Water Quality for Wyoming Livestock & Wildlife - Coming Soon!

y the ongoing search for an appropriate animal model tostudy the human condition. The mechanism(s) putativelyinvolved in the pathogenesis of chronic damage inpeople, i.e. chemical attack by methylated As III metaboliteson cellular macromolecules, do not appear to berelevant in livestock and wildlife. In domestic livestock,as opposed to people, most As is excreted via urine asDMA III . 33 This, together with the shorter observed halflifein these species, suggests that relatively little trivalentAs escapes methylation and excretion to cause cancer.Chronic poisoning in livestock species involves mechanismssimilar to acute poisoning and requires dosagesvery similar to acute poisoning.Given the accumulating evidence that As is a humancarcinogen, the question of residues arises. Can foodanimals consuming As from water accumulate dangerousamounts of As in edible tissues without themselvesshowing signs of toxicity? The literature to date suggestscattle, sheep, etc. eliminate As too quickly for this to bea concern, and a study completed in 2007 by the Universityof Minnesota 89,90 failed to find any evidence of Asaccumulation in milk or edible tissues from dairy cattlewatered from As-contaminated (140 μg/L) wells.The threshold toxic dose in domestic ruminants appearsto be between 1-2 mg/kg BW. This dose is in generalagreement with the NRC 25 , which recommended 30-50ppm dietary As as a maximum tolerated dose and withother reviews. 88,91-93 It is quite distinct from the EUrecommendation of 2 ppm dietary As, for which we havenot been able to discover any justification. Sufficientquantitative data was not found to estimate a similarthreshold for horses, but this dose is similar to that reportedin another monogastric species (dogs) 79 , and previousreviews suggest horses are similar to cattle in sensitivityand/or less frequently affected than cattle under similarconditions. 93,94 Therefore, it seems reasonable that limitssafe for cattle should be adequate for horses. The verylimited data in wild ruminants suggest they are similar tocattle in sensitivity. Therefore, our recommendations arebased upon dosage data from cattle and sheep. Assumingneglible As in feedstuffs, 5 mg As/L in drinking water willprovide the minimum toxic dose of 1 mg As/kg BW tograzing animals in warm weather. Obviously, if animalsare receiving any As from forage or medications, less willbe required to achieve a toxic dose. Although we werenot able to find any significant studies of As in Wyomingforages, limited data from our laboratories suggest naturalbackground concentrations seldom exceed a few ppm,except in areas contaminated by geothermal runoff.Assuming a NOAEL of 0.5 mg/kg BW/day and allowingfor these small forage concentrations, we recommenddrinking water for livestock and wildlife notexceed 1 mg As/L.9