Enteric Redmouth (ERM) Disease, Hagerman disease or Yersinosis is an economically important disease of captive sea-run Atlantic salmon and also young farmed rainbow trout in freshwater aquaculture. The disease is caused by a Gram negative, motile rod-shaped bacterium, Yersinia ruckeri and typical of other Gram-negative septicaemias varies in severity from sub-clinical to subacute / acute infection often with mortality.
Affected fish may appear dark with congestion of the oral area, ulceration, haemorrhage and the presence of cutaneous petechiae. Localised haemorrhage at the tips of the gill filaments may be evident and both unilateral and bilateral exophthalmia with tissue oedema are common. The characteristic reddening of the mouth and jaw area, due to a localised hyperaemia involving the submucosal is apparent during the later stages of a chronic infection.
At necropsy characteristic observations include profound venous and capillary congestion, particularly of the brain and blood vessels, intestinal haemorrhage , with diffuse petechiae occurring in the musculature and hypertrophy of the spleen. The intestine becomes filled with a watery, opaque or blood-tinged fluid and is associated with a yellowish discharge in the vent area.
Histopathology includes bacterial colonization of vascularised tissues and haemorrhage of the gills, kidney, liver, spleen, and heart, as well as muscle. Focal to diffuse necrosis is generally evident in the haematopoietic tissue within the kidney and spleen following bacterial invasion. The spread of the bacteria to the gills, musculature and liver results in capillary dilatation, haemorrhage, tissue oedema and a further spreading necrosis.