Subclinical Mycobacterium Infections in Wild Delta Smelt

This study describes the occurrence and magnitude of latent mycobacterial infections and other pathogens in delta smelt (Hypomesus transpacificus), an endemic fish in the San Francisco Estuary (SFE). Our current knowledge on subclinical infections is characterized by the presence of mycobacterial DNA and absence of clinical signs and Mycobacterium. In our previous studies however, we showed that handling and increased water temperature, stress factors frequently encountered in captivity, have altered infections from benign to severe mycobacteriosis. In the current study, sub adult and adult smelt captured from August 2011 to May 2012 in the SFE were analyzed for presence of bacterial and viral agents (n=741) using standard isolation and culture techniques and Mycobacterium qPCR (n=237). Although mycobacteriosis, Mycobacterium, and virus were not observed in all of the fish examined, 96% of the smelt were positive for mycobacterial DNA. Higher pathogen scores and mycobacterial gene copy numbers were observed in smelt from anthropogenically impacted sites, isolated tributaries, and adult fish. Smelt captured in low salinity zone (1-6 practical salinity units, psu), the preferred habitat in summer and fall, had more infections (higher pathogen scores and mycobacterial gene copy numbers) compared to other salinity regions. Harboring mycobacterial DNA and bacterial pathogens are robust indicators of the cumulative effect of multiple stressors on delta smelt health and the ecological conditions of habitats in the SFE. These findings may provide a foundation in our current understanding of microbial infections in smelt and their potential contribution to health and conservation efforts of this threatened species.