| Could HIV, the virus that causes AIDS,
be weakening? The results of a study conducted in Belgium, at the Institute of
Tropical Medicine in Antwerp, seem to suggest that in one corner of the world it
might be. The report, published in the latest issue of AIDS, a specialist
journal, concludes that HIV's ability to replicate (known technically as its
virulence) may have decreased since the start of the pandemic. Kevin Aden, the
lead author of the paper, stresses that the study is based on a small set of
samples and does not prove that HIV's virulence is attenuating around the world.
However, it does offer new insights into the evolution of the disease. Dr. Arien looked at 24 blood samples collected from untreated patients attending an HIV/ AIDS clinic in Antwerp. A dozen of these samples were taken between 1986 and 1989; the other 12 were collected between 2002 and 2003. First, he analyzed the samples to find their viral load (the number of virus particles per cubic centimeter) and the subtype of virus involved. In Europe and North America, the predominant subtype is B; in sub-Saharan Africa, where the epidemic is at its worst, the predominant subtype is C. Most of Dr. Arien's samples were of subtype B. Having done this analysis, he paired the samples off for a series of replicative "duels". Each sample from the earlier series was matched with the most similar one from the later series, and they were placed in identical cell cultures to see which would multiply the most. The result was that 75% of the viruses from 2002-03 were less virulent than apparently similar counterparts from 1986-89 -- a statistically significant observation. Dr. Arien's caution is sensible, at least until someone replicates the work elsewhere. But his conclusion is not necessarily surprising. Such viral attenuation, as it is known, is one way that vaccines are produced. What causes attenuation in wild viruses, though, is a matter of speculation. Dr Arien believes that in this case the attenuation could be the result of what he calls "serial genetic bottlenecks" during transmission from host to host. These act to reduce the genetic diversity (and thus the replicative fitness) of the virus. Genetic diversity is known to be an important component of HIV's virulence. But what might cause the bottlenecks is still unclear. A second reason for caution besides the small size of the study is, as Geoffrey Garnett, a professor of microparasite epidemiology at Imperial College, London, points out, that the ability of a virus to infect cells in a test-tube is not the same as its ability to cause disease and death in a human host. Nevertheless, Dr Aden's result is intriguing, and surely worth following up in a larger piece of research. |