14 - Emerging Viral Infections

Question 1

Emerging virus

Answer 1

The causative agent of a new or previously unrecognised virus infection in a population

Question 2

Zoonoses

Answer 2

Infections of humans by viruses that pre-exist in stable relationships with nonhuman hosts

Question 3

Four general types of interaction between a virus and its host

Answer 3

• Stable
• Evolving
• Dead end
• Resistant

Question 4

Stable

Answer 4

Both participants survive and reproduce

Question 5

Evolving

Answer 5

Hallmarks are instability and unpredictability

Question 6

Dead end

Answer 6

• Virus is not transmitted to other members of new host species
• Frequent outcome of cross species infection

Question 7

Resistant host

Answer 7

Represents situations in which host blocks infection completely as host cells are not susceptible, permissive or defences are too strong

Question 8

Factors that outcomes of a virus-host interaction depend on

Answer 8

ecological, host, and viral parameters

Question 9

Predominant parameters for spread of infection

Answer 9

• Population density
• Age and health of individuals in that population

Question 10

Incidence

Answer 10

Number of people infected / population

Question 11

Morbidity

Answer 11

Number of individuals who become ill / number of infected

Question 12

Mortality

Answer 12

Number of deaths / the number infected

Question 13

Case fatality ratio

Answer 13

Number of deaths / number of individuals with illness

Question 14

Stable interactions

Answer 14

• 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

Question 15

Evolving host virus interaction

Answer 15

• 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

Question 16

Examples of evolving host virus interactions

Answer 16

• Introduction of measles to natives by colonists
• Attempt to use myxoma virus to rid Aus of rabbits

Question 17

Animal to human transmission

Answer 17

• 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.

Question 18

Common sources of animal to human transmission

Answer 18

Rodents (e.g. Sin Nombre) and bats (Hendra virus)

Question 19

Human vs bat immunity

Answer 19

• 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

Question 20

Dead end hosts

Answer 20

• 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

Question 21

Resistant vs inapparent infection

Answer 21

• 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

Question 22

Example of cross species transmission

Answer 22

Emergence and transmission of H5N1 influenza virus

Question 23

Emergence and transmission of H5N1

Answer 23

• 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

Question 24

Examples of social parameters that facilitate transmission of infection to new hosts

Answer 24

• Deforestation
• Air travel
• Urbanisation
• Day care
• AC
• Xenotransplantation
• Blood transfusion
• Sex
• Drug use

Question 25

Ecological parameters of transmission of infection to new hosts

Answer 25

- 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

Question 26

Coronaviruses

Answer 26

+sense, enveloped, non segmented RNA viruses with largest genomes of RNA viruses (~30kb)

Question 27

SARS-CoV amplifying host

Answer 27

Masked palm civet

Question 28

SARS-CoV reservoir

Answer 28

Horsehoe bat

Question 29

Mpox

Answer 29

Orthopoxvirus following outbreak in monkeys used in research

Question 30

Major developments in methods for virus discovery driving the identification of viruses that infect humans

Answer 30

(A) Discovery by species of virus (B) Discovery by virus family (C) Technological advances through the 20th century to the present

Question 31

% of human infectious diseases originating from animal reservoirs

Answer 31

60%