Proliferative kidney disease is a serious parasitic disease affecting salmonid fish, including, for example, trout, char, and grayling. The disease occurs in both Europe and North America and can cause high mortality rates in both fish farms and wild fish populations. Mortality rates range from several tens of percent to nearly 100 percent, depending on the severity of the infection, especially when fish are weakened by stress or other infections.
The causative agent of the disease is the microscopic parasite Tetracapsuloides bryosalmonae. This parasite requires two hosts for its life cycle—salmonids and freshwater bryozoans (tiny invertebrates living in colonies on submerged rocks, roots, or branches). When conditions are favorable for bryozoan growth—particularly at higher water temperatures and with an abundance of food—the parasite produces infectious stages known as malacospores. These are released into the water and can infect fish. Young fish encountering the parasite for the first time are particularly susceptible. The parasite enters the body through the gills and skin and is subsequently carried via the bloodstream to the kidneys, liver, and spleen.
Water temperature plays a significant role in this. The disease develops primarily at temperatures above 12 °C, which means that the biggest problems occur during the summer and early fall. Experts warn that, due to climate change, the disease is expected to spread further to areas where it has not yet been found, and that the course of the disease is likely to become more severe. “The main target organ is the kidneys, where the parasite multiplies and damages the tissue. This leads to enlargement of the kidneys and disruption of their proper function. However, fish that survive the infection are able to gradually regenerate the damaged tissue and develop a certain degree of resistance to reinfection,” adds Prof. Miroslava Palíková of MENDELU.
The course of the disease also varies among different fish species. While the infection can have serious consequences for rainbow trout, the parasite in its body is unable to produce infectious spores. In contrast, brown trout can be infected without showing significant symptoms, yet they produce infectious stages that are excreted into the aquatic environment via urine, enabling the disease to spread further. The most common signs of the disease include darkening of the body, lethargy, loss of appetite, impaired swimming, enlarged kidneys and spleen, pale gills, or hemorrhages in the skin. In some cases, the disease can lead to mass fish mortality.
Researchers from MENDELU, in collaboration with the Veterinary Research Institute in Brno, staff from the Fisheries and Hydrobiology Research Institute in Vodňany, the Czech Fishing Union, and the Moravian Fishing Union, monitored—among other things—as part of the NAZV project “Assessment of the Hydrological Situation of Trout Streams in the Czech Republic and the Status of Salmonid Populations in Relation to Climate Change.” The principal investigator for MENDELU was Ing. Jan Grmela, Ph.D. A total of 59 sites on 42 streams across the Czech Republic were examined. The results showed that DNA from the PKD pathogen was present in 436 fish, representing 61.8% of all analyzed individuals. The parasite was detected at 46 sites, or 78% of the monitored locations. “Currently, there is no treatment capable of reliably eliminating the parasite from fish.” Some methods to mitigate the effects of the disease have been tested experimentally, such as the use of sodium chloride (table salt), which led to a reduction in mortality and a mitigation of symptoms. However, such measures cannot be practically implemented in natural waters,” says Palíková.
Preventive measures are therefore of crucial importance. In fish farms, the most effective measure is the use of groundwater or well water free of bryozoans and their developmental stages. In the wild, prevention is more difficult; measures that may help include limiting eutrophication (excessive enrichment of water with nutrients), lowering water temperature by using cold water from reservoirs, and maintaining high genetic diversity in fish populations.
The topics of fish health, the prevalence of pathogens, and the prevention of fish diseases are also being discussed at the Second Joint Meeting of the Central and Eastern European Branch of the European Association of Fish Pathologists (EAFP), which is taking place September 1–4 in Křtiny. This conference is held under the auspices of two universities: the University of Veterinary Sciences in Brno and Mendel University in Brno. One of the topics focuses specifically on PKD.