Eosinophilic granular cells (EGC) and histamine responses to Aeromonas salmonicida toxins in rainbow trout.

The function of eosinophilic granular cells (EGCs) in salmonids is unknown. In a previous study of the pathogenesis of A. salmonicida, injection of crude exotoxins into rainbow trout were shown to reproduce the lesions associated with furunculosis and an accompanying lesion, the dispersion and degranulation of EGCs in the intestinal wall was reported. The present study investigated this phenomenon in relationship to the histamine content of the gut. In fish injected i/p with A. salmonicida exotoxins in a dose causing death in six hours, a coincidental decrease in the histamine content of the gut, appearance of histamine in the blood, and degranulation of the EGCs in the intestinal wall was observed 45 minutes post-injection. The fish developed behaviour patterns similar to that described by other workers for fish undergoing systemic anaphylaxis. Other features were pale gills, defaecation and widespread vasodilatation. The possible role of the EGC as a histaminogenic cell in rainbow trout is discussed.