emergence of endemic + pandemic (role of environment/behaviour)
describe 3 ways that the introduction of the virus into cells affects emerging viruses
- genetic instability
- cell tropism
- immune evasion
how does genetic instability affect emerging infections?
allows mutations for rapid evolution to adapt to the ever-changing environment (common for RNA viruses)
how does cell tropism affect emerging infections? (3)
- cell receptors for virus entry
- ability to circumvent innate immune responses
- antigenic immunodominance
4 factors in the establishment and dissemination of the virus in emerging infections
- access to susceptible and permissible cells
- population density and health
- genetic variation in hosts
- virus populations only endure in nature because of serial infections (chain of transmission)
describe infection of SARS-CoV-2 vs Ebola virus
covid can transmit BEFORE symptoms onset whereas ebola can only transmit AFTER symptoms onset
what are 6 factors in cross-species transmission and emergence of endemic+pandemic?
- air travel
- air conditioning
- blood transfusion
- long distance transport of livestock, birds
- deforestation
- hunting
etc.
what is the key determinant of disease emergence?
environment / human activities
how do coronaviruses make the species jump?
subtle changes (in spike protein for ex.) can alter the cell/tissue tropism, host range, and disease manifestation
3 determinants for host-switching
- evolutionary closeness of donor and recipient
- viral quasispecies
- pathogen opportunities
how do we determine the evolutionary closeness of the donor and recipient host?
look at fitness valleys
what are fitness valleys?
reflect the adaptational barriers that need to be crossed
steep vs shallow fitness valley
steep = many mutations must be made
shallow = fewer mutations must be made
how do viruses overcome adaptational barriers? (3)
- point mutations
- recombinations
- reassortment
what are viral quasispecies?
virus evolves as a diverse array of species with diff levels of fitness
benefit of viral quasispecies
enhances adaptation flexibility
how do pathogen opportunities help host-switching?
a virus poorly fit to a host can adapt to a new host if given enough chances
how common is host-switching?
usually dead-end, very rare to produce sustained transmission
can we predict host-switching?
no but we can possibly assess the viruses that are out there and be prepared
5 ways for managing and surveilling emerging infections
- research and development in academia and industry
- early detection and identification technology
- database of viral genomes in ecosystem
- first responder actions + rapid communication
- public health action + widespread compliance
- surveillance and cooperation by governmental and international agencies
what animal is a common source of zoonotic infection?
BATS
describe the diversity of bats
bat species occupy 25% of mammal species
what allows for the transmission of viruses from bats?
bats have wide geographical distribution
how does flight affect bats as a virus host?
bats adapting to flight leads to dampening of inflammation
how does hibernation affect bats as a virus host?
reduced body temperature and metabolic rate suppresses immune response and delays viral clearance
5 things that happen to cause immune tolerance in bats
- STING-dependent type I IFN response was dampened via point mutation
- NLRP3 is dampened
- PYHIN gene family is lost
- reduced activation of inflammatory cytokines
- reduced inflammatory cell deaths
what is STING?
PRR induced by cytosolic DNA
what is NLRP3?
inflammasome sensor that recognizes cellular stresses and pathogen invasion
2 examples of genes in the PYHIN gene family and their role
AIM2 and IFI16 which are inflammasome sensors for intracellular DNA
inflammasome activation in human/mouse vs bats
human/mouse –> RNA viruses activate inflammasome to induce cell death and cytokine secretion
bat –> dampened PRR priming, therefore dampened inflammasome activation and reduced cytokine secretion
