Abstract

Diplomonad flagellates are common commensals of the digestive tract, and less common pathogenic parasites found in the digestive tract and systemically, in many fish species. They also infect shellfish, amphibians, reptiles, birds and mammals. Many aspects of infections are poorly known such as host-flagellate specificity, geographic ranges, and pathogenicity.

Much confusion is due to inadequate determination of genus and species. Although older literature reports Hexamita, Octomitus and Spironucleus from fish, recent studies confirm only Spironucleus. Work by Brugerolle et al. showed that genera can be distinguished by TEM, according to shape of nuclei, location of kinetosomes (flagellar bases) and recurrent flagella relative to nuclei, and presence or absence of flagellar pockets (cytostomal canals). Our recent studies have shown that species can also be reliably distinguished ultrastructurally, by surface omamentations, especially at the posterior end of the body, and pattern of microtubular bands accompanying the flagellar pocket (in transverse section through the mid-body). Both SEM and TEM are essential for robust species descriptions. Light microscopy can supplement electron microscopy, and is useful for comparative diagnosis, but is not, by itself, adequate for showing all features for species characterization. Molecular characterization of fish diplomonads is still in its infancy.

Reliable recognition of several diplomonad species from fish now allows us to begin to compile accurate records of their geographical and host distributions. We now know that 1) all fish diplomonads are not "H. salmonis", 2) some diplomonad species have a wide temperature and salinity tolerance, 3) S. barkhanus is a commensal in its natural salmonid hosts in the wild, but a severe pathogen in novel salmonid hosts in aquaculture. The successful in vitro cultivation of a number of diplomonad species from fish gives us important information about their tolerances and optima of such environmental factors as temperature and pH. This in turn provides some clues about geographic and host ranges, thus helping to explain the distributions observed in nature.

Although many of the factors that trigger pathogenic diplomonad infections remain to be understood, we are beginning to assemble some pieces of the puzzle. We believe that 1) diseases are more likely in captive fish than in wild fish, 2) some diplomonads are inherently more pathogenic than others, and 3) some particular host/diplomonad combinations may be particularly likely to result in disease, especially when infection occurs in a novel species of host.

Acknowledgements

We are delighted to thank Prof. Dr. Christian Steinberg and Dr. Wemer Kloas of the Institute for Freshwater Ecology and Inland Fisheries in Berlin, for their enthusiastic support of this work.