Bartonella henselae (BH) is a high-priority emerging pathogen (RFA-AI-08-002) that was only identified in 1990 and named in 1992. BH is the agent of cat scratch disease (CSD) inmmunocompetent humans. In immunocompromised patients, BH causes potentially fatal vascular lesions of the skin (bacillary angiomatosis [BA]). BH alternates between two very different niches: the bloodstream of the definitive mammalian reservoir (the domestic cat); and the gut of the arthropod vector (the cat flea). The reservoir of infected cats worldwide is tremendous in size: there are 90 million pet cats in the US, and the regional prevalence can be high, e.g., 41% of cats in the SF Bay Area. BH is probably the most common vectorborne bacterial disease in the US, causing substantial morbidity due to CSD (>22,000 cases/yr), and severe morbidity and mortality in immunocompromised patients. Persistent, high-titer bacteremia is a hallmark of Bartonella spp. bacteremia: cats can have titers of >106 cfu/ml of blood, and can remain bacteremic indefinitely, even life long. Strikingly, immunocompromised humans infected with BH (and immunocompetent people with B. quintana) also can maintain persistent, >106 cfu/ml bloodstream infection, leading us to hypothesize that Bartonella utilizes similar mechanisms of persistence in cats and humans. Asymptomatic persistence of Bartonella species in the bloodstream of mammals in such high titers is an extraordinary virulence strategy for a gram-negative organism, and our goal is to understand the underlying mechanisms of persistence, including the host immune response, and the concomitant evasion tactics of Bartonella. Because BH is an emerging pathogen, virtually nothing is known about mechanisms of pathogenesis. To identify underlying correlates of BH bloodstream persistence, we will employ 3 different strategies in parallel. We will draw sequential blood samples from specific pathogen free cats that have been experimentally inoculated with BH, and then 1) use a BH protein microarray chip to identify sequentially recognized antigens, with high-throughput screening of pre-immune and post-infection serial sera; 2) perform sequential 2D SDS-PAGE immunoblotting of BH outer membrane proteins with the serial sera; and 3) use Solexa DNA sequencing technology to identify changes in the BH whole genome profile over time, including characterization of BH quasi-species generated during persistent infection in vivo. Finally, select seroreactive BH antigens identified from the above strategies will be exploited ultimately to develop a diagnostic test for BH. Because so little is known about Bartonella virulence mechanisms, this translational research will address the large void in knowledge about Bartonella pathogenesis.