Exposure of beef cattle to sub-clinical doses of Microcystis aeruginosa: toxin bioaccumulation, physiological effects and human health risk assessment.
Yearling beef cattle were fed 1 x 10(5) cells ml(-1) of toxic Microcystis aeruginosa in their drinking water for a period of 28 d. Feed and water intakes of four control and four treated animals remained unchanged following introduction of M. aeruginosa into the drinking water of the treatment animals, and there were no significant differences in feed and water intakes between control and treated animals. We tested the blood plasma of both control and treated animals twice each week for elevated concentrations of the liver enzymes gamma-glutamyl transferase (GGT), glyceraldehyde dehydrogenase (GADH), amino aspartate transferase (AST) and bilirubin. All tests were negative indicating no measurable liver dysfunction resulting from microcystin intoxication. We also tested for the presence of free microcystin in the liver and blood plasma by HPLC and ELISA and for total microcystin (free+bound) in the liver by GC-MS. If all the ingested microcystin was bioaccumulated within the liver, the concentration would have exceeded 3 microg MCYST-LR g(-1) fresh weight. HPLC and GC-MS analysis of the liver tissue and blood plasma from treated animals failed to detect any microcystins. ELISA analysis of liver tissue extracts from the treated animals indicated microcystin concentrations as high as 0.92 microg MCYST-LR equivalents g(-1) fresh weight although none was indicated in the blood plasma. The microcystin concentrations measured by ELISA in livers of treated animals were more than 1000 times higher than the limit of quantification by HPLC and GC-MS indicating the ELISA results were almost certainly due to cross reactivity with something other than intact MCYST-LR. Based on the detection limits of the HPLC and the per capita daily consumption of beef in Australia, it appears that consumption by beef cattle of water containing M. aeruginosa cell concentrations up to 1 x 10(5)cells ml(-1) for 4 weeks would not produce concentrations of microcystin within the liver or blood plasma that would present an unacceptable risk to human health based on World Health Organization protocols for determining such risks.