This condition results from the ingestion of large amounts of lead-containing substances usually over a short period of time. Affected individuals may be found dead, or they may develop acute neurological signs, or they may appear blind. 

AETIOLOGY

Lead poisoning in cattle occurs when large amounts of lead are ingested over a relatively short period of time. The most readily available sources of this heavy metal include old paint (doors, tins), metallic lead (batteries, shot) and also old engine oil, linoleum and felt. Forage can become contaminated by fall-out from smelters, by the application of sewage sludge, as a result of a high lead content in the soil and from the combustion of leaded petrol.

The susceptibility to lead poisoning depends upon the animal species, type of compound, age and diet. The incidence of disease is high in ruminants perhaps because particulate matter can remain in the stomach where it is converted to soluble lead acetate in the acid environment. Young cattle are more often affected than adults because firstly, they are more inquisitive and tend to lick everything secondly, their tolerance for lead is only about half that per unit body weight of adult cattle and thirdly, their absorptive efficiency is greater (x2). High dietary calcium and phosphorus decreases lead absorption whereas low dietary calcium has the opposite effect. The various forms of lead vary in their solubility with lead acetate being the most soluble. Only about 2% of inorganic lead is actually absorbed because insoluble complexes are formed in the alimentary tract and these are excreted in the faeces.

Although all ages can be affected, the prevalence is comparatively high in young animals because they are so inquisitive. Cases occur in housed and grazing animals, and, although only one or two animals are usually affected, the incidence of disease can occasionally be much higher eg. in cattle eating contaminated silage.

CLINICAL SIGNS

These are the result of firstly, acute central nervous system involvement because a large amount of lead has been ingested over a short time and secondly, intestinal tract involvement as a result of the caustic action of the lead salts on the gut. There is usually a delay of a few days following a toxic dose. Two forms of the disease are recognised in cattle.

Acute Form. This is most common in young animals. The signs develop suddenly and persist for only a short time (12-36 hours). Therefore, many cases are found dead (Sudden Death). Initially, individuals are blind, staggery, with muscle tremors especially of the head and neck. When stimulated, periods of intense excitement (rolling eyes, champing jaws, frothing at the mouth) are followed by depression and head-pressing. Eventually, the animal collapses, develops intermittent convulsions and soon dies because of respiratory failure.

Subacute Form. This is most common in adults. There is extreme dullness, anorexia, drooling of saliva and teeth-grinding. There are signs of abdominal pain, a cessation of ruminal contractions with constipation followed by a foul smelling diarrhoea. Such animals are reluctant to move and, when made to do so, they circle either way because of blindness. Death usually occurs after 3-4 days.

A syndrome characterised by poor growth rate and even weight loss has been recognised in growing cattle grazing over old lead workings. This can be considered to be chronic lead poisoning.

DIAGNOSIS

TREATMENT

The recommended drug is sodium calcium edetate given intravenously. This should be repeated daily for 2-3 days if considered necessary. Blood lead levels must be determined at least 7 days apart after 28 days has elapsed from full clinical recovery.

If the blood lead concentration is in excess of 1.0 ppm then the prognosis is bad. Recovery can take up to 2 weeks, during which time supportive therapy might be necessary. The blindness which usually persists for several days after a general recovery, can be permanent.

After treating the sick animals, the source of the lead should be looked for and if found, disposed of. In some cases, an obvious source may not be found.

PREVENTION