Tuesday, March 17, 2009

Happy to be Wrong

I have spent the last two days on the train from Kigoma to Dar es Salaam, so between electricity supply and internet connection problems in Kigoma and then travelling, I haven't had much opportunity for blogging.

But I did get some time to read a large part of Jared Diamond's Guns, Germs, and Steel, a history of the world that argues for the prominent role played by technology, agriculture, transport, environments and movements of people in the eventual dominance of some nations over others. Diamond has many good ideas and I was especially interested in the chapter on diseases (Chapter 11).

Diamond suggests that old world countries had many diseases that people there had some resistance to and they brought these diseases with them to new world countries. Inhabitants of the new world countries had no resistance to these diseases, so huge numbers of them died.

Most of these new world diseases started as diseases of domestic animals (measles, smallpox, flu, TB, etc). The new world continents had few domestic animals, thus fewer epidemic diseases had developed to infect humans. Therefore, old world invaders did not die in such huge numbers.

HIV (also yellow fever, malaria, etc) is an exception in that it developed from a virus infecting monkeys in the African wild and mutated into a deadly human virus via contact through hunting, eating bushmeat, etc.

Diamond mentions that myxomatosis evolved from a virus that quickly killed most rabbits into a more slow acting virus that infected many rabbits but killed fewer of them. This meant that the virus could establish itself in a large number or rabbits and would have time to infect more rabbits rather than potentially wiping out its host.

Similarly, when syphilis hit Europe in the fifteenth century, it killed people very quickly, in just a few months. But syphilis also evolved into a disease that can be virtually symptomless for a long period, so it can silently infect larger numbers of people. People witnessing the horrors of open pustules and flesh falling off faces, followed by death, would probably act quite quickly to avoid those infected and the virus would be in danger of wiping out its supply of host humans.

This reminded me of something a doctor that I met in Nairobi said. He worked with AIDS patients even before HIV had been identified. He suggested that in the early days, when they still didn't quite know what was happening, patients who turned up seriously ill sometimes died quite shortly after. He said that these were people who had recently been infected with HIV.

HIV has three phases, the first phase immediately follows infection and lasts two or three months. The infected person has no symptoms during this phase but they can suffer a so-called seroconverting illness before entering the second phase. This illness has symptoms similar to other conditions (flu, malaria, etc) and so is treated as such. During this first phase, the infected person is highly infectious, so if they are going through a period of high sexual activity they can infect many other people.

The second phase lasts for up to ten years and, while those infected are not very infectious, they may be sexually active during this period. This would mean that many more people are exposed to the virus. Whether they are infected or not may depend on the presence of other sexually transmitted infections and other factors.

The third phase is where the HIV virus overwhelms the immune system and this phase only lasts a year or so. The patient now has full blown AIDS and they are very infectious but they may not be as sexually active as at previous times.

So if, as this doctor suggested, people were dying of the seroconverting illness, HIV would have very little time to spread beyond the initial host and anyone else they came into contact with in the space of two or three months. A population faced with an incurable sexually transmitted disease that kills in months may well take every precaution to avoid any risks. But this doctor suggested that HIV became less virulent and people do not now die of a seroconverting illness.

But, following Diamond, the fact that HIV does not kill people quickly surely makes it a greater threat. Those who are infected will probably not know that they have been infected until they start to experience the opportunistic infections that characterise the third phase of HIV. They could have infected many more people by that time. A disease that kills the host only after ten or more years is not as likely to give rise to immediate behaviour changes and HIV becomes harder to eradicate.

Nowadays, there are drugs to control HIV progression so that people don't go into the third phase, they are effectively suspended in the second phase. Indeed, the drugs also lower their infectiousness considerably. Uninfected partners of HIV positive people who are on antiretroviral treatment (ART) can remain uninfected indefinitely.

HIV continues to evolve and those on ART need to be monitored regularly to ensure that the virus has not developed resistance to the treatment. But there are alternative drugs they can take if and when resistance arises and HIV positive people receiving ART have a good chance of surviving for many years.

The doctor in Nairobi is optimistic about HIV becoming less and less common and perhaps even dying out, eventually. I don't share his view, for reasons expressed elsewhere on this blog. I don't think we understand many of the most important factors involved in HIV transmission and the countries with the worst epidemics are also worst placed to fight the epidemic spread. But I would be very happy to be wrong in my analysis.


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