Approach and Difficulties in Understanding the Immune Response and Disease Resistance in Wild Fishes

1,2Cheyenne R. Smith, 2Christopher A. Ottinger, 2Vicki S. Blazer, 2Heather L. Walsh

 1West Virginia University, School of Natural Resources, Morgantown, WV; 2United States Geological Survey – Eastern Ecological Science Center at Leetown Research Laboratory, Kearneysville, WV

ABSTRACT

In 2016, we began incorporating functional immune assays into comprehensive fish health assessments of wild smallmouth bass Micropterus dolomieu during spring and fall sampling at multiple sites throughout the Chesapeake Bay watershed. At these sites, smallmouth bass were and continue to face disease and mortality with no single or consistent cause. Pathogens varied from multiple bacterial infections to heavy parasite loads to fungi and even largemouth bass virus with co-infections and visible lesions commonly occurring. We believe the underlying issue to be immunosuppression relating to a complex mixture of stressors making it difficult to resist disease or death. To address this, we developed three functional immune assays using isolated anterior kidney leukocytes from wild smallmouth bass to test distinct aspects of immunity in the laboratory – bactericidal activity, respiratory burst, and lymphocyte mitogenesis. Results have shown seasonal differences in immune responses, correlations between immune responses at some sites but not others, and highly variable responses among individuals within sites. Some factors that seem to be correlated with immune function include chemical concentrations in the water which vary seasonally and annually, disease status of the individual, and surrounding land use. Other factors which may relate to differences in functional immune response which we are just starting to look at include concentrations of perfluorinated compounds in plasma and expression of immune-and contaminant-related genes. Deciphering what the results mean amongst the multitude of confounding factors fish encounter in the wild has been a challenge. We know that stress can impact the immune response and that a good immune response is a balance between the host, environment, and pathogens. Therefore, all aspects must be considered when interpreting results of immune function in wild fish, including complex interactions between water quality, contaminants, climatic factors, pathogens and parasites, genetic changes, and more. What has helped the most in understanding immune function in wild fish has been the ability to monitor over multiple years and collect ancillary data (water for chemical analyses, and other tissues for histopathology, gene expression, chemical analyses). It has been important to be able to consistently incorporate immune function into wild fish health assessments. Long-term monitoring provides a baseline immune response and the ability to collect other tissues and environmental samples gives context to the immune results by providing a more complete picture of health. Without the capability to long-term monitor or look at other aspects of health, understanding immune function in wild fishes is as ineffective and inefficient as banging your head against a brick wall for hours.