The ecology and emergence of diseases in fresh waters

P>1. Freshwater ecosystems, including ponds, lakes, streams and rivers, represent an interaction nexus between environmental change and a wide variety of infectious diseases, including human malaria, salmonid whirling disease, amphibian chytridiomycosis, crayfish plague and many others. However, few studies have explicitly examined patterns of disease in fresh waters and how they are changing over time. 2. Freshwater environments can function as transmission foci for pathogens because of (i) the importance of fresh water for organism survival, (ii) the aquatic life histories of many vectors and intermediate hosts, (iii) the concentrated aggregations of species - both freshwater and terrestrial - in and around freshwater habitats and (iv) the highly altered condition of freshwater ecosystems, which can affect species interactions and disease pathology. 3. To determine whether water-related diseases in wildlife are increasing, we used generalised additive models to quantitatively assess trends in the scientific literature (1970-2009) for major freshwater groups, including amphibians, molluscs, crayfishes, fishes, mammals, reptiles and birds. We further examined what types of pathogens were primarily responsible for observed patterns and whether recurrent groups or transmission modes could be identified. 4. After correcting for research effort and temporal autocorrelation, we find that reports of disease varied over time and across freshwater taxa, with significant increases in amphibians, fishes and crayfishes, a significant decrease in molluscs, and no significant change in freshwater reptiles, birds or mammals. The types and diversity of pathogens varied considerably among groups. Reports of infection in amphibians were dominated by helminths and a chytridiomycete, in crayfishes by viruses and fungi, in molluscs by digenetic trematodes, in birds, fishes and mammals by viruses, protists and helminths and in reptiles by helminths and bacteria. 5. These results provide some of the first quantitative evidence indicative of a long-term increase in disease-related research for freshwater taxa. Managing freshwater ecosystems to reduce or minimise human and wildlife disease risk - arguably one of the most significant ecosystem services - will require enhanced incorporation of ecological approaches alongside medical and veterinary tools.