Malaria transmission dynamics and epidemiology

Question 1

How can there be a thousand fold difference in transmission intensity?

Answer 1

10 people in 100,000 may get malaria from perhaps 20 infectious bites however in some regions, one single person may be bitten by an infected mosquito 20 times per day! 10,000 fold increase in transmission intensity.

Question 2

Describe malalarial immunity (how is it obtained and what is the result)?

Answer 2

Obtained gradually with repeated exposure and IS NEVER COMPLETE.

Question 3

What does R0 describe?

Answer 3

The rate of spread thorugh a population.

Question 4

If R0 is 20 for measels, what does this mean about the critical rate of vaccination? WHy?

Answer 4

Must have 95% vaccination. Because R0 is inverse of vaccination threshold 1/20 =0.05 this gives us the number/ proportion of susceptible people we can have so that it cannot spread.

Question 5

What is the equation for R0 for malaria? What does each term stand for?

Answer 5

R0= ma^2bhp^n/r-lnp
m=no of vectors per host
a=bites per female mosquito per day
b= bites on human producing infection (proportion)
h= bites per vector that give infection (proportion)
p=probability of survival
n= extrinsic incubation period

Question 6

Why is P raised to the power of N? What does this expression mean?

Answer 6

P=survival probability n= extrinsic incubation period- this is the expression because only a fraction of mosquitos survive long enough to get infectious.

Question 7

Why is P raised to the power of N? What does this expression mean?

Answer 7

P=survival probability n= extrinsic incubation period- this is the expression because only a fraction of mosquitos survives long enough to get infectious.

Question 8

Why is a in the R0 equation squared?

Answer 8

Because we need one bite to pick up an infection and another bite to transmit it.

Question 9

Why does vivax penetrate more into temperate areas than falciparum?

Answer 9

Becuase it replicates quicker inside the insect. Also it has a hypnozoite stage so it can survive cold times (e.g. winter).

Question 10

Is vector competence an issue for malaria transmission?

Answer 10

Competence is not much of an issue if the mosqs are long-lived enough and bite humans

Question 11

HOw long are mosquitos infected for once parasites reach the salivary glands?

Answer 11

For life- they never recover.

Question 12

Around how long does it take a female to become infectious?

Answer 12

13 days.

Question 13

Why do DDT and pyrethrins not work as well as we think?

Answer 13

If on walls, the mosquitos will still bite you but simply not rest on the walls afterwards.

Question 14

What is vectorial capacity?

Answer 14

Number of secondary cases of malaria originating per day from a primary case assuming the population is and remains susceptible.

Question 15

Why is there so much more malaria in Africa than in India?

Answer 15

The African vector (gambiae) has a much greater longevity than the Indian vector (culicifacies).

Question 16

What is zooprophylaxis?

Answer 16

Putting animals near humans so they get bitten instead.

Question 17

Why are quadrimaculatis and culicifaces vectors of numbers?

Answer 17

Longevity and anthropophily is not so high so need alrge numbers for effective transmission.

Question 18

What is more effective for control, chemotherapy (one large push) or changes in vectorial capacity?

Answer 18

Changes in vectorial capacity as the incidences slowly decrease but remain low but with chemotherapy, there is a sharp decrease but this has not changed the equilibrium so the numbers gradually increase again.

Question 19

What is the difference between incidence and prevalence?

Answer 19

ncidence should not be confused with prevalence, which is the proportion of cases in the population at a given time rather than rate of occurrence of new cases. Thus, incidence conveys information about the risk of contracting the disease, whereas prevalence indicates how widespread the disease is

Incidence is always a rate per time.

Question 20

What is the equation that describes the dimensions of intensity and instability? (Parasite incidence =?)

Answer 20

API= SPR x ABER (annual parasite incidence = slide positivity rate x annual blood examination rate)

Question 21

In which endemicity locations do we see a lot of malaria case variation?

Answer 21

Mesoendemic areas

Question 22

When does all cause mortality peak in the Gambia?

Answer 22

At the end of the 4 month rainy season.

Question 23

What is efficacy and what is effectiveness?

Answer 23

Efficacy can be defined as the performance of an intervention under ideal and controlled circumstances, whereas effectiveness refers to its performance under ‘real-world’ conditions

Question 24

Why do anopheles have palmate hairs on every segment of the abdomen?

Answer 24

They float parallel to water so use hairs to break the meniscus of the water!