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Back to Malaria as an Infectious Disease

Action of PF on humans

1st stage of growth of PF in human liver

2nd stage of growth of PF in human liver

The first event in catching malaria is to be bitten by a mosquito that is carrying PF (above left). Some PF cells are injected into the bloodstream by the mosquito and these PF cells burrow into a cell in the person's liver (above right). Inside this cell the PF grows until there are many hundreds of cells (below left) and then the liver cell bursts releasing the PF cells into the bloodstream (below right).

3rd stage of growth of PF in human liver 4th stage of growth of PF in human liver

5th stage of growth of PF in human liver 6th stage of growth of PF in human liver

( http://www.tigr.org/tdb/edb/pfdb/images.html
http://www.life.sci.qut.edu.au/LIFESCI/darben/figs/insecta
)

These cells are different to the ones injected by the mosquito and they enter red blood cells where they will grow until there are about 20 cells (below left). The red blood cell bursts and the PF cells each enter a new red blood cell (below right). The infection of red blood cells occurs in regular waves and the periodic fevers are caused by the contents of the burst red blood cells.

 


7th stage of growth of PF in human liver 8th stage of growth of PF in human liver

(http://www.tigr.org/tdb/edb/pfdb/images.html
http://www.life.sci.qut.edu.au/LIFESCI/darben/figs/insecta)

The diagnosis for malaria still depends on seeing PF in red blood cells (as in the photos above).

If the PF is picked up by a feeding mosquito then the mosquito can become infected and transmit the PF to another person.

Do Task 3 now

PF protects itself from a person's antibodies by hiding inside cells.

Why do some diseases persist and others disappear?

The reasons are rarely simply but we can look at any disease and try and understand by using the idea of 'transmission index' (R)

  • If one infected person passes the disease on to two uninfected people on average the transmission index is 2. We say R = 2:

transmission index = 2 one person spreads diseases to two others

two people spread diseases to four others four people spread disease to eight others

  • If one infected person passes the disease on to three uninfected people on average the transmission index is 3 ( R = 3 ).
  • If one infected person passes the disease on to one uninfected person on average then R = 1 and the level of disease in the population remains constant.
  • If, on average, 8 infected people pass on the infectious disease to 4 uninfected people then R = 0.5. Note that this means some infected people do not pass the disease on at all, and eventually the disease will disappear.

So, if R > 1 then the number of people with the infectious disease will increase and if R < 1 the number will decrease and will eventually die out:

disease spread from eight, to four, to two, to one person, then dies out

What happened in Britain in the 1880's?

 

What do you think happened to the transmission index of malaria in Britain in the 1880s?

What factors do you think might have caused this?

 

R < 1. No one knows what caused the decrease in transmission index but some people believe that the appearance of very cheap sheet glass for windows that occurred in the 1880s resulted in less mosquito bites!

In sub-Saharan Africa PF is endemic and transmission is stable - R > 1

Without human intervention, is the transmission index always the same for any one disease?

With human intervention, how could hte transmission index of malaria be reduced in Africa?