The INHAND Project is a joint initiative of the societies of toxicologic pathology from Europe, the United Kingdom, Japan, and North America to standardize diagnostic nomenclature and criteria used in toxicologic studies. The INHAND initiative includes recommended nomenclature for evaluating histologic specimens from nonclinical studies involving laboratory animals including rodents, non-human primates, dogs, minipigs, rabbits, and fish. Specific terminology and criteria are derived from the consensus opinions of senior toxicologic pathologists and subject matter experts who have expertise in the different species of interest. The standardized nomenclature presented in this document is also available electronically on the internet (http://www.goreni.org/). Sources of material included government databases, including the Registry of Tumors in Lower Animals (RTLA), academia, and industrial laboratories throughout the world. This introduction provides context for 14 chapters, arranged by organ system, that define the INHAND nomenclature and diagnostic criteria for fish used in nonclinical studies. Additionally, the current introductory chapter describes both general features of INHAND methodology as well as elements characteristic of toxicologic fish studies. The latter includes fish study design and conduct; euthanasia, sampling, the histologic processing of fish tissues, and a general approach to evaluating fish studies.

Understanding how environmental changes affect the health of organisms and ecosystems is complex, but recent interdisciplinary advances and the recognition of immune function as a dynamic mediator offer exciting progress. Environmental immunotoxicology in teleost fishes is evolving beyond cataloguing stressors towards a mechanistic, integrative framework that leverages omics, in vivo tracking and cross-disciplinary modelling. However, knowledge gaps in immune mechanisms, toxicokinetics and multi-stressor interactions remain. The present work highlights these gaps, advocating for immune function as both a mechanistic lens and an integrative health indicator. Such a framework can improve predictive risk assessments, management strategies and our understanding of contaminant effects on resilience, disease susceptibility and population viability. While challenges remain, the field is poised for significant growth through collaborative innovation and advancing technology.

Contamination with pesticides, rising water temperatures and pathogen pressure represent a multiple stressor scenario relevant to surface water ecosystems globally. This study investigated the combination of three environmental stressors on juvenile brown trout, Salmo trutta. Fish acclimatised to either 12 or 15 °C, were exposed for 14 days to one (at 12 °C) or two (at 15 °C) sublethal concentrations of a pesticide mixture (fluopyram, epoxiconazole, diuron, chlorpyrifos, λ-cyhalothrin), then were maintained in clean water for three months, with half exposed to T. bryosalmonae, the causative agent of proliferative kidney disease (PKD). Pesticide exposure alone neither caused mortality nor changes in growth, hematocrit, or organ indices. However, the transcriptome (RNA-Seq) and gene expression (RT-qPCR) in brain and liver were significantly altered. No interactions between temperature and pesticide exposure were observed on apical, physiological, or qPCR endpoints and susceptibility to PKD was not affected by pesticide exposure. However, transcriptomic analysis revealed that the number of differentially expressed genes (DEGs) induced by pesticides was greater at 15 °C. Moreover, the number of DEGs affected by temperature in the brain was strongly reduced in the presence of pesticides. In addition, fish exhibited decreased basal oxygen consumption 2.5 months after exposure to the higher pesticide concentration at 15 °C suggesting potential metabolic trade-offs. Overall, our findings emphasize the need for ecotoxicological studies to incorporate multi-stressor scenarios and the importance of considering sublethal effects in understanding the response of fish populations to pesticide contamination in a changing climate.

The purpose of Chapter 6 is to provide a general overview of our current understanding of how xenobiotics target the immune system of teleostean fish and explore new frontiers toward understanding mechanisms of action and how these mechanisms may address the question of ecotoxicological consequences. There are over 20,000 different species of fishes, so it is impossible to draw simple or generalized conclusions about the immunobiology or immunotoxicology of all fish, because the differences between immune systems of major groups and between species within each group are substantial. Herein, we focused on those species of fish commonly used in research relevant to aquaculture, environmental health, and as models for human and veterinary biomedicine. Although seemingly primitive in comparison with mammals, the immune systems of fish are highly evolved to meet the demands of the environment in which they live. Surrounded by water of optimal temperature, fish exist in a relatively stable environment. Water, however, is teaming with microorganisms, some of which are pathogenic, while others may share a commensal or mutualistic relationship with the host, and they may form part of the microbiotic community (microbiome) on the epithelial surfaces of fish. The immune system must distinguish between these relationships. In addition, the water column contains dissolved inorganic and organic substances, some of which may affect immunocompetence. The importance of integument as an immune organ is maintained throughout the evolution of vertebrates, culminating in an extremely complex component of immunity and homeostasis in all modern vertebrates.

Immunotoxicity refers to the perturbation of immune system structures and functions induced by exposure of the organisms to chemical or physical (e.g., UV light) stressors. Many environmental contaminants are able to modulate functioning and development of the immune system of fish. As a consequence, fish health and population status may become impaired. The impact of contaminants on the fish immune system can occur either directly, for instance, by interference with receptor signaling pathways in immune cells, or indirectly, for instance, through interactions with other physiological systems or with the microbiome. Current understanding of the physiological mechanisms and ecological outcomes of fish immunotoxicity is still in its infancy.