The polyethers yessotoxin (YTX) and okadaic acid (OA) are two marine algal toxins frequently associated as edible shellfish contaminants. Seafood contamination by these compounds, also at low concentrations and for a long period of time, can increase the possibility of their simultaneous and repeated ingestion, with possible synergistic toxic effects. Thus, in vivo toxicity by repeated oral exposure to a combination of fixed doses of YTX and OA (1 mg YTX/kg and 0.185 mg OA/kg, daily for 7 days) was investigated in mice, in comparison to that of each toxin alone. No mortality, signs of toxicity, diarrhea or hematological changes was induced by the toxins co-administration or by the single toxins. Light microscopy revealed changes at gastric level (multifocal subacute inflammation, erosions and epithelial hyperplasia) in 2/5 mice co-administered with the toxins. In animals dosed only with OA, epithelial hyperplasia of forestomach and slight focal subacute inflammation of its submucosa were noted. No changes were induced by the treatment with YTX. Ultrastructural analysis of the heart revealed some cardiomyocytes with "loose packing" of myofibrils and aggregated rounded mitochondria in mice co-administered with the toxins or with YTX; OA-treated mice showed only occasional mitochondrial assemblage and dilated sarcomeres. Thus, the combined oral doses of YTX (1 mg/kg/day) and OA (0.185 mg/kg/day) did not exert cumulative or additive toxic effects in mice, in comparison to the single toxins.
