Modelling and controlling soil-transmitted helminths

Question 1

Why transmission modelling in STHs?

Answer 1

To reduce disease in order to eliminate, no risk of bounce back. Can use Rx on children alone?

Question 2

What needs to be considered when calculating the number of worms exposed to Rx?

Answer 2

Demography - proportion of population are school aged children Behaviour - proportion attending school Epidemiology - worm load in children

Question 3

When forming a model for STH what factors must be included?

Answer 3

Pop. of mature worms, Production of eggs/larvae, Pop. of infective stages of infected vectors/intermediate hosts Also include environmental effects, density dependence, behavioural effects and immune effects (p27 of Deidre lecture)

Question 4

What is the impact of transmission dependent on?

Answer 4

Extent transmission from children to adults and vice versa

Question 5

What is a consequence of aggregation of egg intensity?

Answer 5

A large change in intensity could result in little change in prevalence.

Question 6

What does heterogeneous modelling with different exposure and deposit rates show?

Answer 6

Children over-contribute to transmission. Starting intensity in untreated group is lower, larger impact on this group of treating the children.

Question 7

What is use of modelling a disease?

Answer 7

• Identification of weakpoints for intervention to control infection (non-linearity)
• What to measure in the field
• Reveals counter intuative results

Question 8

What logic should be applied to modelling for vaccine coverage for eradiation of a disease?

Answer 8

If the number of secondary infections is less than one the infection will eventually die out.

Question 9

1. Arethmetically show conditions required to model of vaccine coverage/disease persistance
2. Based on this how could you show the numbers of secondary infections if people are vaccinated?
3. What conditions are neded to eradicate disease?

Answer 9

(1.) v < 1 - 1/Ro

v = proportion vaccinated

Ro = number of secondary infections

therefore 1 - v is the proportion still succeptable

2. Ro(1-v)
3. Ro(1-v) < 1

Question 10

What assumptions are made when modelling for vaccine coverage?

Answer 10

Vaccines are 100% effective

Lifelong immunity

Question 11

What are the consequences of modelling vaccine coverage and implementing findings?

Answer 11

• A resulting increased age at first infection
• How can secondary infection be measured?
• Is the required scale of vaccine coverage attainable?

Question 12

What is the SIR model?

Answer 12

S = number Susceptable

I = number Infectious

R = number Recovered (immune)

A simple model for infectious disease which records the dynamic interaction between a directly transmitted microparasite and its host population.

Question 13

What factors nees to be included when forming an SIR model for disease?

Answer 13

Susceptibility, infection, immunity, age, time, ‘dynamic’ movement between all.

More sophistaicated models will include age specifc host dealth rate, per capita recovery rate, per capita death rate, force of infection at time, and latent infection

Question 14

What are the principles of the Ross-Macdonald model?

Answer 14

Cycling between secondary (human) host and a primary vector, it illustrates the dynamics between the two. It’s the key basic equation for vector borne disease.

Question 15

What variables are included in the Ross-Macdonald model?

Answer 15

a = mosquito biting rate

b = mosquito to human transmission probability, per bite

c = human to mosquito transmission probability, per bite

m = number of female mosquitoes per human host

ξ human recovery rate: average duration of human infection

δ mosquito death rate: average duration of mosquito infection

Question 16

What assumptions are made in the Ross-Macdonald model (for malaria)?

Answer 16

• There is no human immunity
• There are homogenous populations of mosquitos and humans
• Mosquito behaviour and population dynamics consistant (no biting preference, no difference in seasonal behaviour)
• Plasmodium species contracted the same.
• No stochasticity (random events)

(Leads to an asymmetry in the transmission term)

Question 17

Advantages of modelling.

Answer 17

Objective way of investigating dynamics

Cheap and easy way to get insights into basic epidemiology (non-linearities)

Examine imaginary scenarios

Can plug-in unknown parametre values to find whether they have a big impact

Models often produce counter-intuative results

Question 18

Disadvantages of modelling.

Answer 18

• Many people hostile to models, prefer own intuition
• Often have little idea about the basic biology required to calibrate the models
• Aura of scientific precision and accuracy that is usually unfounded
• People may select parametres to obtain answers they wish
• Maths error