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“If we want to stop the HIV epidemic, then we must know the mechanisms by which HIV uses human sex to spread,” said principal investigator Davey Smith, MD, MAS, associate professor of medicine in UCSD’s Division of Infectious Diseases and in the Veterans Affairs San Diego Healthcare System, and director of the CFAR Viral Pathogenesis Core.

It is known that most HIV infections worldwide result from exposure to the HIV virus in semen, made up of seminal cells and the fluid around these calls, called seminal plasma. HIV virus particles contain RNA and exist in the plasma, while infected seminal cells contain HIV DNA.

Using a method of comparing genetic characteristics, called phylogenetic analysis, the researchers studied a group of men who had sexually transmitted their HIV virus to other men. Phylogenetic models allow researchers to estimate the dates of origin of various groups of viruses; in this way the team was able to determine the source of rapidly mutating HIV viruses by analyzing the viral sequences extracted from the blood and semen of HIV transmitting partners. The team found that recipients shared a more recent common ancestor with virus from the seminal plasma than with virus found in the seminal cells of their source partner.

“Until now, it had not been established whether HIV RNA or DNA is transmitted during sex,” said Smith. “By analyzing the genetic differences between these two forms and the virus that was ultimately transmitted to newly infected individuals we found that it was the HIV RNA form present in seminal plasma that was transmitted.”

“The findings from this study will help direct prevention strategies to address the virus in the seminal plasma,” Smith said. “By knowing the origin of the transmitted virus, scientists may be able to develop new vaccines, vaginal microbicides and drugs to prevent the spread of sexually transmitted HIV.”

Smith notes that because the study involved pairs of men who have sex with men, the findings do not comment directly on HIV transmission to women. “Since the vast majority of women are infected with HIV by exposure to the virus in semen, HIV RNA in the seminal plasma is the likely culprit, but this needs to be confirmed,” he said.

The results of this paper, “HIV and Tuberculosis Co-infection Among Hispanics in Southern California: An Increasing Health Disparity,” will appear in the February edition of the American Journal of Public Health.

“While the overall numbers are modest, our study shows that what used to be mostly a disease of white and black patients in San Diego is now largely a disease of Hispanics,” said Timothy Rodwell MD, PhD, MPH, associate physician/fellow in the Division of Global Public Health at UC San Diego. “This indicates that the benefits of prevention and treatment of TB and HIV over the last decade have been uneven in the different ethnic/racial groups in this region.”

HIV is a potent risk factor for TB disease. It increases the risk of latent TB infection reactivation, the rate of disease progression and the risk of new infections. TB also accelerates HIV disease progression, increasing infectivity and reducing HIV treatment efficacy.

“The synergy of TB and HIV has created a worldwide public health crisis and has significantly complicated attempts to eliminate TB in both the industrialized and developing worlds,” noted Rodwell.

Rodwell’s team analyzed San Diego County TB surveillance data from 1993 through 2007, grouping cases by HIV status: positive, negative or unknown. Of the 5,172 TB cases analyzed, 8.8 percent were also infected with HIV. The number of new co-infected cases did not change significantly during this period, but the proportion of cases among Hispanics did increase significantly, while the proportion of cases among non-Hispanic white and black patients decreased.

Hispanics now account for more than 80 percent of all TB–HIV co-infection cases in the county, further widening existing health disparities in this region. Co-infected patients are significantly more likely to be Hispanic 30 to 49 year-old male injection drug users than any other patient group. Researchers recommend that since the burden of TB and HIV in San Diego has shifted to Hispanics, the disparity must be addressed with focused binational TB and HIV prevention efforts.

Rodwell explained, “With more than 40,000 people crossing the border between Mexico and San Diego on a daily basis, and our finding that the majority of new TB–HIV co-infection cases occurred among Hispanics that were born in Mexico, it is clear that future interventions to address this health disparity will need to be binational in nature.”