Following a 1998 paper that linked the gene deletion with bubonic plague, "bubonic plague had been cited as a classical example of a historical selection pressure acting on a clinically important locus," she said. That classic example now changes, with smallpox replacing the plague.
The gene produces a receptor, called CCR5, that is the main entry port for HIV-1 into T cells and macrophages. While most people around the world have two CCR5 genes or alleles, about 10 percent of Europeans, on average, lack one of the alleles. They thus produce fewer CCR5 receptors, which hinders initial infection by HIV-1 and slows spread within the body once an immune cell has been infected. Those lacking both alleles produce no CCR5 receptor.
Based on a population genetics model, Galvani and Montgomery Slatkin, a UC Berkeley professor of integrative biology, argue that bubonic plague could not have caused such a rapid spread of this genetic mutation throughout Europe. Even though the Black Death pandemic killed off 25-40 percent of all Europeans during its run through the continent between 1346 and 1352, bubonic plague was historically a sporadic disease with an average annual death rate of only a few percent during the 400-year period it afflicted Europe. Transmitted by fleas infesting rats, it killed people of all ages.
Smallpox, on the other hand, was a continuous presence in Europe for 2,000 years, and almost everyone was exposed by direct person-to-person contact. Most people were infected before the age of 10, with the disease's 30 percent mortality rate killing off a large part of the population before reproductive age.
"When you remove children from a population, you remove more of the reproductive potential for the species, compared to losing older people, who are not reproducing," Galvani said.
Other diseases common at the time, including measles, polio, whooping cough, rubella, scarlet fever, chicken pox and influenza, also targeted children, but fatalities were typically only a few percent and could not have exerted strong influence on the frequency of this genetic mutation.
The smallpox virus also has more biological similarities to HIV-1 than does bubonic plague, the authors point out. Plague is a bacterial disease, and there is no evidence that the bacterium, Yersinia pestis, uses the CCR5 receptor in infection. The bacteria actually reproduce outside immune cells.
Smallpox, on the other hand, is a virus based on RNA, just like HIV, she said. And there is some evidence that smallpox, Variola major, uses chemokine receptors like CCR5 to enter cells.
The researchers said the geographic distribution of smallpox also correlates better with the frequencies of the CCR5 deletion in Europe.
Re: Magic's appearance...like I said in an earlier
thread
I wonder if any of you guys have ever actually seen a person who is on HAART long term.