Malaria as an Infectious Disease

What causes people to become ill?

We can all think of lots of different illnesses, some we’ve read about, some that have affected family members and some we’ve had ourselves. For thousands of years people have wondered what caused illness. The answers may seem obvious to us; if not, I hope they will be in a couple of hours, but without modern knowledge, this is a very difficult question to answer.

(http://www.biosci.ohio-state.edu/~parasite/anopheles.html
copyright held by BIODIDAC http://biodidac.bio.uottawa.ca/info/browse.htm)

In the past many cultures have produced volumes of work on how to treat illnesses, some very effective, some less so. The indigenous peoples of South America used the bark of a tree that contained quinine to prevent malaria - it really does work. Until two hundred years ago, many illnesses in Britain were treated by bleeding – not so good!

The old works on how to treat illnesses contain very few accurate observations on what caused the illness in the first place. The reason is that the authors had a very limited understanding of how a body works when it is not ill. It’s easier to understand this if we imagine someone being asked to explain why a car would not start when he did not know how the car worked in the first place.

It is in only in the recent past that we have developed an understanding of how illnesses arise. The knowledge has come from the study of the body as opposed to illness, and started with the drawings of Leonardo da Vinci, which described the way a body is put together. This process has continued right up to the completion of the sequence of the human genome, which describes the digital plan for a body.

So, lets begin by trying to answer the question ‘what causes illnesses?’

Start by making a list in your minds of a few different illnesses, and then think if they are all totally different to one another, or whether they have some similarities.

What is an infectious disease?

• a disease that can pass from one individual to another

How does an infectious disease pass from one individual to another?

• by transfer of an infectious agent

The disease is caused by the growth of an infectious agent in the infected person

What sort of thing is an infectious agent?

• prion
• virus
• bacteria
• protozoan (a single cell animal)
• other animals

They are all different sorts of organisms from simple viruses to insects or worms – in other words infectious agents come in all shapes and forms.

• prion - BSE, vCJD
• virus - HIV infection, cold, influenza
• bacteria – pneumonia, tuberculosis
• protozoan (a single cell animal) malaria
• other animals – intestinal worms, head lice

How do infectious agents spread?

• water - contaminated drinking water
• contaminated food
• air (aerosol) – coughs and sneezes spread diseases but only some
• carried by another living organism – especially those that drink blood
• bodily fluids – direct transfer

How does the body defend itself against infectious reagents?

The immune system makes special protein molecules called antibodies that recognise, for example, the bacteria. The antibodies recognise the bacteria by binding to it very tightly. Special cells in the body (white blood cells) then detect the bound antibody and kill the bacteria by engulfing it.

Let's look at one disease in particular - Malaria

Why is malaria important?

• Malaria is one of the remaining infectious diseases that has been unaffected by the modern era
• Malaria kills between one million and three million people a year, that’s the same as three thousand to ten thousand each day or up to five hundred during the one hour and a bit we’ll be talking
• 90% of cases are in Africa and 90% of deaths are children under 5 years of age

Is malaria just a disease of distant places?

• No, relatively recently malaria was much common in Britain especially in the marshy areas in the south, such as East Anglia.

Malaria and its association with wetlands was well known to Shakespeare:

‘All the infections that the sun sucks up
From bogs, fens, flats, on Prosper fall and make him
By inch-meal a disease’

The Tempest, Shakespeare 1611

It’s interesting that The Tempest was written when the average temperature was several degrees colder than today, cold enough to freeze the River Thames in winter. So, it is not too cold for malaria to return.

How severe was malaria in Britain?

The effect of the disease in parts of Britain was then very much the same as the effect it has in Africa today. This can be seen by comparing death rates of children up to 5 years old in two parishes in Essex in the 1800s:

• In a parish with marshes 95 per thousand
• In a nearby dry parish 45 per thousand

(http://www.biosci.ohio-state.edu/~parasite/anopheles.html
copyright held by BIODIDAC http://biodidac.bio.uottawa.ca/info/browse.htm)

The occurrence of malaria in Britain declined very sharply around the1880s and the last case was in the 1950s. Nobody is sure why but we'll discuss that later.

The infectious agent that causes most human malaria is called Plasmodium falciparum, but we'll call it PF.

• How does PF get from one person to another?
• What does it do once it is inside a person?

Gives them periodic fevers that are so acute they can lead to death in young children or in adults that catch it for the first time.

‘As one who has the shivering of the quartan so near,
that he has his nails already pale
and trembles all, still keeping the shade,
such I became when those words were uttered’

The Inferno, Dante (1265-1321)

If you are born in an area where malaria is common and live to 5 years of age, then it is unlikely to be fatal, but remains debilitating - why?

• Because the immunity is very slowly acquired and is very complex.