Proliferative Kidney Disease (PKD) of Salmonids: A Disease Guide

Proliferative kidney disease, caused by Tetracapsuloides bryosalmonae, severely swells the kidney of young salmonids. It is temperature-driven and worsens as waters warm.

Overview

Proliferative kidney disease (PKD) is one of the most serious parasitic diseases of salmonids in Europe and North America. It is caused by Tetracapsuloides bryosalmonae, a myxozoan parasite of the class Malacosporea. The disease causes a marked proliferative reaction in the kidney of young fish and can result in losses of up to 90% in infected populations. PKD is strongly temperature-dependent, and its impact is expected to increase as freshwaters warm.

Hosts and life cycle

Tetracapsuloides bryosalmonae has a two-host life cycle that alternates between freshwater bryozoans and salmonid fish. The bryozoan hosts include species such as Fredericella sultana and members of the genus Plumatella. Infected bryozoans release spores into the water, which enter fish through the gills and skin. Affected salmonids include rainbow trout (Oncorhynchus mykiss), brown trout (Salmo trutta), and Atlantic salmon (Salmo salar), with young fish most severely affected.

Clinical signs

Markedly swollen, mottled kidney (the defining lesion)
Swelling of the spleen
Anaemia with pale gills, especially in later stages
Exophthalmia (bulging eyes) and darkening of the body
Abdominal swelling and bloody ascites
Lethargy and increased mortality during warm-water periods

The temperature link

Water temperature is central to PKD. Clinical disease and mortality increase with rising water temperature, with severe outbreaks associated with temperatures above roughly 15 degrees C, and higher parasite burdens and more pronounced signs at warmer temperatures. Conversely, recovery of fish is accelerated as water temperatures fall. Because of this, warming rivers and lakes are expected to expand and intensify PKD.

Diagnosis

Diagnosis is based on the gross appearance of grossly swollen, mottled kidneys together with histopathology that detects the sporogonic and extrasporogonic stages of Tetracapsuloides bryosalmonae in renal tissue. PCR and immunohistochemistry are used for specific confirmation of the parasite.

Treatment, control and prevention

There is no licensed treatment for PKD. Management focuses on reducing temperature-related stress and limiting exposure to the parasite, and many fish that survive the warm season recover as temperatures decline.

Keep water temperatures as low and stable as practical, especially for young fish
Avoid or treat water supplies carrying bryozoans and infective spores
Reduce stocking density and handling stress during warm periods
Use spring or borehole water and screening or filtration where feasible
Monitor bryozoan populations and water temperature as part of risk assessment