Disease and host population regulation

Question 1

What is an epidemic fade-out?

Answer 1

epidemic fade-out: extinction because number of susceptible hosts are so low following initial epidemic - the diesease is so infectious that it burns through the susceptibles pool and then fades out because there are no moe susceptibles to infect

Question 2

What is an endemic fade out?

Answer 2

endemic fade out- extinction occurring because endemic levels are so low it is possible for small stochastic fluctuations to remove all parasites - not enough parasites to susceptible hosts

Question 3

What is can migration between places of bigger population and smaller population cause?

Answer 3

you can get infection waves coming from high density populations (like large cities) that will reinfect the smaller populations (like countryside) that had previously experienced fade outs

Question 4

What did Metcalf et al 2013 discover?

Answer 4

the more unvaccinated children you have the less likely a disease is to go extinct; although it is easier to get rid of them on islands as the population sizes are smaller

Question 5

How do infections with high beta behave?

Answer 5

infections with high beta are likely to burn themselves out in all but the largest populations

Question 6

How do infections with long infectious periods behave?

Answer 6

infections that have a long infectious period can persist in smaller populations because this increases the probability that an infectious person contracts a susceptible and transmits

Question 7

How does the human behaviour change measles transmission in Niger?

Answer 7

• in Niger during the rainy season people move away from big cities → we dontt have the critical community size for the infection
• when everybody comes back to the city combined with the fact that the birth rate is very high in Niger gives as a new pool of susceptibles so you have high density in the cities + fresh babies = big epidemics

Question 8

What types of density dependent relationships can you have, draw the graphs

Answer 8

-directly transmitted
-sexually transmitted
-VB

Question 9

Describe the relationship between population density and the number of contacts in a VBD

Answer 9

At first contacts increase with density - having more hosts increases the probability that a vector encounters a host at the right time and bites them

At some point this starts to level off - all the vectors are contacting hosts so adding more hosts does not increase the number of contacts

Finally If you have too many hosts the probability of a vector biting an infected host starts to decline

Question 10

What is the 20/80 rule

Answer 10

20% of hosts contribute to 80% of transmission potential

Question 11

What are two sources of heterogeneity in infectiousness?

Answer 11

• variants in number of contacts
• variation in the likelihood of transmission during a contact

Question 12

What is the formula for v?

Answer 12

infectiousness * contact rate * infectious period

Question 13

Do asymptomatic cases cause more or less infections?

Answer 13

asymtopmatic infections tend to be less infectious BUT they tend to cause more infections because the contact rate is higher as the host doesn’t think that there is something wring with them

Question 14

What are the source of variation in individual R0?

Answer 14

• environment - crowding, other hosts, state of medical knowledge
• pathogen - variation in transmissibility
• host - disease severity and shedding rate