Freshwater fish are vulnerable to a parasite called Tetracapsuloides bryosalmonae. This parasite, which has no effect on humans, causes proliferative kidney disease (PKD) – which is highly contagious and can wipe out entire fish populations. In Switzerland, this seasonal disease has been detected in rainbow trout, brown trout and char. It has been identified as one of the main causes of the decline in fish populations in recent decades and has played a major role in the extinction of local endemic species.

Researchers from EPFL, the Swiss Federal Institute of Aquatic Science and Technology (Eawag) and the University of Bern’s Center for Fish and Wildlife Health (FIWI) spent three years studying the 50-km-long Wigger River – which crosses the cantons of Aargau and Lucerne in the Alp foothills – and its watershed. In their work, they show how both spatial and environmental features of river systems can be used to better understand the epidemiology and dynamics of waterborne diseases. They also developed a mathematical model that predicts how and where T. bryosalmonae will contaminate its environment.

Complex life cycle

In creating their epidemiological model, the scientists had to take into account the parasite’s complex life cycle. It appears first in bryozoans – tiny animals attached to stones on the river floor. The parasites are then excreted into the river water as spores and enter fish through their gills and mucous membranes, where the disease first breaks out before attacking the kidneys. The spores are subsequently released through fish urine and can, in mutated form, infect colonies of bryozoans. This means the parasite needs both bryozoans and fish to multiply and mutate. T. bryosalmonae contamination generates few symptoms in cold water but becomes apparent when the water temperature stays above 15 degrees Celsius for several weeks. Its deadly effect on both farmed and wild fish therefore lasts from the summer through early fall. Infected fish that manage to survive may continue excreting spores for several seasons. The researchers had to look at all such biological factors in order to model outbreaks.