Sarah L. Poynton1; Erik Sterud2
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
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
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.