HSS is a condition of unknown aetiology, which affects Salmo salar leading to dramatic pathological changes. The disease occurs in pre smolts, smolts and post smolts. Affected fish typically have good body condition, without obvious weight loss; they show lethargy, superficial swimming and dark colouration.
Bilateral exophthalmia and increased respiratory rate can sometimes be observed
The external gross findings include pale gills, with haemorrhages in the branchial arches and lamellae, plus petechiae and ecchymoses ventrally and at the base of the fins.
Internally, generalized visceral petechiae and ecchymoses are seen involving gastrointestinal tract, swim bladder and peritoneum, visceral adipose tissue, heart and somatic musculature. The liver appears yellowish, sometimes with petechiae and ecchymoses. Additionally, ascites and hydropericardium can be recorded.
Differential diagnosis include: Infectious Salmon Anemia (ISA), Infectious Pancreatic Necrosis (IPN), Atypical Furunculosis, Haemorrhagic Viral Septicaemia (VHS) and Infectious Haematopoietic Necrosis (IHN).
- Nylund., H. Plarre., K Hodneland., M. Devold, V. Aspehaug., M. Aarseth., C Koren., K Watanabe. Haemorrhagic smolt syndrome (HSS) in Norway: pathology and associated virus-like particles.
- Rodger, H. D., & Richards, R. H. (1998). Haemorrhagic smolt syndrome: a severe anaemic condition in farmed salmon in Scotland. Veterinary Record, 142(20), 538–541. doi:10.1136/vr.142.20.538.