Heterosexual intercourse during periods of acute HIV infection may account for a substantial amount of transmission of the virus, according to a study published in the May 15 issue of The Journal of Infectious Diseases, now available online. The study found that HIV levels in semen rise and fall with HIV levels in blood, indicating an increased risk of transmission during acute infection, when HIV levels in the blood are known to be higher than those in chronic infection. These finding suggest one important mechanism by which the HIV pandemic is sustained.
The study, led by Dr. Christopher Pilcher, from the University of North Carolina at Chapel Hill, hypothesized on the basis of past studies that viral burden in semen rises in accordance with HIV levels in blood. Previous studies examining data on sexual behaviors calculated that probable infection rates were too low to sustain an epidemic alone, suggesting that other avenues of transmission were involved. However, Pilcher noted that the existing studies did not take into account biological factors that could increase or decrease viral levels in the blood. Pilcher and his colleagues then postulated that if viral levels rose in the bloodstream, as they do during acute HIV infection, viral levels in the genital tract would rise as well, thereby increasing the rate of HIV transmission during sex.
To assess the relationship between virus concentrations in semen and blood, a large group of investigators (including scientists from the University of North Carolina, Duke University, and the multinational Quest study group) collaborated to examine samples donated by HIV-infected men with acute and chronic infections. The researchers observed that the viral load in the semen of the acutely infected men was significantly higher than that of the chronically infected men. Furthermore, statistical analysis found that there was a direct correlation between the viral levels of blood and semen: as the viral level in the blood rises and falls, so does the level in semen.
With this established, the group then used a probabilistic model to evaluate the effect that semen from acutely affected individuals would have on transmission rates during heterosexual sex. They suggest that individuals are hyper-infectious before symptoms of infection occur, and that this state continues for approximately six weeks. In this period, transmission rates during heterosexual sex are likely to be far higher than previously thought, as the older studies examining transmission rates took into account only viral levels during chronic infection.
Pilcher and his colleagues warn that their estimates of the likelihood of transmission during these times are necessarily conservative, due to the nature of their statistical model. Moreover, factors such as infection with other STDs, absence of antibodies in the body fluids of acutely infected individuals, and heightened partner susceptibility or riskier sexual behaviors would all increase the rate, sometimes drastically. Overall, increased infectiousness during acute HIV infection may account for substantially more HIV transmission than previously thought, and refocusing prevention and treatment efforts toward acutely infected individuals may help slow the spread of the virus in the future.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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