Lead is a naturally occurring toxic metal. Its widespread use has resulted in extensive environmental contamination, such that lead poisoning in animals and people is of major concern worldwide. In veterinary medicine, lead poisoning following opportunistic or accidental ingestion, is most common in dogs and cattle. Lead exposure may have serious consequences for human health, particularly for young children. Even low levels of exposure can affect brain development, resulting in reduced intelligence, and behavioural changes such as shortened attention span and increased antisocial behaviour. These effects are believed to be irreversible. Because of the hazards associated with ingesting lead, Australian food-producing animals that have been exposed to lead sources, and may therefore pose a risk if their meat is eaten, are subject to movement and slaughter restrictions to ensure food safety and product integrity.
Lead is the most commonly reported source of heavy metal poisoning in livestock, with severely affected animals showing a variety of nervous system signs. These may include teeth grinding, blindness (stumbling, walking through fences or crashing into solid structures), muscle spasms, lack of co-ordination, head pressing in corners or against trees, and eye rolling. Signs of acute lead poisoning occur within 12-24 hours of consumption, with severely affected animals quickly dying from seizures and breathing paralysis. Lower level toxicity may occur more slowly, with animals surviving for several days; blindness, aimless wandering or staggering and gastro-intestinal upsets may be seen. These signs are not, however, specific to lead poisoning, and a diagnosis should be made by a veterinarian to rule out other causes of brain illness. Lead poisoning may present with signs that require tissue sampling and testing, in order to differentiate the illness from other conditions such as:
- tetanus (early stage)
- botulism (late stage)
- polioencephalomalacia (vitamin B deficiency)
- nervous ketosis
- infections of the brain
- other poisonings (e.g. salt, mercury or arsenic), and
- exotic diseases such as rabies and BSE (mad cow disease).
Cattle in the NT occasionally succumb to lead poisoning when they are able to access station dump sites containing discarded lead-acid car batteries. Exposure to sunlight and chemical corrosion of the batteries over time, leads to shattering of the chemical cell housing, so animals can easily access the lead. In a discharged battery, the lead is present as lead sulphate, a salt which is tasty to cattle and easy to consume. Other sources of lead poisoning may include mine tailings, water contaminated in lead-lined pipes or ground poisoning from contamination with materials such as sump oil or lead shot. Producers should make every effort to prevent animals accessing potential sources of lead such as metal dumps or tailings dams, as the consequences of lead exposure are long lasting and expensive. Station dumps should be securely fenced to prevent stock access, and discarded lead and other heavy metals should preferably be recycled through accredited metal recyclers, rather than discarded on site.
It is important to know that not all animals that have ingested lead will have symptoms; severely lead poisoned animals may simply be the sentinels which indicate that a herd has been exposed to a lead source. Some animals may have consumed some lead and show no symptoms, but their blood and tissue levels of lead may still be above the Maximum Level (ML) allowable under the Australian & NZ Food Standards Code. For this reason, if lead poisoning is diagnosed in a single animal on a property, further investigation and testing of herd cohort animals is likely to be necessary, to ensure that animals with lead contamination do not enter the food chain. 95 per cent of ingested lead is stored in the bones of the animal, from where it may be released at different times. This is why affected animals may be temporarily or permanently restricted from entering the food chain. DPIR livestock biosecurity officers and field veterinary officers are able to provide support and advice on herd management in the event that lead exposure is detected on your property.
Last updated: 12 December 2019