VIMS researchers have found the Pfiesteris shumwayae, a member of the toxic Pfiesteria comples (TPC), kills fish by feeding directly on their skin, not by releasing a potent toxin into the water, as has been widely reported. Their results appear in the August 5, 2002 issue of Nature.
VIMS researchers investigated the pathogenicity of P. shumwayae using an innovative larval fish bioassay developed at the institute. "This bioassay allows us to investigate the interaction between Pfiesteria and fish in great detail," says Dr. Wolfgang Vogelbein, the Nature article's lead author. Using a P. shumwayae culture known to kill fish, the scientists performed controlled fractionation studies in which they used centrifugation and filtration to remove the dinoflagellates from the water, presumably leaving any toxin(s) the organisms may have secreted. They also used permeable membranes to separate the dinoflagellates from the fish during exposure studies. "Only in treatments where fish were in direct physical contact with the dinoflagellate did we see fish dying," says Vogelbein.
Further evidence against secretion of a potent toxin by P. shumwayae comes from observations of the dinoflagellate and fish using videomicrography, high-resolution microscopy, and electron microscopy direct observation of the assays with microscopy allowed us to observe dinoflagellates in large numbers swarming toward the fish and rapidly attaching and feeding directly on the fish skin," says Vincent Lovko, a VIMS graduate student who helped to develop the new bioassay.
"Standard bioassay methods used by others cannot distinguish between toxigenicity and killing by micropredatory feeding," says Vogelbein. The VIMS researchers thus recommend that all strains and species of Pfiesteria be re-evaluated for toxicity using these sensitive new methods. "This is the only way to determine if there are, in fact, any bona fide toxic strains out there," Vogelbein says.
The role of Pfiesteria in aquatic animal and human health remains controversial. "This new research clearly demonstrates that Pfiesteria shumwayae, even though it does not secrete any potent toxins, can kill fish in closed systems, under controlled laboratory conditions," says Dr. Jeffrey Shields, another co-author of the Nature article. "Does Pfiesteria do this in the natural environment? That question remains to be answered."
The new research has important implications for scientists trying to understand both the basic biology of Pfiesteria species and the possible role these dinoflagellates may play in estuarine fish kills and fish-lesion events. The findings are important to government agencies charged with protecting environmental and human health. Research efforts designed to answer these questions continue at VIMS, with grant support from EPA, NOAA, and the Centers for Disease Control and Prevention.