External & Internal Signs

Clinical Signs

External signs are non-specific and may include:
Body darkening
Gross abdominal swelling
Exophthalmos
Gill pallor
Loss of equilibrium and respiratory distress

Internal signs:
Granulomatous renal swelling, particularly of the posterior (up to ×10 of the normal size)
Spenomegaly
Generalised pallor

Microscopic findings:
Renal haematopoietic hyperplasia followed by granulomatous interstitial nephritis
Presence in kidney of extrasporogonic and sporogonic parasite stages

Diagnosis
Although not pathognomonic, characteristic clinical and gross pathological signs could support a putative diagnosis of PKD in conjunction with clinical history - especially the season of the year and records of previous farm outbreaks. However, definitive diagnosis relies upon the detection of T. bryosalmonae in samples of fish tissue. Extrasporogonic stages - often encircled by phagocytes - can be readily identified by methylene blue, May-Grünwald-Giemsa or Leishman-Giemsa staining of kidney smears (Figure 4), or in fixed paraffin-embedded sections stained with haematoxylin and eosin or by immunohistochemistry (Figure 5) using T. bryosalmonae specific monoclonal antibodies such as P01 (Aquatic Diagnostics Ltd., Stirling, UK). Molecular techniques such as in situ hybridisation and polymerase chain reaction assays have also facilitated reliable diagnosis.