Abstract

Diagnosticians (aquarists, prosectors, veterinarians, pathologists) understand that collection and histopathologic examination of the heart is an essential component of a complete post mortem examination. Many diagnosticians do not appreciate the anatomic and functional differences within the heart and the potential impact that examination of these varying sites may have on ones ability to make a diagnosis of infectious disease. The fish heart is generally considered to be a two chambered system, however there are actually four distinct components: the sinus venosus, the atrium, the ventricle, and the bulbus arteriosus. The sinus venosus is difficult to identify and is rarely of any diagnostic significance. The ventricle is anatomically and functionally similar to the ventricle in other species and may be a target for degenerative, toxic or infectious diseases. Due to its size and prominence, the ventricle is a commonly sampled/examined component of the heart. The atrium and bulbus arteriosus are less commonly sampled/examined but their unique anatomic and functional characteristics make examination of these sites essential. In many fish species, the atrial lumen is lined by endothelial cells that function as a component of the reticuloendothelial system. These cells function in the innate immune response and are capable of phagocytosis of particulate foreign material and, more importantly, infectious agents such as bacteria and viruses. In my experience, histopathologic examination of the atrium (specifically the atrial endothelium) can be key to making a diagnosis of infectious disease in fish.Hypertrophy of the endothelial cells may occur in response to systemic disease and may clue the diagnostician or pathologist in to the presence of systemic infectious disease elsewhere in the body. Bacterial phagocytosis by these endothelial cells has been identified associated with systemic infections of Vibrio sp. and Mycobacterium sp. Viral infection of these cells has been identified in numerous fish infected with iridovirus. The other component of the fish heart that is unique is the bulbus arteriosus. This structure is unique from an anatomic perspective in that the bulbus arteriosus is composed of a complex meshwork of elastic fibers and smooth muscle that may serve to trap circulating infectious agents. Examination of this structure has proved useful in the diagnosis of systemic fungal infection. Fungal infection of the bulbus arteriosus has been noted in two separate cases (one unidentified fungus, one case of Fusariomycosis). In one of these cases, fungal elements were only identified in the bulbus arteriosus and death was attributed to obstruction of blood flow out of the heart. The fish heart is a simple organ that is easily sampled during a routine post mortem examination. Due to the size of the organ, it is often possible to collect either the whole heart or a significant portion of the heart in formalin for histopathologic examination. As a diagnostician or prosector it is important to understand the unique anatomical and functional differences among components of the heart. In addition to the prominent ventricle, the atrium and bulbus arteriosus should be routinely sampled and examined as these components may often provide significant diagnostic information.

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Acknowledgements

I would like to thank Dr. Roy Yanong at the University of Florida Tropical Aquaculture Laboratory for submission of case material and Drs. Mark Stetter and Donald Neiffer of Disney's Animal Programs for case material and editorial and presentation advice/assistance.

References

1.  Ellis, A.E., Munro A.L.S. and Roberts, R.J. 1976. Defense mechanisms in fish. 1. A study of the phagocytic system and the fate of intraperitoneally injected particulate material in the plaice (Pleuronectes platessa). Journal of Fish Biology 8: 67-78.

2.  Roberts, R.J. (ed). 1989. Fish pathology. W.B. Saunders, Philadelphia, PA. 467 pp.

3.  Stoskopf, M.K. (ed). 1993. Fish medicine. W.B. Saunders, Philadelphia, PA. 881 pp.