The Disease

Introduction
Proliferative kidney disease (PKD) has been recognised as a parasitic disease of great economic significance to salmonid aquaculture. Although primarily regarded as a condition affecting first season rainbow trout (Oncorhynchus mykiss), all salmonids can become infected during freshwater stages with varying severity. Although the name PKD was first coined by Roberts & Shepherd in 1974, reports of a similar syndrome affecting trout date to at least 50 years previously. The disease is endemic in large areas of Western Europe and North America, but has not been recognised in the Southern hemisphere to date.

Etiology
The identity of the enigmatic causative agent of PKD – originally denoted as PKX - remained elusive until the late 1990s when it was established to be a myxozoan whose spores possessed four distinctive polar capsules (Figure 1) and which also parasitized freshwater Bryozoa, invertebrates colloquially known as ‘moss animals’ (Figure 2). The parasite was consequently named Tetracapsuloides bryosalmonae in reference to its two known hosts. Various freshwater phylactolaemate bryozoans including Fredericella sultana have since been found to become infected with T. bryosalmonae resulting in the development of spherical sacs (Figure 3) which release spores. These are released into the surrounding water where they can infect salmonid fish. Massive numbers of spores can be produced from relatively small volumes of bryozoans, with recent research suggesting that very low numbers are capable of infecting fish to result in disease.