ABSTRACT
Fifteen hundred 12−15-month-old tropically adapted heifers inadvertently grazed a paddock which had a refuse dump in it containing burnt out vehicle batteries. The cattle grazed this paddock for approximately seven days. Subsequently these cattle were managed as two cohorts (cull and potential replacement breeding animals). Deaths commenced in the cull heifer group approximately 18 days after initial exposure to the refuse dump during relocation to a feedlot. Mortalities continued for 12 days, with other heifers showing clinical signs of marked central nervous system dysfunction requiring euthanasia. Necropsy of several clinically affected cattle plus blood sampling for lead analysis confirmed a diagnosis of lead intoxication. The crude mortality rate in the cull heifers was 6.6% (n = 685). Following confirmation of the diagnosis most of the potential replacement heifers (second cohort) were also relocated to the feedlot. The estimated crude mortality rate in this cohort was 5.8% (n = 815). All possible lead intoxication deaths occurred within 34 days of initial exposure, and apparently after day 16 at the feedlot no further heifers showed any clinical signs which could be attributed to lead intoxication. Longitudinal monitoring of blood lead concentrations was used to identify cattle suitable for slaughter. Overall, 70% of heifers initially blood sampled (n = 1408) had no detectable lead in their blood, however 16% had markedly elevated blood lead concentrations (> 0.7µmol/L) which persisted, and 2% had above the maximum normal threshold 1.5 years later. These latter cattle were subsequently euthanized, and necropsy revealed that visible pieces of lead were still present in the reticulum of several animals. At no time did any of these heifers with persistently high blood lead concentrations show clinical signs of lead intoxication.
