14 - Emerging Viral Infections Flashcards
Emerging virus
The causative agent of a new or previously unrecognised virus infection in a population
Zoonoses
Infections of humans by viruses that pre-exist in stable relationships with nonhuman hosts
Four general types of interaction between a virus and its host
- Stable
- Evolving
- Dead end
- Resistant
Stable
Both participants survive and reproduce
Evolving
Hallmarks are instability and unpredictability
Dead end
- Virus is not transmitted to other members of new host species
- Frequent outcome of cross species infection
Resistant host
Represents situations in which host blocks infection completely as host cells are not susceptible, permissive or defences are too strong
Factors that outcomes of a virus-host interaction depend on
ecological, host, and viral parameters
Predominant parameters for spread of infection
- Population density
- Age and health of individuals in that population
Incidence
Number of people infected / population
Morbidity
Number of individuals who become ill / number of infected
Mortality
Number of deaths / the number infected
Case fatality ratio
Number of deaths / number of individuals with illness
Stable interactions
- Both participants survive and reproduce
- Essential for continued existence of virus
- Infected individuals can become ill, recover, develop immunity, or die, yet in the long run, both virus and host populations survive
- Interactions are dynamic, fragile and rarely reversible
- Viral populations may become more or less virulent, if such a change enables them to be maintained in the population
Evolving host virus interaction
- Selective forces are applied to both host and virus, and are magnified when host populations are small
- Virus in a stable relationship may acquire a new property that increases its virulence or spread
- Introduction of virus in new geographic location
Examples of evolving host virus interactions
- Introduction of measles to natives by colonists
- Attempt to use myxoma virus to rid Aus of rabbits
Animal to human transmission
- Most hemorrhagic viruses are endemic on rodents
- The viruses establish a persistent infection, and the rodents show few, if any, ill effects.
- Substantial numbers of virus particles are excreted in urine, saliva, and faeces to maintain the virus in the rodent population.
- Humans become infected when they happen to come in contact with rodent excretions that contain infectious virus particles.
Common sources of animal to human transmission
Rodents (e.g. Sin Nombre) and bats (Hendra virus)
Human vs bat immunity
- The Type I IFN-alpha, a major innate anti-viral immune response, is constantly active in bats.
- In humans this response is only activated when a virus infection is detected
- In humans this persistent activation would induce immunopathology
- In bats the inflammatory response is down-modulated
Dead end hosts
- The virus reproduces in species of bird and/or in the mosquito.
- Disease is likely to be nonexistent or mild in these species, as these hosts have adapted to the infection.
- A third, dead end host (e.g. horses or humans) occasionally is infected when bitten by a mosquito that previously fed on an infected bird.
- Contribute little to spread but suffer from illness
- Mosquito can also spread disease between humans
Resistant vs inapparent infection
- Resistant: Destroyed on contact or organism produces some virus particles but is cleared without activation of acquired immune system
- Inapparent infection: IR is mounted but individual exhibits no signs of disease
Example of cross species transmission
Emergence and transmission of H5N1 influenza virus
Emergence and transmission of H5N1
- Birds infected with avian influenza viruses generally experience no overt pathogenesis.
- While virulent mutants do arise occasionally, These properties indicate that influenza virus is in
evolutionary stasis in birds - The avian hosts provide the stable reservoir for influenza virus gene sequences that emerge as recombinants capable of transspecies infection
Examples of social parameters that facilitate transmission of infection to new hosts
- Deforestation
- Air travel
- Urbanisation
- Day care
- AC
- Xenotransplantation
- Blood transfusion
- Sex
- Drug use
Ecological parameters of transmission of infection to new hosts
- Contact with bodily fluids of infected hosts
- Sharing resources with different species
- Being host to same insect vector
- Encroachment by one species into habitat of another
Coronaviruses
+sense, enveloped, non segmented RNA viruses with largest genomes of RNA viruses (~30kb)
SARS-CoV amplifying host
Masked palm civet
SARS-CoV reservoir
Horsehoe bat
Mpox
Orthopoxvirus following outbreak in monkeys used in research
Major developments in methods for virus discovery driving the identification of viruses that infect humans
(A) Discovery by species of virus
(B) Discovery by virus family
(C) Technological advances through the 20th century to the present
% of human infectious diseases originating from animal reservoirs
60%
