Cyanobacteria are a big group of photosynthetic prokaryotic organisms inhabiting fresh and sea waters and damp soils. Cyanobacteria produce secondary metabolites – cyanotoxins, which are potentially dangerous for humans and animals if present in drinking water, and pose real danger for waterfowl, fish, and zooplankton. Therefore, cyanobacteria are often considered as pathogenic organisms albeit they are unable to develop in animals’ organisms. As reported by the UNESCO international hydrological program CYANONET, cyanobacteria and cyanotoxins were found in surface waters in 65 countries. To study the occurrence of cyanobacteria and cyanotoxins in European lakes, two international cooperation programs were implemented, namely CYANOCOST in 2012–2016 and EMLS (European Multi Lake Survey) in 2015. Cyanotoxins are divided into groups according to their action on human organisms (hepatotoxins, neurotoxins, dermatoxins, and cytotoxins). Lately, the health impacts of harmful cyanobacteria have been more frequently detected and reported. However, reports of human and animal illnesses or death associated with harmful cyanobacteria tend to be investigated separately. One Health is an approach that recognizes that the health of people is closely connected to the health of animals and our shared environment. This review illustrates how the investigation of cyanotoxin-associated animal illnesses and death can be used to reduce the risks of human
exposure to harmful cyanobacteria. The One Health approach is an environmental risk management method that requires the integration of efforts of different specialists.