br Results br Discussion Geography can either

Results
Discussion Geography can either limit or facilitate the spread of infectious diseases, like HIV. The proximity and close cultural ties between populations in Mexico and the southwestern US, have linked these groups for many years. Thus, it is not surprising that in the early days of the HIV epidemic, nearly all identified HIV infections in Mexico were acquired in the US (Valdespino-Gomez et al., 1995). In this study, we explored the connectedness of the two cross-border epidemics over time and found, similar to our previous work (Mehta et al., 2010), which used a much smaller dataset and less detailed methods, that the two epidemics were mostly separate. In this new larger analysis we were able to better characterize viral migrations between San Diego and Tijuana, and we demonstrated that HIV may now also be flowing from Mexico to the US, likely because of mixing between high risk MSM (Ritieni et al., 2006) and PWID populations (Brouwer et al., 2009). Given the estimated size of the HIV infected populations of San Diego (~12,000) (AIDSVu, 2014) and Tijuana (~1800–5500) (Brouwer et al., 2006), our bi-national dataset of 1106 unique viral sequences represented approximately 7% of infected individuals in the border region, and up to 12% of newly diagnosed infections in San Diego County (County of San Diego HaHSA, 2012). Although our analysis focused on the 843 sequence with associated demographic data collected from ten separate studies, each of which employed a different recruitment approach and was focused on a specific Dioscin cost (geographically and/or by risk factor), we still were able to obtain and include data across risk groups and geography. Although only 7.2% of our sequences were obtained from individuals residing in Tijuana, we identified 14 putative transmission clusters involving individuals from Tijuana and five clusters involving individuals reporting residence on both sides of the border. These cross-border clusters had higher proportions of females and heterosexuals than the rest of the transmission clusters, with >50% of members participating in transactional sex. This observation highlights the importance of the sex trade in HIV transmission across the region\'s border. Interestingly, two of the four cross border clusters included PWID from Tijuana and clients of female sex workers (FSW) from San Diego. Given that the focus of sampling in Tijuana was in high-risk FSW and PWID, this finding underscores the importance red light districts as a “melting pot” for risk groups, resulting in the bridging of different types of transmission networks. As expected the majority of our putative transmission clusters were predominantly males who were MSM, since the majority of participants were from HIV research programs focused on the MSM population of San Diego. However, in transmission clusters comprised predominantly of heterosexuals, there was a predominance of females. This lack of males in putatively inferred transmission clusters has been demonstrated previously (Hue et al., 2014). Although injection drug use could lead to female–female transmission, unidentified infected male contacts of our sampled FSW are likely to be involved in these transmission clusters. Identification of these men will require different recruitment methods than those currently being used and underscores the importance of identifying and testing partners of high-risk individuals. This cross-border molecular epidemiologic study has several limitations, the most important of which is sampling bias. Different methodologies were used to collect data across the different studies ranging from respondent driven sampling (Detection of HIV in Latinos, El Cuete, SD PIRC, and STAHR (Garfein et al., 2012)), passive enrollment (SD PIRC), venue based recruitment (SDPIC, Detection of HIV in Latinos, and STAHR-II (Robertson et al., 2014)), time location sampling (El Cuete and STAHR-II), partner identification (Amigos, SD PIRC), and convenience sampling (Hombre Seguro, Amigos, Proyecto Parejas, Mujer Segura and Mujer Más Segura, STAHR and STAHR-II). Variations in the depth of sampling across the region and collection of risk factor data also likely contributed to the study\'s overall sampling bias. A more comprehensive sample of the infected population in the border region would likely have resulted in more putatively identified transmission links, a higher rate of overall clustering, and a improved understanding of viral migration across the border. Despite this limitation, this study was large and demonstrated linkage of transmission networks across different risk groups and geographically separate populations in the border region. Sampling across risk groups in the Tijuana border region also highlighted the role of such unique geographic areas in bridging transmissions across risk groups, and suggests the need for future research that is attentive to how social, sexual, and drug use networks interact with the built environment. Finally, this analysis highlights the potential usefulness of continuous molecular epidemiologic monitoring of HIV transmission networks, specifically by: 1) providing information about which sexual and PWID networks contribute disproportionately to new infections, and 2) identifying which important socio-demographic or risk groups might be missed in current identification strategies, resulting in improved targeting of prevention efforts.