Re: Why does malaria infect red blood cells?

Dear Mary,

Your question is a very good one and the answer requires us to think about the life styles of the parasites involved in causing colds and those involved in causing malaria.

The short answer to your question: "Wouldn't it be easier for malaria if it infected your nose and throat like a cold?" is No, it wouldn't. The rest of this answer attempts to explain why.

We can consider a successful species to be one whose overall population size over time remains at least constant, or increases.

Successful species are often specialised for narrow ecological niches. This means they have a very restricted set of conditions under which they can survive. Humans are one of the exceptions, being very good generalists: we can live at different altitudes, latitudes, in forests or deserts; we can survive on extremely varied diets, etc. However, adaptation to restricted environments is more common in nature. And a good place to see examples of this is to look at the life histories of parasites.

When we get ill, what we often really mean is that we, or some part of ourselves, have been (often temporarily) colonised by parasites. Different cell types, or areas of our bodies, can be seen as specialised environments and parasites will be adapted to take advantage of certain of these environments. For example, bacteria and viruses that inhabit our intestines are able to thrive in acidic surroundings.

So lets take a look at the environmental niches of the parasites that cause colds and those that cause malaria so that we can see why it is that malaria would not do well if it was transmitted in the same manner as colds.

Viruses (of the genus Rhinovirus) are the cause of most common colds. These viruses infect cells of our upper respiratory tract. To become infected it is necessary for the virus to come into contact with cells in this area of your body. The obvious way to do this is through your nose and mouth. So, these parasites are effectively transmitted through touch or airborne transmission. (If it's by touch, the virus is transmitted when you touch your hand to your mouth or nose.) Viruses are obligate parasites (that is, they need a host in order to survive and replicate), however rhinoviruses can survive for hours to days on surfaces around us, (e.g. tables or books), and are quite tolerant of temperatures between 24 and 37 degrees centigrade. As they are transmitted through the air and touch, it is good for them that they can survive a while outside a host while they wait to get "picked up".

Malaria in humans is caused by one of four species of the protozoan genus called Plasmodium. Protozoans are single-celled organisms and are extremely complex compared to viruses. Plasmodium is also an obligate parasite, and it has 2 hosts that it colonises at different stages in its life cycle: a mosquito and a vertebrate (for example: humans).

Many people know that malaria parasites infect our red blood cells, because this is the phase of infection when humans fall ill. However, this could in fact be considered one of the final stages in the Plasmodium life cycle. Before it gets to that point, a parasite has had to:

• infect a mosquito
• have sex and create progeny
• the progeny had to mature in the gut of the mosquito
• once mature the progeny had to get to the salivary glands of the mosquito and be injected into a human host when the mosquito fed
• within about a minute, an infecting parasite made it to the liver and invaded a hepatocyte (liver cell) there
• over a period of about 2 weeks, these parasites matured and divided, creating thousands of progeny which then:
  • erupted into the blood stream where they infect red blood cells
  • these parasites mature and produce progeny, which then infect more red blood cells. This cycle continues for some time.

A few of the parasites inside red blood cells will become sexual stages, a male or a female. As yet, we do not know what triggers certain parasites to do this. It is these sexual stages, at the right stage of maturity, which have to be picked up by a feeding mosquito, thus beginning the whole cycle again.

So, at different times, malarial parasites would appear to be well adapted for a number of environments including:

• mosquito gut
• hepatocytes (liver) cells
• red blood cells

So, even armed just with the brief information above, we can see several reasons why malaria would not have an easy time of it if it infected your nose and throat. Firstly, to do so would probably mean being exposed to the open air, and Plasmodium would die in this environment. The parasites need to be able to access sources of "food" that they are adapted to use, which means inhabiting particular types of cells. Given that the first place a malarial parasite needs to be when it infects a human is in a liver cell, how can it get there if it enters your nose and throat? When a Plasmodium-infected mosquito bites, a transmitted malarial parasite is passed into the bloodstream where it has rapid access to the liver.

Now that you have a basic idea of the life cycle of Plasmodium, you can probably think of some other problems for the parasite if it was transmitted through the air (apart from the fact that its not hardy enough to survive anyway!)

To recap the general gist of all that was said above: Both Rhinoviruses and Plasmodium parasites are successful occupiers of human bodies. They occupy and utilise different parts of our bodies. In other words, they inhabit different niches.

I have missed out a lot of important and interesting information about the parasites above, much of which would answer your question in better detail. The best place to get basic information about Rhinoviruses is probably any general text book on Virology, and for Plasmodium, try and any general Parasitology text book. Hopefully your school or local library will have books in these categories. If not, you could try a general Biology textbook.

I have listed a few links to sources of information should you want to read more about either parasite family on-line. Thanks for sending in your question!

General Information about Plasmodium falciparum:
http://www.cdfound.to.it./html/pla1.htm

Plasmodium falciparum infecting the liver:
http://www-ermm.cbcu.cam.ac.uk/dcn/swf001dcn.htm

Info about Rhinoviruses and colds:
http://www.discovery.com/area/skinnyon/skinnyon980206/skinnyon.html

Pictures of Rhinoviruses http://www.bocklabs.wisc.edu/Rhinomovies.html