Lead poisoning in livestock

Lead is a common cause of poisoning of domestic animals throughout the world. The Queensland situation is no exception with about 20 cases of lead poisoning diagnosed at DPI&F laboratories each year (based on submissions from 1980 to 1995). This is an unknown proportion of the actual number of poisonings from lead ingestion. Nevertheless, it indicates that stock owners should be aware of the possibility of lead poisoning occurring, and have an understanding of its effects on farm animals.

Cattle are the most susceptible livestock, with calves the most likely victims. However, lead poisoning can occur in all domestic animals including horses, birds/poultry and dogs. Pigs are the least susceptible.

Major sources of lead on the farm

Livestock may find lead in rubbish dumps and around farm buildings and machinery. It can occur in:

• discarded sump oil
• dumped lead batteries
• flaking lead-based paint or old paint tins
• linoleum, grease, putty, oil filters, metallic lead and other sources.

Stock may find sump oil and other sources of lead attractive as lead compounds can have a sweet taste. Contributing factors may include boredom from confinement, and depraved appetites from malnutrition and phosphorus deficiency. However, predisposing factors are not essential.

Lead shot may be a source of poisoning of domestic poultry and wild birds. It is most often seen in water fowl, such as ducks and geese, which swallow lead shot and fisherman´s sinkers from the bottom of lakes and ponds.

Clinical signs

A combination of gastro-intestinal and nervous signs may occur. The gastro-intestinal effects may produce either constipation or diarrhoea.

In cases of acute poisoning, stock may be found dead, or display combinations of several signs for a few hours before death: colic, staggering gait, rolling eyes, slobbering, muscle spasms, blindness, uncoordinated attempts to climb obstacles, excessive response to external stimuli, head pressing and convulsions.

Animals with subacute poisoning may display dullness, loss of appetite, abdominal pain and diarrhoea.

In chronic poisoning animals show wasting, loss of appetite, anaemia, constipation and perhaps recumbency. Paralysis and death may occur. Sometimes acute attacks may occur during the course of chronic poisoning. Horses may have difficulty breathing.

Differential diagnoses

Similar nervous system effects can be induced by various diseases affecting the brain including:

• cerebral babesiosis - tick fever
• nutritional and metabolic deficiencies eg hypomagnesaemia, ketosis, thiamine deficiency, vitamin A deficiency
• other poisons eg mercury, organophosphates, organochlorines
• plant poisoning eg Noogoora burr, pyrrolizidine alkaloid poisoning
• enterotoxaemia eg Clostridium perfringens type D
• viral infections eg infectious bovine rhinotracheitis (IBR), sporadic bovine encephalomyelitis (SBE), and bovine malignant catarrh (BMC)
• bacterial infections eg listeriosis

Blood concentrations of lead over 0.5 mg/kg, and liver or kidney concentrations over 10 mg/kg are considered diagnostic in adult cattle. Faecal concentrations over 50 mg/kg indicate substantial exposure. However, age and species influence susceptibility.

Specimens for laboratory examination for lead

Clotted blood and faeces should be submitted from live animals. From dead animals submit liver and kidney, unpreserved. Preserved liver/kidney, EDTA blood and a blood smear may also assist differential diagnosis. In addition, supply samples of any suspected sources of lead.

Treatment

1.      Treatment with calcium versenate as a chelating agent, by intravenous injection, is recommended. Treatment over several days may be necessary. Treated animals will need to be under the care of a veterinary surgeon.

2.      Supportive therapy includes fluids, and sedatives to combat convulsions.

Disposal of lead sources

Lead batteries may be sold to scrap metal merchants. In some areas, local garages may accept sump oil and batteries for disposal. Such wastes should not be disposed of in general rubbish dumps. Council dumps often have provision for toxic waste disposal. If other alternatives are not feasible, toxic material should be buried under 2m of soil in a position where ground water from the area will not contaminate water supplies.

Food safety of animals with elevated lead concentrations

Livestock that access a source of lead may survive but may have lead concentrations that prevent their sale into the food chain. Exposed animals may have lead concentrations exceeding the Food Standards Code regardless if the animals show clinical signs of lead poisoning.

According to part 1.4 Contaminants and residues, Standard 1.4.1 Contaminants and natural toxicants of the Food Standards Code, the levels set for lead in tissues of stock are:

Lead

Edible offal of cattle, sheep, pig and poultry 0.5 mg/kg

Meat of cattle, sheep, pig and poultry (excluding offal) 0.1 mg/kg

Contaminated stock may be restricted from sale or movement so that any public health and safety risk is adequately managed. The detection of significant levels of lead in meat may also impact on access to overseas markets.