The element molybdenum (Mo) is found in all living organisms and is considered to be an essential or beneficial micronutrient. However, the molybdenum poisoning of ruminants has been reported in at least 15 States and 8 foreign countries.
Molybdenum is used primarily in the manufacture of steel alloys. Its residues tend to be elevated in plants and soils near Mo mining and reclamation sites, fossil-fuel power plants, and Mo disposal areas. Concentrations of Mo are usually lower in fish and wildlife than in terrestrial macrophytes.
Molybdenum chemistry is complex and inadequately known. Its toxicological properties in mammals are governed to a remarkable extent through interaction with copper and sulfur; residues of Mo alone are not sufficient to diagnose Mo poisoning. Domestic ruminants, especially cattle, are especially sensitive to Mo poisoning when copper and inorganic sulfate are deficient. Cattle are adversely affected--and die if not removed--when grazing on pastures where the ratio of copper to Mo is <3, or if they are fed low copper diets containing Mo at 2 to 20 mg/kg diet; death usually occurs when tissue residues exceed 10 mg/kg body weight. The resistance of other species of mammals tested, including domestic livestock, small laboratory animals, and wildlife, was at least 10X that of cattle. Mule deer (Odocoileus hemionus), for example, showed no adverse effects at dietary levels of 1,000 mg/kg.
Additional research is needed in several fields: the role of Mo on inhibition of carcinomas and dental caries; the establishment of minimum, optimal, and upper daily requirements of Mo in aquatic and wildlife species of concern; the improvement in diagnostic abilities to distinguish molybdenum poisoning from copper deficiency; and the determination of sensitivity of early developmental stages of fishes to Mo insult.
Source: Ronald Eisler , U.S. Fish and Wildlife Service, Patuxent Wildlife Research Center, Laurel, MD 20708
Aquatic organisms are comparatively resistant to Mo salts: adverse effects on growth and survival usually appeared only at water concentrations >50 mg Mo/l. But in one study, 50% of newly fertilized eggs of rainbow trout (Oncorhynchus mykiss) died in 28 days at only 0.79 mg Mo/l. High bioconcentration of Mo by certain species of aquatic algae and invertebrates--up to 20 grams of Mo/kg dry weight--has been recorded without apparent harm to the accumulator; however, hazard potential to upper trophic organisms (such as waterfowl) that may feed on bioconcentrators is not clear. Data on Mo effects are missing for avian wildlife and are inadequate for mammalian wildlife. In domestic birds, adverse effects on growth have been reported at dietary Mo concentrations of 200 mg Mo/kg, on reproduction at 500 mg/kg, and on survival at 6,000 mg/kg.