Histopathological analysis of carbaryl-induced toxicity in the spleen of Levantine frog, Pelophylax bedriagae (Anura: Ranidae)

The amphibian's spleen as a source of biomarkers for ecotoxicity assessment: Historical review and trends

Amphibians are very sensitive to many environmental changes, so these animals are considered good bioindicator models for ecotoxicology. Given the importance of the amphibian spleen for hematopoietic and immune responses, this can be a key organ for the evaluation of biomarkers to monitor the health of individuals in nature or in captivity. In this systematic review, we searched databases and summarized the main findings concerning the amphibian spleen as a source of possible biomarkers applied in different scientific fields. The searches resulted in 83 articles published from 1923 to 2022, which applied the use of splenic samples to evaluate the effects of distinct stressors on amphibians. Articles were distributed in more than twenty countries, with USA, Europe, and Brazil, standing out among them. Publications focused mainly on anatomical and histomorphological characterization of the spleen, its physiology, and development. Recently, the use of splenic biomarkers in pathology and ecotoxicology began to grow but many gaps still need to be addressed in herpetological research. About 85 % of the splenic biomarkers showed responses to various stressors, which indicates that the spleen can provide numerous biomarkers to be used in many study fields. The limited amount of information on morphological description and splenic anatomy in amphibians may be a contributing factor to the underestimated use of splenic biomarkers in herpetological research around the world. We hope that this unprecedented review can instigate researchers to refine herpetological experimentation, using the spleen as a versatile and alternative source for biomarkers in ecotoxicology.

Characteristics of tissue distribution, metabolism, effects on brain catecholamines, and environmental exposure of frogs to neonicotinoid insecticides

Pesticide exposure is considered to be one important factor responsible for declining amphibian populations worldwide. The usage of neonicotinoid insecticides (NNIs) has markedly increased in recent years, and there are concerns regarding the effects of NNI-induced toxicity on the development and behavior of amphibians. However, there have been few reports on the metabolism, distribution, and neurotoxicity of NNIs in amphibians. In this study, we exposed the Western clawed frog (Silurana tropicalis) to clothianidin (CLT) in water. After 24 h of exposure, the highest concentrations were detected in the skin, indicating that frogs are at a high risk of absorbing CLT through their skin along with water. Excretion of CLT was estimated based on the concentrations of CLT metabolites in the water until 48 h of exposure. The findings showed that frogs had higher CLT metabolic ability than zebrafish. Serotonin levels in the brain were lower in the high-concentration CLT exposure group than in the control group, although the difference was not statistically significant. This suggested that catecholamine-related effects of CLT on the brain cannot be disregarded. In addition, quantitative analyses of NNI residue in wild frogs, soil, and water in agricultural areas in Hokkaido, Japan, were performed and four NNIs were detected. These results indicated the possible risk of NNI-induced toxicity in frogs. This is the first report of the characteristics of tissue distribution and metabolism of NNIs in frogs, which may facilitate the design of appropriate conservation programs for amphibians.

Ecological Risks Due to Immunotoxicological Effects on Aquatic Organisms

The immunotoxic effects of some anthropogenic pollutants on aquatic organisms are among the causes of concern over the presence of these pollutants in the marine environment. The immune system is part of an organism's biological defense necessarily for homeostasis. Thus, the immunotoxicological impacts on aquatic organisms are important to understand the effects of pollutant chemicals in the aquatic ecosystem. When aquatic organisms are exposed to pollutant chemicals with immunotoxicity, it results in poor health. In addition, aquatic organisms are exposed to pathogenic bacteria, viruses, parasites, and fungi. Exposure to pollutant chemicals has reportedly caused aquatic organisms to show various immunotoxic symptoms such as histological changes of lymphoid tissue, changes of immune functionality and the distribution of immune cells, and changes in the resistance of organisms to infection by pathogens. Alterations of immune systems by contaminants can therefore lead to the deaths of individual organisms, increase the general risk of infections by pathogens, and probably decrease the populations of some species. This review introduced the immunotoxicological impact of pollutant chemicals in aquatic organisms, including invertebrates, fish, amphibians, and marine mammals; described typical biomarkers used in aquatic immunotoxicological studies; and then, discussed the current issues on ecological risk assessment and how to address ecological risk assessment through immunotoxicology. Moreover, the usefulness of the population growth rate to estimate the immunotoxicological impact of pollution chemicals was proposed.