Hematological analysis is commonly used to assess the physiological state of fish. It includes red blood cell parameters, white blood cell parameters, and the number of thrombocytes per blood volume unit. Hematological analysis is one of the basic tools (often accompanied by biochemical and histopathological analyses) to assess the influence of organic and inorganic substances on fish. It is, therefore, applicable in both ecotoxicology and pharmacotoxicology. The advantages of this research method are the lack of need for specialized laboratory equipment and low costs, and the limitations are the need for extensive experience among the personnel performing the tests. One of the recommended methods of supplementing routinely determined hematological parameters is assessing the cellular composition and activity of hematopoietic tissue. As there is very little scientific data available on the issue of the effects of xenobiotics on the cellular structure of fish head kidney hematopoietic tissue, filling this gap should be considered an urgent need. Therefore, we recommend conducting research with the simultaneous use of hematological and hematopoietic analysis as reliable and complementary methods of assessing the impact of toxic substances on fish.

Antibacterial agents are commonly present in aquatic environment at low concentrations. Terrestrial animal farms, human medicine and aquaculture are main sources of water contamination with antibacterials. Antibiotics were proved to be directly toxic to fish causing oxidative stress, general stress response, histopathological lesions, hematological, metabolic, and reproductive disorders, as well as immunosuppressive and genotoxic effects. Environmentally realistic low concentrations of antibiotics also disturb aquatic bacterial communities causing alterations in fish symbiotic microbiota and induce emergence of antibiotic-resistant pathogenic bacteria by exerting selective pressure on spread of antibiotic-resistance genes.

Blood-based biochemical assays are used as predictive and diagnostic methods to evaluate fish welfare in aquaculture and research. The variations of blood biochemical parameters in fish are commonly used as biomarkers of exposure to toxic agents. Blood biochemical parameters can help identify the magnitude of toxicity and the mechanisms by which particular toxic agents act on the organisms. Some parameters typically measured in the blood can also be evaluated in the whole body in the early developmental stages of fish (embryos and larvae) that are often used in toxicological studies. This review assessed the usefulness of various blood biochemical indices as toxicity biomarkers. Analysis of multiple studies showed that toxicity-induced changes in most blood biochemical parameters in fish often depend on toxic agent concentration and exposure duration. Also, various parameters manifest different sensitivity to intoxication, and diverse directions of changes may occur. Among biochemical parameters, some are biomarkers of general physiological stress, while others indicate dysfunctions of particular organs. Moreover, hormonal endpoints seem to be sensitive but nonspecific biomarkers of intoxication in fish.

Exposure to xenobiotics causes pathophysiological changes in fish, including oxidative stress and neurotoxicity. Here, we describe the biochemical mechanisms underlying oxidative stress (i.e., redox imbalance) and the biochemical markers commonly used to assess its level. Neurotoxicity biomarkers used in fish include behavioral, histological, molecular, neurotransmitter-related, and enzymatic parameters, among which acetylcholinesterase (AChE) activity is the most commonly measured. We therefore also review the changes in AChE activity in fish exposed to common xenobiotics. In most cases, AChE activity decreased in a concentration- and time-dependent manner, although some studies reported no change or even an increase. We emphasize the relevance of all the parameters discussed in the context of fish toxicology studies.

Hematological, biochemical and hematopoietic effects of therapeutic doses of two antibiotics, oxytetracycline (OTC) and gentamicin (GEN), in clinically healthy common carp juveniles were studied. The fish were divided into four groups: controls 1 and 2 (untreated or injected with 0.6% NaCl solution), and two groups treated with antibiotics (orally with 75 mg/kg OTC four times every two days or injected with a single dose (4 mg/kg) of GEN dissolved in 0.6% NaCl). Blood and head kidneys were sampled from all fish 3 days post-treatments for hematological, biochemical and hematopoietic tissue analyses. No major alterations in the values of hematological and serum biochemical parameters occurred following administration of OTC or GEN. Glucose concentrations were significantly lower in both groups of fish subjected to injections (Control 2 and GEN), while the oxidative metabolic activity of phagocytes increased in the antibiotic-treated groups (significantly in OTC). More alterations were observed in hematopoietic tissue. Immunocytochemical analysis revealed that G caused a significant increase in the rate of cell proliferation (PCNA-positive cells) and an increase in the frequency of apoptotic cells (caspase-positive). The frequency of lymphoid lineage decreased, which was related to a decrease in the abundance of mature lymphocytes in GEN-treated fish. Percentages of neutrophilic lineage were significantly elevated in OTC and GEN groups compared to controls. The obtained results showed no considerable hematotoxicity or hepatotoxicity of therapeutic doses of OTC and GEN to carp.