Friday, 10 April 2015


Catechism time!
Q. What kills black babies in sub-Saharan Africa? 
A. It is the unholy trinity of tuberculosis [TB], infectious diarrhoea and malaria. 
Actually TB should get first prize because that is caused by a single species of bacteria Mycobacterium tuberculosis, while the others are portmanteau diseases.  Infectious diarrhoea, for example, is caused by a huge range of bacteria, including Escherichia coli and apicomplexans [a class of single celled organisms with defined nuclei] like Cryptosporidium.  I'm never sure whether they include Vibrio cholerae in the I.D. statistics or treat cholera as a separate way of draining people of their vital fluids.  Malaria also is caused by a grab-bag of different apicomplexans but all in the genus Plasmodium.

It is important to recognise the difference among these Plasmodium species because they occur in different habitats, are very different in the severity of their symptoms, and are amenable or not to different therapeutics.  I've had occasion before to emphasise the correct identification of similar species, but it is only conservationists who really care which species of vulture or elephant we are talking about.  Being wrong about the species of Plasmodium can kill you or the small child under your care. Malaria's epidemiology is complicated by the fact that there are other essential players in the game - mosquitoes of the genus Anopheles which carry the Plasmodium from person to person and spread the disease.

The scale of the problem is enormous: malaria infects about 200 million people worldwide and kills much more than half-a-million each year. Most of the deaths are among children and that is a tragedy for each family but the loss of work days, the frightening-off of tourists and the costs of medication is draining $20 billion out of the African economy each year.  Controlling malaria could provide the energy to put Africa back on its feet but we have to box clever and think of the most efficient and cost-effective way to intervene because we of the West would rather buy a new iPhone than send $100 to Oxfam or the Gates Foundation. Solutions have included drugs, mosquito nets and DDT. There is something appealing in the idea of swatting the fly that is carrying the parasite before it can bite anyone else - those solutions don't please the shareholders of Big Pharma but they can work really well if carried out systematically - we don't have malaria in Europe any more.

One of the interesting aspects of swatting theory is that Anopheles needs to have water in which to lay its eggs and that water needs to exist for at least eight days which is the adult-to-adult life-cycle for these mosquitoes. In Europe, draining swamps was singularly effective in breaking the cycle. One of the long-standing mysteries is what happens in the dry season when there is no standing water;  how and where do the mosquitoes hunker down to burst out in epidemic disaster as soon as the rains return and provide the puddles in which the mosquitoes breed. Nobody knows where the mosquitoes go when they are not breeding and sucking blood.  If we could track them to their lair and kill them there it would be mighty.  There are two theories - aestivation and migration.  Aestivation is when the mosquitoes find somewhere quiet to hang out locally - although nobody really knows where that is; while the migration has to happen over distances of 100s of km because google-maps shows that the nearest water is that far away from villages in the Sahel where malaria is endemic in the wet season. As neither scenario looks likely there have been some ding-dong word-wars between aestivists and migrationists.

In the Christmas issue of Nature last year there was a News and Views to interpret a research paper that addressed the controversy by gathering lots data. Both those are behind the Nature pay-wall - sorry. Dao et al. stayed in a village in Mali for 5 years and collected mosquitoes - more than 50,000 of them and tracked their abundance through the whole year.  What was interesting is that they showed that "Anopheles gambiae" is really three species which are impossible to differentiate by looking at. One of these species Anopheles colluzzi could be found all through the dry-season and showed fluctuations in abundance of 10x to 100x even during these times. The real Anopheles gambiae which they now call sensu strictu or s.s. cannot be found locally and shows a peak of activity that lags behind Anopheles colluzzi when the mosquito season hots up after the rains. It now looks likely that A. gambiae is migrating incredible distances while A. colluzzi is hiding out in a local refuge.  The third species Anopheles arabiensis is not so abundant but behaves partly like A. gambiae s.s and partly like A. colluzzi.  So both sides were right but about different things and you could have wished that, instead of blaggarding each other in print and at conferences they could have devoted their energy to solving the problem.

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