The proximity of California to Mexico, where tuberculosis (TB) is endemic, has resulted in a higher incidence of the disease across the border from Tijuana to San Diego than in the entire state of California. Adding to the crisis, Tijuana has an ongoing HIV epidemic among drug (injecting) abusers and female sex workers. Advancing HIV disease leads to the reactivation of latent TB infection in these groups, resulting from their compromised immune systems. Exacerbating the epidemic by co-infection, TB is a leading cause of death in these individuals.
In a 2007 Centers for Disease Control and Prevention (CDC) study of 503 participants, researchers determined the prevalence of TB in high-risk populations by using the interferon-gamma release assay (IGRA), QuantiFERON® TB Gold In-Tube (QFT) assay (Cellestis Ltd., Carnegie, Victoria, Australia) to detect M. tuberculosis infection and the Determine® Rapid HIV Antibody Test (Abbott Laboratories, Boston, MA) to detect HIV infection. Unlike screening with the TB skin test (TST) in the US, populations in Mexico vaccinated with bacille Calmette-Guerin (BCG) require a more accurate whole blood assay (an IGRA) performed on persons (including children > 5 years) at high risk for TB disease. This is because skin tests performed on people vaccinated with BCG can yield false positive reactions.
The study was limited by use of AFB smear microscopy without the specificity of TB culture and susceptibility testing. However, adding chest x-ray exams, researchers found the prevalence of TB was four times higher than reported in Baja California alone. Another limitation was the small number of study participants, resulting in an underestimate of the actual numbers of co-infected individuals.