Intersecting epidemics: spatial patterns of HIV, syphilis infection and methamphetamine use among injection drug users in Tijuana, Mexico
M.L.A. Rusch1, K.C. Brouwer1, R. Lozada2, A. Vera1, C. Magis-Rodriguez3, S.A. Strathdee1
1University of California San Diego, Medicine, San Diego, United States, 2Patronato Pro-COMUSIDA, A.C., Tijuana, Mexico, 3CENSIDA, Mexico City, Mexico
Background: The intersection of injection and sexual risks and the environment in which these behaviors take place may impact transmission of HIV and sexually transmitted infections. We examined spatial patterns of HIV and syphilis infection among injection drug users (IDUs).
Methods: Respondent-driven sampling (RDS) was used to recruit 1056 IDUs who had injected in the last month and resided in Tijuana during 2006-07. Participants underwent interviews and testing for HIV, syphilis and TB. HIV and syphilis (positive TPPA and RPR>1:8) were mapped by residence and drug use (DU) locations using ArcGIS. Global Morans I statistics were calculated to assess clustering of these infections overall, and by drug type and route of administration.
Results: Among 1056 IDUs, RDS-adjusted baseline prevalence of HIV, syphilis, and HIV/syphilis co-infection was 3.0% (95% CI: 1.8-3.7%), 5.1 % (95% CI: 3.8-6.6%), and 0.8% (95% CI: 0.4-1.4%), respectively. Although over 90% reported daily heroin use, 62% also reported methamphetamine use and 19% cocaine use in past 6 months. HIV and syphilis clustered (p< 0.01) when mapped by residence, while only HIV clustered by DU location. Stratifying by drug type, HIV and syphilis clustered by DU location for methamphetamine users (HIV, p< 0.01; syphilis, p=0.03); however, only HIV clustered by residence (p< 0.01). No disease clustering was observed among cocaine users (residence, p=0.36; DU, p=0.28), or heroin users who did not use methamphetamine (residence, p=0.57; DU, p=0.93). Syphilis continued to clustered at DU locations among those injecting methamphetamine in combination with heroin/cocaine (p=0.02), but not among those smoking methamphetamine (p < 0.63).
Conclusions: Specific clustering of syphilis by DU location but not by residence highlights the potential importance of drug use environment for sexual risks among methamphetamine injectors. Interventions tailored for locations such as shooting galleries need to incorporate sexual prevention messages and address sexual risk in the context of drug use.
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