Author: Ng Elizabeth
Date: August 2007
An important step in combating infectious diseases like HIV is exploring the origin and evolution of the disease. Simian immunodeficiency virus (SIV) is the equivalent of HIV for monkeys and chimpanzees. It is believed that HIV arose from SIV, which was then transmitted to humans by contact with chimpanzees. In a study published in PLoS Pathogens, scientists from the University of Arizona in Tucson have found that SIV may have infected the African green monkey population much later than previously thought.
"Understanding how emerging infectious diseases evolve in their natural host organism helps us understand the disease's possible trajectory," said author Joel Wertheim, a doctoral candidate in the UA department of ecology and evolutionary biology.
SIV may have become less deadly over a shorter period of time than predicted by older evolutionary models. Although SIV may have been very deadly to monkeys millions of years ago, the green monkeys of today rarely become sick from SIV.
This finding "may indicate a future decrease in virulence of HIV," Wertheim added.
Wertheim and Michal Worbey, a UA assistant professor of ecology and evolutionary biology, unconvinced by previous work on the green monkey evolutionary trees, decided to perform an in-depth analysis of mitochondrial DNA to create a more accurate family tree of four species of African green monkeys. They compared this phylogenic tree to those of four different strains of SIV. If SIV had infected green monkeys prior to speciation, the family trees of the virus and the monkeys should match; they did not.
This indicated not only that SIV had infected monkeys later than expected, but also that SIV had been transmitted horizontally between different species of green monkeys, rather than vertically through time. Another result of this research was a better understanding of the geographic spread of SIV.
The four green monkey species studied occupied four different geographical regions: western Africa, central Africa, south-eastern Africa, and north-east Africa.
By determining how closely each strain of SIV was related, and then examining the timeframe of infection, Wertheim and Worbey were able hypothesize that the infection began in the westernmost species and then spread to the central then south-eastern and north-eastern species.
Researchers will be able to use these findings to better understand the future evolution of HIV.
Citing some laboratory research that suggested HIVs from the late 1980s were more virulent than HIVs from the 2000's, Worobey added, "for HIV, the really cool thing is that these changes can take place on a more rapid timeline than previously thought."
By Elizabeth Ng