Book excerpt: Due to the recent rapid development of freshwater aquaculture in the Caucasus Region, many new and previously known fish diseases have appeared. One of the most prominent features of the region’s aquaculture is that it is mostly based on the rearing of cyprinids, mainly the common carp (Cyprinus carpio), as well as a few other predatory fish species. As a result, this book focuses on the diseases that affect these and other important warmwater fish species. Although this field guide covers the diseases of warmwater fish of Central and Eastern Europe, the Caucasus and Central Asia, it also draws upon the extensive knowledge base available for the countries of Central Europe and the former Soviet Union, as well as recent research findings from the Islamic Republic of Iran and from Turkey. The major warmwater fish species cultured in the region and their health status are discussed, and two major categories of disease are recognized: biotic and abiotic diseases. Although there are numerous biotic diseases, abiotic factors (e.g. lack of oxygen, temperature, feeding mistakes) remain the main cause of losses in aquaculture. The best practices for the field and laboratory examination of disease outbreaks are reviewed, and the importance of accurate and detailed data recording emphasized. Prevention as a key factor in avoiding the spread of disease is highlighted, and actions to prevent the spread of diseases between farms, regions, countries and continents are discussed. Possible methods for the treatment of each disease are reviewed; unfortunately, the chemicals available for use in aquaculture are now rather limited, as many of them are hazardous to both the environment and human health. Of the viral diseases discussed, spring viraemia of carp (SVC) and koi herpesvirus (KHV) pose the greatest threats to the world’s carp populations. Of the bacterial diseases, ulcer disease is still the main problem in carp culture, while among the parasites, Ichthyophthirius multifiliis, the cause of white spot disease, is among the most important. Exotic parasites such as various Thelohanellus species, as well as tapeworms belonging to the genera Bothriocephalus and Khawia, are responsible for a considerable amount of damage. Some diseases of unknown aetiology are also discussed.