abstract
Diplomonad flagellates, of the genus Spironucleus (previously, and mistakenly named Hexamita), are commonly found in the digestive track of numerous fish species. Less commonly the parasites are found in systemic infections. Most of the literature reports pathogenic intestinal infections in farmed fish, especially juvenile salmonids and cichlids, with chronic clinical signs including poor appetite and weight loss. Within the last 15 years, there have been several reports of pathogenic systemic infections in farmed adult salmonids, with acute signs including muscle lesions and ascites. In 2007 there has been the first report of pathogenic systemic infection in wild salmonids in North America - a novel finding bringing cause for concern.
The diligent application of modern methods of investigation such as molecular taxonomy and phylogeny, and quantitative approaches to investigating pathogenesis, are yielding answers to some fundamental questions - why do these fish develop acute or chronic infections, when do we see morbidity and when do we see mortality, and what are the host and geographic ranges?
Among the numerous parasites adversely affecting farmed rainbow trout, Oncorhynchus mykiss, is the diplomonad flagellate Spironucleus salmonis. The flagellate which is commonly found in the lumen of the pyloric region of the intestine can be a significant pathogen of young fish. The disease is manifest as chronic morbidity and mortality, with affected fish having poor appetite, and significantly lower weight gain than unaffected fish. There are reports of excess mucus in the intestine of affected fish, but the mechanisms of this excess mucus production, and its possible link with the clinical signs, have not been determined. In order to better understand the pathogenesis of S. salmonis infection, we undertook a quantitative study of enteropathy, with focus on mucus production.
We compared the condition of the intestine in two groups of juvenile rainbow trout from the same farm, uninfected fish (n = 15), and infected fish with moderate density of infection (n = 15). For each fish, transverse histological sections were made of the anterior pyloric region, 5 µm thick sections were stained with hematoxylin and eosin, and 3 µm thick sections were stained with combined Alcian blue/PAS. The intestinal architecture was examined for the entire cross section, and then five representative folds were selected, and evaluated quantitatively for the number, size, and release of mucus by goblet cells, and enterocyte histopathology.
In infected fish, there was a higher incidence of vacuolization of enterocytes than in uninfected fish. In addition, while there was no evidence of hyperplasia of goblet cells in infected fish, there was significant hypertrophy and release of mucus by goblet cells compared to uninfected fish. This new data suggests that hyperproduction of mucus in S. salmonis infected fish is not due to goblet cell hyperplasia, but is primarily due to hypertrophy and increased release of mucus by goblet cells. These alterations in enterocytes and goblet cells may decrease nutrient absorption and underlie impaired growth in infected rainbow trout.
acknowledgements
We gratefully acknowledge the Promotionsförderung des Landes Berlin--NaFöG for the award of a Ph.D. Scholarship to M. Reza Saghari Fard, and the Deutsches Forschungsgemeinschaft for the award of a Mercator Visiting Professorship to Sarah L. Poynton.